For list of authors, see Credits (Chapter 18).
This is a DRAFT proposal. Some version of this document will become version 0.99.
Comments on this document can be sent to the MNG specification maintainers at one of the following addresses:
Distribution of this memo is unlimited.
At present, the latest version of this document is available on the World Wide Web from
ftp://swrinde.nde.swri.edu/pub/mng/documents/.
This document defines the MNG (Multiple-image Network Graphics) format. It also defines the MNG-LC (Low Complexity), MNG-VLC (Very Low Complexity), and JNG (JPEG Network Graphics) formats, which are proper subsets of MNG.
MNG is a multiple-image member of the PNG (Portable Network Graphics) format family that can contain animations, slide shows, or complex still frames, comprised of multiple PNG or JNG single-image datastreams.
The MNG and JNG formats use the same chunk structure that is defined in the PNG specification, and they share other features of the PNG format. Any MNG decoder must be able to decode PNG or JNG datastreams.
The MNG format provides a mechanism for reusing image data without having to retransmit it. Multiple images can be composed into a "frame" and a group of images can be used as an animated "sprite" that moves from one location to another in subsequent frames. "Palette animations" are also possible. MNG (but not MNG-LC or MNG-VLC) can also store images in a highly compressible "Delta-PNG" format, defined herein.
A MNG frame normally contains a two-dimensional image or a two-dimensional layout of smaller images. It could also contain three-dimensional "voxel" data arranged as a series of two-dimensional planes (or tomographic slices), each plane being represented by a PNG or Delta-PNG datastream.
A Delta-PNG datastream defines an image in terms of a parent PNG or Delta-PNG image and the differences from that image. This has been demonstrated to provide a much more compact way of representing subsequent images than using a complete PNG datastream for each.
This document includes examples that demonstrate various capabilities of MNG including simple movies, composite frames, loops, fades, tiling, scrolling, storage of voxel data, and converting GIF animations to MNG format.
If "231" looks like
the number "231"
instead of 2 raised to the power
31, your viewer is not
recognizing the HTML 4.0 <SUP> tag; you need
to look at the HTML 2.0, ASCII text, or PostScript
version of this document instead.
This specification defines the format of a MNG (Multiple-image Network Graphics) format. It also defines low-complexity and very-low-complexity versions (MNG-LC and MNG-VLC), which are proper subsets of MNG.
Note: This specification depends on the PNG (Portable Network Graphics) [PNG] and the JPEG (Joint Photographic Experts Group) specifications. The PNG specification is available at the PNG web site,
http://www.libpng.org/pub/png/
A MNG datastream describes a sequence of zero or more single frames, each of which can be composed of zero or more embedded images or directives to show previously defined images.
The embedded images can be PNG, JNG, or Delta-PNG datastreams. MNG-LC and MNG-VLC datastreams do not contain JNG datastreams, but MNG-LC and MNG-VLC applications can be enhanced to recognize and process those as well.
A typical MNG datastream consists of:
MNG is fundamentally declarative; it describes the elements that go into an individual frame. It is up to the decoder to work out an efficient way of making the screen match the desired composition whenever a nonzero interframe delay occurs. Simple decoders can handle it as if it were procedural, compositing the images into the frame buffer in the order that they appear, but efficient decoders might do something different, as long as the final appearance of the frame is the same.
Images can be "concrete" or "abstract". The distinction allows decoders to use more efficient ways of manipulating images when it is not necessary to retain the image data in its original form or equivalent in order to show it properly on the target display system.
MNG is pronounced "Ming."
When a MNG datastream is stored in a file, it is recommended that ".mng" be used as the file suffix. In network applications, the Media Type "video/x-mng" can be used. Registration of the media type "video/mng" might be pursued at some future date.
The MNG datastream begins with an 8-byte signature containing
138 77 78 71 13 10 26 10
(decimal) which is similar to the PNG signature with "\212 M N G" instead of "\211 P N G" in bytes 0-3.
MNG does not yet accommodate sound or complex sequencing information, but these capabilities might be added at a later date, in a backward-compatible manner. These issues are being discussed in the mng-list@ccrc.wustl.edu mailing list.
Chunk structure (length, name, data, CRC) and the chunk-naming system are identical to those defined in the PNG specification. As in PNG, all integers that require more than one byte must be in network byte order.
The chunk copying rules for MNG employ the same mechanism as PNG, but with rules that are explained more fully (see below, Chapter 7). A MNG editor is not permitted to move unknown chunks across the SAVE and SEEK chunks, across any chunks that can cause images to be displayed, or into or out of a IHDR-IEND or similar sequence.
Note that decoders are not required to follow any decoding models described in this specification nor to follow the instructions in this specification, as long as they produce results identical to those that could be produced by a decoder that did use this model and did follow the instructions.
Each chunk of the MNG datastream or of any embedded object is an independent entity, i.e., no chunk is ever enclosed in the data segment of another chunk. MNG-compliant decoders are required to recognize and decode independent PNG or JNG datastreams.
Because the embedded objects making up a MNG are normally in PNG format, MNG shares the good features of PNG:
In addition:
See also the glossary in the PNG specification.
In MNG-VLC datastreams, each frame (except for the first, which also includes the background layer) contains a single layer, unless the framing rate (from the MHDR ticks-per-second field) is zero. When the framing rate is zero, the entire datastream describes a single frame.
When the layers of a frame do not cover the entire area defined by the width and height fields from the MHDR chunk, the layers are composited over the previous frame to obtain the new frame.
When the frame includes the background layer, and the background layer is transparent, the transparent background is composited against the outside world and the remaining layers are composited against the result to obtain the new frame.
`object_id=N'".
In MNG-LC and MNG-VLC, only image 0 is permitted.
Note that a layer can be completely empty if the image is entirely outside the clipping boundaries.
A layer can be thought of as a transparent rectangle with the same dimensions as the frame, with an image composited into it, or it can be thought of as a rectangle having the same dimensions (possibly zero) and location as those of the object after it has been located and clipped.
The layers in a MNG datastream are gathered into one or more subframes for convenience in applying frame parameters to a subset of the layers (see the definition of "subframe" below).
An embedded visible PNG or JNG datastream generates a single layer, even though it might be interlaced or progressive. If the background consists of both a background color and a background image, these are combined into a single layer.
object_id is
an unsigned sixteen-bit number that serves as the identifier of a set of
object attributes. In MNG-LC
and MNG-VLC
only object 0 is permitted.
do_not_show flag to zero,
for
on-the-fly
display while the embedded image that defines it is being
cloned or
decoded.
do_not_show flag to zero.
An "object", which is identified by
an object_id, is an
image or it is a nonviewable entity that is created by the BASI
chunk. The object_id is an unsigned sixteen-bit number that
serves as the identifier of a set of object attributes.
An "image" is a viewable object.
Object 0 is a special object whose pixel data is not available for later use (see below).
An embedded object is:
Objects have object attributes that can be defined and modified by the contents of various MNG chunks. Decoders are responsible for keeping track of them. The simplest decoder might establish a 65,536-element array for each attribute, but real applications will undoubtedly use a more memory-efficient method. Object attributes include:
A nonzero object ceases to exist when it does not have the "frozen" attribute and
object_id replaces it
without an intervening DEFI chunk. In this case, the new object
inherits the object attribute set from the previous object with the same
object_id.
Object 0 always exists.
do_not_show byte of the DEFI or CLON chunk
that introduced it. The "potential visibility" of viewable objects
can be changed by the SHOW chunk. When an embedded object is
"potentially visible," it can be displayed "on-the-fly"
as it is being
decoded. Later, the SHOW chunk can direct that a "potentially
visible" viewable object be displayed.
It is permitted to change the potential visibility of "frozen"
objects; if this is done, the potential visibility must be restored to
its "saved&rdquo condition by the encoder prior to the end of the
segment.
A nonviewable object becomes viewable immediately when the decoder receives a viewable object buffer or when an image delta makes it viewable, and if the object is potentially visible it can be displayed "on-the-fly" while the object buffer is being decoded or updated. Note that object 0 is only viewable while its embedded image is being decoded and displayed on-the-fly, after which it becomes nonviewable again because no object buffer is ever created for object 0.
An object buffer is created by the appearance of an embedded object
in the datastream, with a nonzero object_id, or by the
appearance of a CLON chunk that specifies a "full clone".
The contents of an object buffer can be modified by processing an image
delta or a PAST chunk.
Object buffers contain a 2D array of pixel data and can contain additional information. In addition, decoders are responsible for keeping track of some properties of the data in the object buffer:
Object 0 conceptually never has an object buffer. Decoding applications can create one for their own convenience, but such an object buffer must never be made available to the rest of the MNG datastream or be considered viewable after it has been processed.
When the "stored objects" flag (bit 9 of the simplicity
profile) is 0
and valid (i.e., bit 6 is 1 and bit 9 is 0), an object buffer need not be
created even when an embedded object with a nonzero object_id
appears, since the flag promises that the object buffer will never be used
again. There is no requirement not to create an object buffer; no
harm will be done except for some unnecessary memory consumption.
When a partial clone is made of an object via the CLON chunk, the reference count for the object buffer is incremented, and no new object buffer is created.
When an object is discarded, and it points to an object buffer that has a nonzero reference count, that reference count is decremented, and the object buffer is also discarded if the resulting reference count is zero.
Object 0 is a special object that has a set of object attributes that control its location, clipping, and visibility properties, and also has a set of magnification factors and methods, but does not have an object buffer. The object attributes and magnification data, which can be modified by the DEFI, MOVE, CLIP, and MAGN chunks, are applied to future embedded objects whose object_id is zero. The pixel data for object 0 is available only for on-the-fly display and not available for later use. If at the end of any segment the attribute values or magnification data are different from the default/saved values, they become undefined when a SEEK chunk appears.
This chapter describes chunks that can appear at the top level of a MNG datastream. Unless otherwise specified in the Delta-PNG chapter of this specification, they need not be recognized within a Delta-PNG datastream.
This section describes critical MNG control chunks that MNG-compliant decoders must recognize and process ("processing" a chunk sometimes can consist of simply recognizing it and ignoring it; some chunks have been declared to be critical only to prevent them from being relocated by MNG editors).
The MHDR chunk is always first in all MNG datastreams except for those that consist of a single PNG or JNG datastream with a PNG or JNG signature.
The MHDR chunk contains exactly 28 bytes:
Frame width: 4 bytes (unsigned integer).
Frame height: 4 bytes (unsigned integer).
Ticks per
second: 4 bytes (unsigned integer).
Nominal layer
count: 4 bytes (unsigned integer).
Nominal frame
count: 4 bytes (unsigned integer).
Nominal play
time: 4 bytes (unsigned integer).
Simplicity
profile: 4 bytes:(unsigned integer).
bit 0:
0: Absence of any features is
unspecified. All other bits
of the simplicity profile must
be zero (i.e, all other even
numbers are invalid).
1: Absence of certain features is
specified by the remaining bits
of the simplicity profile.
(must be 1 in MNG-LC and MNG-VLC
datastreams)
bit 1:
0: Simple MNG features are absent.
1: Simple MNG features may be present.
(must be 0 in MNG-VLC datastreams)
bit 2:
0: Complex MNG features are absent.
1: Complex MNG features may be present.
(must be 0 in MNG-LC and MNG-VLC
datastreams)
bit 3:
0: Transparency is absent or can
be ignored. All images in the
datastream can be rendered as
opaque without affecting the
final appearance of any frame.
1: Transparency may be present.
bit 4:
0: JNG and JDAA are absent.
1: JNG or JDAA may be present.
(must be 0 in MNG-LC and MNG-VLC
datastreams)
bit 5:
0: Delta-PNG is absent.
1: Delta-PNG may be present.
(must be 0 in MNG-LC and MNG-VLC
datastreams)
bit 6: Validity flag for bits 7, 8, and 9
0: The absence of background transparency,
semitransparency, and stored objects
is unspecified; bits 7, 8, and 9
have no meaning and must be 0.
1: The possible presence of background
transparency is expressed by bit 7,
semitransparency by bit 8, and the
possible presence of stored objects by
bit 9.
bit 7:
0: Background transparency is absent
or can be ignored
(i.e., the first layer fills the
entire MNG frame with opaque pixels).
1: Background transparency may be
present.
bit 8:
0: Semitransparency (i.e., an image
with an alpha channel that has
values that are neither 0 nor the
maximum value) is absent.
1: Semitransparency may be present.
bit 9:
0: Object buffers need not be stored.
1: Object buffers must be stored.
(must be 0 in MNG-LC and MNG-VLC
datastreams)
bits 10-15:
Reserved for public expansion. Must
be zero in this version.
bits 16-30:
Available for private or experimental
expansion. Undefined in this version
and can be ignored.
bit 31:
Must be zero.
Decoders can ignore the "informative" frame-count, layer-count, play-time, and simplicity-profile fields.
The frame_width and frame_height fields
give the intended display size (measured in
pixels) and provide
default clipping boundaries
(see Recommendations for encoders, below).
It is strongly recommended that these be set to zero if
the MNG datastream contains no visible images.
The ticks_per_second field gives the
unit used by the FRAM chunk to specify interframe delay and timeout.
In MNG-VLC datastreams, it gives the framing rate.
It must be nonzero if the datastream contains a sequence of images.
When the datastream contains exactly one frame,
this field should be set to zero.
When this field is zero, the length of a tick is infinite, and
decoders will ignore any
attempt to define interframe delay, timeout, or any other variable that
depends on the length of a tick. If the frames are intended to be
displayed one at a time under user control, such as a slide show or
a multi-page FAX, the tick length can be set to any positive number
and a FRAM chunk can be used to set an infinite interframe
delay and a zero timeout. Unless the user intervenes, viewers will only
display the first frame in the datastream.
When ticks_per_second is nonzero,
and there is no other
information available about interframe delay,
viewers should display the sequence of frames
at the rate of one frame per tick.
If the frame-count field contains a zero, the frame
count is unspecified. If it is nonzero, it contains the number
of frames that would be displayed, ignoring the
fPRI chunks and the
TERM chunk. If the frame count is greater
than 231-1,
encoders should write 231-1, representing an infinite
frame count.
In MNG-VLC datastreams, the frame count is the same as the number of
embedded images in the datastream (or one, the background layer, if there are
no embedded images).
If the layer-count field contains a zero, the layer
count is unspecified. If it is nonzero, it contains the number of
layers (including
all background layers)
in the datastream, ignoring any effects of the
fPRI chunks and the
TERM chunk.
If the layer count is greater than 231-1, encoders
should
write 231-1, representing an infinite layer count.
In MNG-VLC datastreams, the layer count is the number of embedded images,
plus one (for the background layer).
If the nominal-play-time field contains a zero, the
nominal play time is unspecified. Otherwise, it gives the play time,
in ticks, when the file is displayed ignoring the
fPRI chunks and the
TERM chunk.
Authors who write this field should choose a
value of "ticks_per_second" that will allow the nominal play time
to be expressed in a four-bit integer. If the nominal play time is greater
than 231-1 ticks, encoders should write 231-1,
representing an infinite nominal play time.
In MNG-VLC datastreams, the nominal play time is the same as the frame count,
except when the ticks_per_second is zero, in which case the nominal play
time is also zero.
When bit 0 of the simplicity profile is zero, the simplicity (or complexity) of the MNG datastream is unspecified, and all bits of the simplicity profile must be zero. The simplicity profile must be nonzero in MNG-LC and MNG-VLC datastreams.
If the simplicity profile is nonzero, it can be regarded as a 32-bit profile, with bit 0 (the least significant bit) being a "profile-validity" flag, bit 1 being a "simple MNG" flag, bit 2 being a "complex MNG" flag, bits 3, 7, and 8 being "transparency" flags, bit 4 being a "JNG" flag, bit 5 being a "Delta-PNG" flag, and bit 9 being a "stored objects" flag. Bit 6 is a "validity" flag for bits 7, 8, and 9, which were added at version 0.98 of this specification. These three flags mean nothing if bit 6 is zero.
If a bit is zero, the corresponding feature is guaranteed to be absent or if it is present there is no effect on the appearance of any frame if the feature is ignored. If a bit is one, the corresponding feature may be present in the MNG datastream.
Bits 10 through 15 of the simplicity profile are reserved for future MNG versions, and must be zero in this version.
Bits 16 through 30 are available for private test or experimental versions. The most significant bit (bit 31) must be zero.
When bit 1 is zero ("simple" MNG features are absent), the datastream does not contain the DEFI, FRAM, MAGN, or global PLTE and tRNS chunks, and filter method 64 is not used in any embedded PNG datastream.
When bit 2 is zero, the datastream does not contain any "complex MNG
features". These are the BASI,
CLON, DHDR/IEND, PAST, DISC,
MOVE, CLIP, and SHOW
chunks, or any future chunk that defines or uses stored objects. If
the DEFI chunk is present, it only defines object 0.
If the BACK chunk is present, it does not define
a background image.
If the LOOP chunk is present, it has iteration_min=1.
A MNG with a "complex MNG feature" (which has a simplicity
profile that has bit 2 set to 1) may contain
at least one of these chunks. A simple
decoder can display "simple" MNGs (which have a simplicity
profile with bit 2 set to 0) without having to store any
objects or dealing with the SAVE/SEEK mechanism, and it can ignore
the LOOP and ENDL chunks and execute all loops
exactly once.
"Transparency is absent or can be ignored" means that either the MNG or PNG tRNS chunk is not present and no PNG or JNG image has an alpha channel, or if they are present they have no effect on the final appearance of any frame and can be ignored (e.g., if the only transparency in a MNG datastream appears in a thumbnail that is never displayed in a frame, or is in some pixels that are overlaid by opaque pixels before being displayed, the transparency bit should be set to zero).
"Semitransparency is absent means that if the MNG or PNG tRNS chunk is present or if any PNG or JNG image has an alpha channel, they only contain the values 0 and the maximum (opaque) value. It also means that the JDAA chunk is not present. The "semitransparency" flag means nothing and must be 0 if bit 3 is 0 or bit 6 is 0.
"Background transparency is absent" means that the first layer of every segment fills the entire frame with opaque pixels, and that nothing following the first layer causes any frame to become transparent. Whatever is behind the first layer does not show through.
When "Background transparency" is present, the application is responsible for supplying a background color or image against which the MNG background layer is composited, and if the MNG is being displayed against a changing scene, the application should refresh the entire MNG frame against a new copy of the background layer whenever the application's background scene changes. The "background transparency" flag means nothing and must be 0 if bit 6 is 0. Note that bit 3 does not make any promises about background transparency.
The "stored objects" flag is only useful when bit 2 is nonzero (i.e., "complex MNG features" are present). This flag promises that even though such features are present, no chunk will ever use the information in an existing object buffer; therefore it is not necessary to store an object buffer for any object. An object attributes set is necessary for each object, however. Therefore, the MOVE, CLIP, DISC, deterministic LOOP, partial CLON, and immediately-displayed BASI chunk are permissible. The "stored objects" flag means nothing and must be 0 if bit 2 is 0 or bit 6 is 0.
A MNG-LC (i.e., a "low-complexity MNG") datastream must have a simplicity profile with bit 0 equal to 1 and all other bits except possibly for bits 1, 3, 6, 7, and 8 ("simple MNG" MNG features and transparency) equal to zero. If bit 4 (JNG) is 1, the datastream is a "MNG-LC that might contain a JNG" datastream carrying an image or an alpha channel. MNG-LC decoders are allowed to reject such datastreams unless they have been enhanced with JNG capability.
A MNG-VLC (i.e., a "very low-complexity MNG") datastream must have a simplicity profile with bit 0 equal to 1 and all other bits except possibly for bits 3, 6, 7, and 8 (transparency) equal to 0. If bit 4 (JNG) is 1, the datastream is a "MNG-VLC with JNG" datastream. It might contain a JNG datastream carrying an image or an alpha channel. MNG-VLC decoders are allowed to reject such datastreams unless they have been enhanced with JNG capability.
Encoders that write a nonzero simplicity profile should endeavor to be accurate, so that decoders that process it will not unnecessarily reject datastreams or avoid possible optimizations. For example, the simplicity profile 351 (0x15f) indicates that JNG, critical transparency, semitransparency, and at least one "complex" MNG feature are all present, but Delta-PNG, stored objects, and background transparency are not. This example would not qualify as a MNG-LC or a MNG-VLC datastream because a "complex" MNG feature might be present. If the simplicity profile promises that certain features are absent, but they are actually present in the MNG datastream, the datastream is invalid.
The MEND chunk's data length is zero. It signifies the end of a MNG datastream.
The LOOP chunk provides a "shorthand" notation that can be used to avoid having to repeat identical chunks in a MNG datastream. The LOOP chunk can be ignored by MNG-LC and MNG-VLC decoders, along with the ENDL chunk. Its contents are the first two or more of the following fields. If any field is omitted, all remaining fields must also be omitted:
Nest level: 1 byte (unsigned integer).
Iteration count: 4 bytes (unsigned integer),
range [0..2^31-1].
Termination
condition: 1 byte (unsigned integer).
Must be omitted if termination_condition=0,
which means Deterministic, not cacheable,
or if iteration_count=0.
1: Decoder discretion, not cacheable.
2: User discretion, not cacheable.
3: External signal, not cacheable.
4: Deterministic, cacheable.
5: Decoder discretion, cacheable.
6: User discretion, cacheable.
7: External signal, cacheable.
Iteration min: 4 bytes(unsigned integer). Must be
present if termination_condition
is 3 or 7. If omitted, the default
value is 1.
Iteration max: 4 bytes (unsigned integer). Must be
present if termination_condition is
3 or 7; must be omitted if iteration_min
is omitted; if omitted, the default value
is infinity.
Signal number: 4 bytes (unsigned integer). Must be
present if termination_condition is 3 or 7.
Additional
signal number: 4 bytes. May be present if
termination_condition is 3 or 7.
...etc...
Decoders must treat the chunks enclosed in a loop exactly as if they
had been repeatedly spelled out. Therefore, during the first iteration
of the loop, the parent objects for any Delta-PNG datastreams in the
loop are the images in existence prior to entering the LOOP
chunk, but in subsequent iterations these parent objects might have been
modified. The termination_condition field can be used to
inform decoders that it is safe to change the number of loop iterations.
Simple decoders can ignore all fields except for the
iteration_count.
When the LOOP chunk is present, an ENDL chunk
with the same nest_level must be present later in the MNG
datastream. Loops can be nested. Each inner loop must have a higher
value of nest_level than the loop that encloses it, though
not necessarily exactly one greater.
The termination condition specifies how the actual number of iterations is determined. It is very similar to the termination condition field of the FRAM chunk, and can take the same values:
iteration_min nor more than
iteration_max. If the decoder has no reason to choose its
own value, it should use the iteration count. One example of a decoder
wishing to choose its own value is a real-time streaming decoder
hovering at a loop while waiting for its input buffer to fill to a
comfortable level.
iteration_min and iteration_max limits. Some
decoders might not be able to interact with the user, and many decoders
will find that nested user-discretion loops present too great of a
user-interface challenge, so the <user-discretion> condition
will probably usually degenerate into the <decoder-discretion>
condition.
iteration_min nor more than iteration_max. The
exact number can be determined by the arrival of a signal whose number
matches one of the signal_number fields.
When the value of the termination condition flag is 4 or more, the loop is guaranteed to be "cacheable", which means that every iteration of the loop produces the same sequence of frames, and that all objects and object buffers are left in the same condition at the end of each iteration. Decoders can use this information to select a different strategy for handling the loop, such as storing the composited frames in a cache and replaying them rather than decoding them repeatedly.
The iteration_min and iteration_max
can be omitted. If the condition is <deterministic> the
values are not used. Otherwise,
defaults of 1 and <infinity> are used. The iteration_count,
iteration_min, and iteration_max can be any
non-negative integers or <infinity>, but they must satisfy
iteration_min <= iteration_count <= iteration_max.
Infinity is represented by 0x7fffffff. If all of the loops in
a MNG datastream have iteration_min=1, the datastream
can qualify as a "simple" MNG for the purpose of setting bits
1 and 2
of the "simplicity profile" to zero, unless there are other
reasons for setting them to one.
If iteration_count is zero,
the termination_condition, and the remaining fields must be
omitted, and the loop is done zero times.
Upon encountering a LOOP chunk with iteration_count=0,
decoders simply skip chunks until the matching ENDL chunk is
found, and resume processing with the chunk immediately following it.
The signal_number can be omitted only if the termination
condition is not <external-signal>. There can be any number
of signal_number fields. Signal_number = 0 is
reserved to represent any input from a keyboard or pointing device,
and 1-255 are reserved to represent the corresponding character code,
received from a keyboard or simulated keyboard, and values 256-1023 are
reserved for future definition by this specification.
An infinite or just overly long loop could give the appearance of having locked up the machine. Therefore a decoder should always provide a simple method for users to escape out of a loop or delay, either by abandoning the MNG entirely or just proceeding to the next SEEK chunk (the SEEK chunk makes it safe for a viewer to resume processing after it has jumped out of the interior of a segment).
MNG editors that extract a series of PNG or JNG files from a MNG datastream
are expected to execute the loop only iteration_min times, when
the termination condition is not <deterministic>.
The ENDL chunk ends a loop that begins with the LOOP chunk. It contains a single one-byte field:
Nest_level: 1 byte (unsigned integer),
range [0..255].
When the ENDL chunk is encountered, the loop
iteration_count is decremented, if it is not already zero. If
the result is nonzero, processing resumes at the beginning of the loop.
Otherwise processing resumes with the chunk immediately following the
ENDL chunk.
When the ENDL chunk is present, a LOOP chunk with
the same nest_level must be present earlier in the MNG
datastream. See below.
Loops must be properly
nested: if a LOOP chunk with higher nest_level
appears inside a LOOP/ENDL pair, a matching ENDL chunk
must also appear to close it.
The SAVE and SEEK chunks are not permitted inside a LOOP-ENDL pair. To rerun an entire datastream that includes these chunks, use the TERM chunk instead. See below (Paragraph 4.3.6).
The DEFI chunk sets the default object attribute
set (object_id, do_not_show flag,
concrete_flag, location, and clipping boundaries) for
any subsequent images that are defined with IHDR-IEND,
BASI-IEND,
or JHDR-IEND datastreams.
Bit 1 of the MHDR simplicity profile can be used to promise that the DEFI chunk is not present.
The DEFI chunk contains 2, 3, 4, 12, or 28 bytes. If any field is omitted, all remaining fields must also be omitted.
Object id: 2 bytes (unsigned integer) identifier
to be given to the objects that
follow the DEFI chunk.
This field must be zero in MNG-LC
files.
Do_not_show
flag: 1 byte (unsigned integer)
0: Make the objects potentially
visible.
1: Make the objects not potentially
visible.
Concrete flag: 1 byte (unsigned integer)
0: Make the objects "abstract" (image
cannot be the source for a
Delta-PNG)
1: Make the objects "concrete" (object
can be the source for a Delta-PNG).
MNG-LC decoders can ignore this flag.
X_location: 4 bytes (signed integer).
The X_location and Y_location fields can
be omitted as a pair.
Y_location: 4 bytes (signed integer).
Left_cb: 4 bytes (signed integer). Left clipping
boundary. The left_cb, right_cb,
top_cb, and bottom_cb fields can be
omitted as a group.
Right_cb: 4 bytes (signed integer).
Top_cb: 4 bytes (signed integer).
Bottom_cb: 4 bytes (signed integer).
If the object number for an object is nonzero, subsequent chunks can use this number to identify it.
When the object number for an object is zero, its object buffer
can be discarded immediately after it has been processed, and it can
be treated as an "abstract" image, regardless of the contents of the
concrete_flag field.
Negative values are permitted for the X and Y location and clipping boundaries. The left and top boundaries are inclusive, while the right and bottom boundaries are exclusive. The positive directions are downward and rightward from the frame origin (see Recommendations for encoders, below).
Multiple IHDR-IEND, JHDR-IEND, and
BASI-IEND objects can follow a single DEFI chunk.
When object_id is nonzero, the DEFI chunk values
remain in effect until another DEFI chunk or a SEEK
chunk appears, unless they are modified by SHOW, MOVE,
or CLIP chunks.
The object_id and concrete_flag
can only be changed by using another DEFI chunk. If no
DEFI chunk is in effect (either because there is none in
the datastream, or because a DISC or SEEK chunk has
caused it to be discarded),
the decoder must use the following default values:
Object_id = 0
Do_not_show = 0
Concrete_flag = 0
X_location = 0
Y_location = 0
Left_cb = 0
Right_cb = frame_width
Top_cb = 0
Bottom_cb = frame_height
The object attributes for all existing unfrozen objects except for object 0 become undefined when a SEEK chunk is encountered.
The object attributes for object 0 become undefined when a SEEK chunk is encountered, only if they have been reset to values other than these defaults. It is the encoder's responsibility to reset them explicitly to these values prior to the end of every segment in which they have been changed, or to include a full DEFI chunk prior to embedding object 0 in any segment.
These default values are also used to fill any fields that were omitted from
the DEFI chunk, when an object with the same object_id
has not been previously defined or a DISC or SEEK chunk has
caused it to be discarded.
An object attributes set is created or an existing one is modified when
the DEFI chunk appears, but an object buffer is neither created
nor discarded.
If object_id is an identifier that already exists when
a DEFI chunk appears, the object attribute set (except for
the pointer to the object buffer) is immediately replaced, but the
contents of the object buffer do not change until and unless an IHDR,
JHDR, or BASI chunk is encountered; then all of the contents
of the object buffer previously associated with the identifier are discarded
and the data from the new embedded object is stored in the object buffer.
Note that if the object has partial clones, the object buffer of the clones is naturally affected because it is shared, but the object attribute sets of the clones are not affected.
The PLTE chunk has the same format as a PNG PLTE chunk. It provides a global palette that is inherited by PNG datastreams that contain an empty PLTE chunk.
The tRNS chunk has the same format as a PNG tRNS chunk. It provides a global transparency array that is inherited along with the global palette by PNG datastreams that contain an empty PLTE chunk.
If a PNG datastream is present that does not contain an empty PLTE chunk, neither the global PLTE nor the global tRNS data is inherited by that datastream.
If the global PLTE chunk is not present, each indexed-color PNG in the datastream must supply its own PLTE (and tRNS, if it has transparency) chunks.
The global PLTE chunk is not permitted in MNG-VLC datastreams.
A PNG (Portable Network Graphics) datastream.
See the PNG specification [PNG] and the Extensions to the PNG Specification document [PNG-EXT] for the format of the PNG chunks.
The IHDR and IEND chunks and any chunks between them are written and decoded according to the PNG specification, except as extended in this section. These extensions do not apply to standalone PNG datastreams that have the PNG signature, but only to PNG datastreams that are embedded in a MNG datastream that begins with a MNG signature. Nor are they allowed in MNG-VLC datastreams.
64: Adaptive filtering with five basic types
and intrapixel differencing.
The intrapixel differencing transformation, which is a modification of a method previously used in the LOCO image format [LOCO], is
S0 = Red - Green (when color_type is 2 or 6) S1 = Green (when color_type is 2 or 6) S2 = Blue - Green (when color_type is 2 or 6) S3 = Alpha (when color_type is 6)
in which S0-S3 are the samples to be passed to the next stage of of the filtering procedure.
The transformation is done in integer arithmetic in sufficient precision to hold intermediate results, and the result is calculated modulo 2^sample_depth. Intrapixel differencing (subtracting the green sample) is only done for color types 2 and 6, and only when the filter method is 64. This filter method is not permitted in images with color types other than 2 or 6.
Conceptually, the basic filtering is done after the intrapixel differencing transformation has been done for all pixels involved in the basic filter, although in practice the operations can be combined.
To recover the samples, the transformation is undone after undoing the basic filtering, by the inverse of the intrapixel differencing transformation, which inverse is
Red = S0 + S1 Green = S1 Blue = S2 + S1 Alpha = S3
As in the forward transformation, the inverse transformation is done in integer arithmetic in sufficient precision to hold intermediate results, and the result calculated modulo 2^sample_depth.
Applications that convert a MNG datastream to a series of PNG datastreams must convert any PNG datastream with the additional filter method 64 to a standard PNG datastream with a PNG filter method (currently 0 is the only valid filter method).
The extra filter method can also be used in PNG datastreams that is embedded in Delta-PNG and BASI datastreams.
It is suggested that encoders write a "nEED MNG-1.0" chunk if they use this feature, for the benefit of pre-MNG-1.0 decoders.
Applications must not write MNG-VLC datastreams or independent PNG datastreams (with either the .png or .mng file extension) with the new filter method, until and unless it should become officially approved for use in PNG datastreams.
Note that the top-level color space chunks are used only to supply missing color space information to PNG or JNG datastreams when the datastreams are being decoded. They do not have any effect on already-decoded objects.
If do_not_show=0 for the image when the IHDR
chunk is encountered, a viewer can choose to display the image while
it is being decoded, perhaps taking advantage of the PNG interlacing
method, or to display it after decoding is complete.
If object_id=0, there is no need to store the
pixel data after decoding it and perhaps displaying it.
If concrete_flag=1 and object_id != 0,
the decoder must store the original pixel data losslessly, along
with data from other recognized PNG chunks, because it is possible
that a subsequent Delta-PNG datastream might want to modify it. If
concrete_flag=0, the decoder can store the pixel data in any
form that it chooses.
If the "stored objects" flag in the simplicity profile is
valid and zero, there is no need to store the pixel data and other chunk
data after decoding and perhaps displaying the image.
If an object already exists with the same object_id, the
contents of its object buffer are replaced with the new data.
A JNG (JPEG Network Graphics) datastream.
See the JNG specification below (Chapter 5) for the format of the JNG datastream.
The JHDR and IEND chunks and any chunks between them are written and decoded according to the JNG specification.
The remaining discussion in the previous paragraph about PNG datastreams also applies to JNG datastreams.
MNG-LC and MNG-VLC applications are not expected to process JNG datastreams unless they have been enhanced with JNG capability.
The BASI chunk introduces a basis object that, while it might be incomplete, can serve as a parent object to which a delta image can be applied.
The first 13 bytes of the BASI chunk are identical to those of the IHDR chunk. The next 8 bytes, which can be omitted, provide sixteen-bit {red, green, blue, alpha} values that are used to fill the entire basis object when the IDAT chunk is not present, and a 1-byte "viewable" flag can also be present.
Width: 4 bytes (unsigned integer).
Height: 4 bytes (unsigned integer).
Sample_depth: 1 byte (unsigned integer)
1, 2, 4, 8, or 16.
Color_type: 1 byte (unsigned integer).
0: Gray, 2: RGB, 3: indexed
color, 4: Gray-alpha, 6: RGBA
Compression_method: 1 byte (unsigned integer).
0: zlib with deflate
Filter_method: 1 byte (unsigned integer).
0: five basic filter types.
64: intrapixel differencing
and five basic filter types.
Interlace_method: 1 byte (unsigned integer).
0: none, 1: Adam7
Red_sample or
gray_sample: 2 bytes (unsigned integer).
Green_sample: 2 bytes (unsigned integer).
Blue_sample: 2 bytes (unsigned integer).
Alpha_sample: 2 bytes (unsigned integer).
Viewable: 1 byte (unsigned integer).
0: Basis object is not viewable.
1: Basis object is viewable.
The sample depth, color type, compression method, and interlace method must be valid PNG types, and the width and height must be within the valid range for PNG datastreams. The filter method must be one of the filter methods allowed in PNG datastreams (currently only 0) or the additional filter method (64) allowed in PNG datastreams that are embedded in MNG datastreams.
The alpha_sample can be omitted if the viewable
field is also omitted. If so, and the color_type is one
that requires alpha, the alpha value corresponding to an opaque
pixel will be used. If the color samples are omitted, zeroes will
be used. If the viewable field is omitted, the object is not
viewable.
The decoder is responsible for converting the color and
alpha samples to the appropriate format and sample depth for the
specified color_type.
The color and alpha samples are written as four sixteen-bit samples
regardless of the color_type and sample_depth.
When the sample_depth is less than sixteen, the least
significant bits are used and the remaining bits must be zero
filled. When color_type=3, the least significant byte
of each color sample and the alpha sample is used and the upper byte must
be zero.
When color_type=0 or color_type=4, the green and blue
samples must be present but must be ignored by decoders.
When color_type=0 or color_type=2,
only the values 0 and 2sample_depth should be written.
Any other alpha value must be interpreted as fully opaque.
When color_type=3, the
decoder must generate a palette of length 2sample_depth,
whose first entry contains the given {red_sample, green_sample,
blue_sample} triple, and whose remaining entries are filled with
zeroes. It must also generate an alpha array whose first entry is the
given alpha sample and the rest are opaque
(i.e., if the alpha sample is not opaque, it creates a one-entry tRNS
chunk containing the least significant byte of the given alpha sample).
The BASI datastream contains PNG chunks, but is not necessarily a PNG datastream. It can be incomplete or empty and it can deviate in certain ways from the PNG specification. It can serve as a parent object for a Delta-PNG datastream, which must supply the missing data or correct the other deviations before the image is displayed. The end of the datastream is denoted by an IEND chunk.
The permitted deviations from the PNG format in a BASI datastream are:
color_type=3. If so, the subsequent Delta-PNG that uses this
as the parent object can supply a complete replacement PLTE chunk,
if the single-entry palette that is generated is not desired.
This deviation is only permitted when the object is concrete and not viewable.
The BASI chunk can be used to introduce such things as a library of iCCP chunks from which one or another can be selected for use with any single image, or it can be used to introduce a simple blank or colored rectangle that will be immediately displayed or into which other images will be pasted by means of the PAST chunk.
A BASI chunk appearing in a MNG datastream receives its
object_id, location, and potential visibility from the
preceding DEFI chunk, if one is present, or the default
values for DEFI, if one is not present. The
concrete_flag can be either 0 (abstract) or 1 (concrete),
depending on whether the basis image is intended for subsequent use by
a Delta-PNG datastream or not. When it is abstract it must also be
viewable. When it is viewable, the
resulting object, after the pixel samples are filled in, must be identical
to an object that would have been obtained by decoding a legal
PNG datastream. If viewable=1 and
do_not_show=0, a viewer is expected to display
it immediately, as if it were decoding a PNG datastream.
If an object already exists with the same object_id, the
contents of its object buffer are replaced with the new data.
Top-level gAMA, sRGB, cHRM, bKGD, sBIT, pHYs, iCCP, and sPLT chunks are inherited by a BASI datastream in the same manner as by a PNG datastream.
No provision is made in this specification for storing a BASI datastream as a standalone file. A BASI datastream will normally be found as a component of a MNG datastream. Applications that need to store a BASI datastream separately should use a different file signature and filename extension, or they can wrap it in a MNG datastream consisting of the MNG signature, the MHDR chunk, the BASI datastream, and the MEND chunk.
Create a clone (a new copy) of an image, with a new
object_id. The CLON chunk contains 4, 5, 6, 7, or
16 bytes. If a field is omitted, all remaining fields must also be
omitted.
Source_id: 2 bytes (nonzero unsigned integer).
Identifier of the parent object to be
cloned.
Clone_id: 2 bytes (nonzero unsigned integer).
Identifier of the child object that is
created.
Clone_type: 1 byte (unsigned integer).
0: Full clone of the object attributes
set and the object buffer.
1: Partial clone; only object attributes
set (the location, clipping boundaries,
and potential visibility) are copied
and a link is made to the object
buffer.
2: Renumber object (this is equivalent to
"CLON source_id clone_id 1
DISC source_id").
If this field is omitted, the clone_type
defaults to zero (full clone).
Do_not_show: 1 byte (unsigned integer).
0: Make the clone potentially visible
and display it immediately.
1: Make the clone not potentially
visible.
2: Retain the potential visibility of
the parent object.
When this field is omitted, the object retains
the potential visibility of the parent object.
Concrete_
flag: 1 byte (unsigned integer).
0: Concrete_flag is the same as
that of the parent object.
1: Make the clone "abstract"
(concrete_flag=0).
When this field is omitted, the object retains
the concrete flag of the parent object.
Loca_
delta_type: 1 byte (unsigned integer)
0: Location data gives X_location and
Y_location directly.
1: New positions are determined by
adding the location data to the
position of the parent object.
This field, together with the X_location
and Y_location fields, can be omitted
as a group. When they are omitted, the
clone has the same location as the parent
object.
X_location or delta_
X_location:4 bytes (signed integer).
Y_location or delta_
Y_location:4 bytes (signed integer).
The source_id must be an existing object identifier, and
the clone_id must not be an existing object identifier.
Negative values are permitted for the X and Y position. The positive directions are downward and rightward from the frame origin.
The clone is initially identical to the parent object except for the
location and potential visibility. It has the same clipping boundaries
as the parent object. Subsequent DHDR, SHOW,
CLON, CLIP, MOVE, PAST, and
DISC chunks can use the clone_id to identify it. If
the parent object is not a viewable image, neither is the clone.
Subsequent chunks can modify, show, or discard a full clone or modify its potential visibility, location and clipping boundaries without affecting the parent object, or they can modify, show, or discard the parent object or modify its object attribute set without affecting the clone.
The concrete_flag byte must be zero or omitted when the
clone_type byte is nonzero.
If an object has partial clones, and the data in the object buffer of a parent object or any of its partial clones is modified, the parent object and all of its partial clones are changed. Decoders must take care that when the parent object or any partial clone is discarded, the object buffer is not discarded until the last remaining one of them is discarded. Only the location, potential visibility, and clipping boundaries can be changed independently for each partial clone.
If viewable=1 and
do_not_show=0, the resulting image is displayed immediately.
A Delta-PNG datastream.
See The Delta-PNG Format (Chapter 6),
below, for the
format of the Delta-PNG datastream. Any chunks between DHDR
and IEND are written and decoded according to the Delta-PNG
format. The object_id of the Delta-PNG DHDR
chunk must point to an existing parent object. The resulting image
is immediately displayed if its do_not_show=0. The parent
object must be concrete (i.e., it must have concrete_flag=1).
Paste an image or images identified by source_id,
or part of it, into an existing abstract image identified by
destination_id.
The PAST chunk contains a 2-byte destination_id
and 9 bytes giving a "target location", plus one or more 30-byte source
data sequences.
Destination_id: 2 bytes (unsigned integer).
Target_
delta_type: 1 byte (unsigned integer).
0: Target_x and target_y are given
directly.
1: Target_x and target_y are deltas
from their previous values in a
PAST chunk with the same
destination_id.
2: Target_x and target_y are deltas
from their previous values in the
previous PAST chunk regardless of
its destination_id.
Target_x: 4 bytes (signed integer), measured
rightward from the left edge of the
destination image.
Target_y: 4 bytes (signed integer), measured
downward from the top edge of the
destination image.
Source_id: 2 bytes (unsigned nonzero integer).
An image to be pasted in.
Composition_
mode: 1 byte (unsigned integer).
0: Composite over.
1: Replace.
2: Composite under.
Orientation: 1 byte (unsigned integer).
The source image is flipped to
another orientation.
0: Same as source image.
2: Flipped left-right, then up-down.
4: Flipped left-right.
6: Flipped up-down.
8: Tiled with source image. The
upper left corner of the assembly
is positioned according to the
prescribed offsets.
Offset_origin: 1 byte (unsigned integer).
0: Offsets are measured from the
{0,0} pixel in the destination
image.
1: Offsets are measured from the
{target_x,target_y} pixel in the
destination image.
X_offset: 4 bytes (signed integer).
Y_offset: 4 bytes (signed integer).
Boundary_origin: 1 byte (unsigned integer).
0: PAST clipping boundaries are
measured from the {0,0} pixel in
the destination image.
1: PAST clipping boundaries are
measured from the {target_x,target_y}
pixel in the destination image.
Left_past_cb: 4 bytes (signed integer).
Right_past_cb: 4 bytes (signed integer).
Top_past_cb: 4 bytes (signed integer).
Bottom_past_cb: 4 bytes (signed integer).
...etc...
The destination image must have the "abstract" property
(concrete_flag=0). When destination_id=0, the
resulting image is "write-only" and therefore only "composite-over"
(composition_mode=0) operations are permitted.
The source images can be "abstract" or "concrete" and have any
color_type and sample_depth. They must
have the "viewable" property. The number of source images is
((chunk_length-11)/30).
The x_offset and y_offset distances and the
PAST clipping boundaries are measured, in pixels, positive
rightward and
downward from either the {0,0} pixel of the destination image
or the {target_x, target_y} position in the destination
image. They do not necessarily have to fall within the destination
image. Only those pixels of the source image that fall within the
destination image and also within the specified clipping boundaries
will be copied into the destination image. The coordinate
system for offsets and clipping is with respect to the upper lefthand
corner of the destination image, which is not necessarily the same
coordinate system used by the DEFI, MOVE and CLIP
chunks.
If the source image has been flipped or rotated, X_offset and
Y_offset give the location of its new upper left hand corner.
When it is tiled, the offsets give the location of the upper left hand
corner of the upper left tile, and tiling is done to the right and down.
The PAST left and top clipping boundaries are inclusive, while the
right and bottom clipping boundaries are exclusive
(see Recommendations for encoders, below).
When composition_mode=0, any non-opaque pixels in the
source image are combined with those of the destination image. If
the destination pixel is also non-opaque, the resulting pixel will be
non-opaque.
When composition_mode=1, all pixels simply replace those
in the destination image. This mode can be used to make a transparent
hole in an opaque image.
When composition_mode=2, any non-opaque pixels in the
destination image are combined with those of the source image. If the
source pixel is also non-opaque, the resulting pixel will be non-opaque.
The order of composition is the same as the order that the
source_ids appear in the list (but a decoder can do the
composition in any order it pleases, or all at once, provided that the
resulting destination image is the same as if it had actually performed
each composition in the specified order). Decoders must be careful when
the destination image equals the source image--the pixels to be drawn
are the ones that existed before the drawing operation began.
The clipping information from the DEFI, MOVE
or CLIP chunks associated with the
destination_id and the source_ids is not used in
the PAST operation (but if a decoder is simultaneously updating
and displaying the destination_id, the clipping boundaries
for the destination_id are used in the display
operation).
This chunk provides mandatory magnification factors for existing objects and/or for subsequent embedded images whose object id is 0.
The chunk contains 0 to 18 bytes. If any field is omitted, all remaining fields must also be omitted.
First magnified object_id: 2 bytes. If omitted,
any previous MAGN chunk is nullified.
Last magnified object_id: 2 bytes. If omitted,
last object_id = first object_id.
X_method: 1 byte
0 or omitted: No magnification
1: Pixel replication of color and
alpha samples.
2: Magnified intervals with linear
interpolation of color and alpha
samples.
3: Magnified intervals with replication
of color and alpha samples from the
closest pixel.
4: Magnified intervals with linear interpolation
of color samples and replication of
alpha samples from the closest pixel.
5: Magnified intervals with linear interpolation
of alpha samples and replication of
color samples from the closest pixel.
MX: 2 bytes. X magnification factor, range 1-65535.
If omitted, MX=1. Ignored if X_method is 0
and assumed to be 1.
MY: 2 bytes. Y magnification factor. If omitted,
MY=MX.
ML: 2 bytes. Left X magnification factor. If omitted,
ML=MX.
MR: 2 bytes. Right X magnification factor. If
omitted, MR=MX.
MT: 2 bytes. Top Y magnification factor. If
omitted, MT=MY. Ignored if Y_method is 0
and assumed to be 1.
MB: 2 bytes. Bottom Y magnification factor. If
omitted, MB=MY.
Y_method: 1 byte. If omitted, Y_method is the same
as X_method.
The MAGN chunk causes the contents of the object buffers pointed to
by the specified range of objects to be immediately and irreversibly magnified.
The first object_id can be zero. If so, any subsequent embedded
objects with object_id=0 must be magnified immediately when they
appear in the datastream. Magnification factors and methods for object 0 are
updated by the appearance of a subsequent MAGN chunk whose
first object_id is 0, and magnification of object 0 is turned off
by the appearance of an empty MAGN chunk or by a MAGN
chunk whose first object_id is zero and whose X_method and Y_method
are zero, explicitly or by omission.
The magnification factor for object 0 becomes undefined when a SEEK
chunk appears. Therefore, it is the encoder's responsibility either
to include a MAGN chunk that turns off magnification of object 0
prior to the end of any segment in which object 0 was magnified, or to
include a MAGN chunk for object 0 prior to the first embedded object 0
in every segment that contains an embedded object 0.
The last object_id must be greater than or equal to the
first object_id. It
is not an error to include a nonexistent object or an existing
"frozen"
object in the range; decoders must do nothing to any such objects. If an
object is potentially visible and viewable, it is displayed immediately
after it is magnified. If any object_id is nonzero the result
of magnifying that object is stored in place of the its original object buffer
for later use.
If the MAGN chunk is present, all existing objects in the
specified range
must conceptually be magnified immediately in accordance with the given
magnification factors and methods (decoders may wish to save the magnification
factors and delay the magnification until display time, or until the object
is used as the parent object of a Delta-PNG, to save memory. There is
nothing preventing this, provided that the end effect is the same as if the
magnification had been accomplished immediately). If object
0 is in the specified range, then any subsequent embedded objects with
object_id=0 must be magnified immediately when they appear in
the datastream.
When X_method is 0, all X magnification factors in the MAGN chunk are ignored and can be assumed to be 1.
When X_method is 1, X magnification is done by simple pixel replication. The leftmost pixel of each row is replicated ML-1 times. If the original width is greater than 1, the rightmost pixel is replicated MR-1 times. If the original width is greater than 2, the original interior pixels are replicated MX-1 times. The magnified width W is
W = ML; if (width > 1) W = W + MR; if (width > 2) W = W + (width-2)*MX;
When X_method is 2, X magnification is done by linear interpolation between pixels. If the original width of the image is greater than 1, the interval between the leftmost pixel and the second pixel of each row is subdivided into ML intervals by inserting ML-1 pixels with color and alpha values that are obtained by linear interpolation. If the original width is 1, then the pixel is simply magnified as if X_method is 1. If the original width is greater than 2, the rightmost interval is subdivided into MR intervals. If the original width is greater than 3, each original interior interval is subdivided into MX intervals. The magnified width W is
/* The orginal pixels: */
W = width;
/* Add the new pixels in the left interval: */
if (width > 1) W = W + ML-1;
/* Add the new pixels in the right interval: */
if (width > 2) W = W + MR-1;
/* Add the new interior pixels: */
if (width > 3) W = W + (width-3)*(MX-1);
When X_method is 3, intervals are subdivided as in X_method 2, and the color and alpha values for the new pixels are obtained by replicating the closest original pixel, with ties being broken by replicating the pixel to the left. The magnified width is calculated in the same manner as in X_method 2.
When X_method is 4, the color samples are magnified as in X_method 2 and the alpha samples are magnified as in X_method 3.
When X_method is 5, the color samples are magnified as in X_method 3 and the alpha samples are magnified as in X_method 2.
When Y_method is 0, all Y magnification factors in the MAGN chunk are ignored and can be assumed to be 1.
When Y_method is 1, Y magnification is done by simple pixel replication. The topmost pixel of each column is replicated MT-1 times. If the original height is greater than 1, the bottom pixel is replicated MR-1 times. If the original height is greater than 2, the original interior pixels of each column are replicated MY-1 times. The magnified height H is
H = MT; if (height > 1) H = H + MB; if (height > 2) H = H + (height-2)*MY;
When Y_method is 2, Y magnification is done by linear interpolation between pixels. If the original height of the image is greater than 1, the interval between the topmost pixel and the second pixel of each column is subdivided into MT intervals by inserting MT-1 pixels with color and alpha values that are obtained by linear interpolation. If the original height is 1, then the pixel is simply magnified as if Y_method is 1. If the original height is greater than 2, the bottom interval is subdivided into MB intervals. If the original height is greater than 3, each original interior interval is subdivided into MY intervals. The magnified height H is
H = height; if (height > 1) H = H + MT-1; if (height > 2) H = H + MB-1; if (height > 3) H = H + (height-3)*(MY-1);
When Y_method is 3, intervals are subdivided as in Y_method 2, and the color and alpha values for the new pixels are obtained by replicating the closest original pixel, with ties being broken by replicating the pixel above. The magnified width is calculated in the same manner as in Y_method 2.
When Y_method is 4, the color samples are magnified as in Y_method 2 and the alpha samples are magnified as in Y_method 3.
When Y_method is 5, the color samples are magnified as in Y_method 3 and the alpha samples are magnified as in Y_method 2.
When the image being magnified is a concrete object, it must not be a JNG or indexed-color PNG (the latter could be promoted to RGB or RGBA via a Delta-PNG PROM chunk first). The result of the magnification is also a concrete object. The Method 2 magnification is conceptually done first in the vertical (Y) direction, the results rounded to the sample depth, then in the horizontal (X) direction. Linear interpolation must be done on the raw pixels, prior to any color correction, using integer arithmetic, to ensure that the result is deterministic. For each channel, the m-1 interpolated samples s[i] are obtained from the two samples s0 and s1 by the following ISO C code or by any other method that obtains the identical results:
if(s1 == s0)
for (i=1; i < m; i++)
s[i] = s0;
else
for (i=1; i < m; i++)
s[i] = ((2*i*(s1-s0)+m)/(m*2) + s0;
Signed arithmetic in a precision large enough to hold the intermediate results must be used, and the final results must be modulo the sample depth.
When the image being magnified is an abstract object (which is always true of object 0), interpolation can be done by any means that achieves a visually similar but not necessarily identical result, such as rounding the results to the sample depth later, using video hardware that is capable of interpolation, or using floating point addition in the loop instead of integer multiplication and division as in:
float delta = ((float)(s1-s0)/(float)m);
float sf= (float)s0;
for (i=1; i < m; i++) {
sf = sf+delta;
s[i]=(int)(sf+0.5);
}
If the abstract object being magnified is being stored in an indexed representation, interpolation must be accomplished by a method that achieves a similar result to that obtained by interpolating between RGB or RGBA pixels.
Note that if an object and partial clones of it appear in the range of objects to be magnified, the object buffer will be magnified repeatedly.
Because the MAGN chunk was added late in the development of
MNG-1.0, it is recommended that encoders place an empty MAGN chunk or
a nEED MAGN
chunk early in the datastream, so that pre-MNG-1.0 applications that do not
recognize the MAGN chunk will encounter one quickly.
The DISC chunk can be used to inform the decoder that it
can discard the object data associated with the associated object
identifiers. Whether the decoder actually discards the data or not, it
must not use it after encountering the DISC chunk.
The chunk contains a sequence of zero or more two-byte object
identifiers. The number of objects to be discarded is the chunk's data
length, divided by two.
If the DISC chunk is empty, all nonzero objects except those
preceding the SAVE chunk (i.e., except for
the "frozen" objects)
can be discarded. If a SAVE chunk has not been encountered, all
objects can be discarded. Note that each appearance of a SEEK
chunk in the datastream implies an empty DISC chunk.
If the DISC chunk is not empty, the listed objects can be
discarded.
When an object is discarded, any location, potential visibility, and
clipping boundary data associated with it is also discarded.
It is not an error to include an It is an error to name explicitly any "frozen" object in the
DISC list.
When the object is a partial clone or is the source of a partial
clone that has not been discarded, only the object attribute set
(location, potential visibility, clipping boundaries, etc.) can be
discarded. The data in the object buffer must be retained until the
last remaining partial clone is discarded.
The BACK chunk suggests or mandates a background
color, image, or both
against which
transparent, clipped,
or less-than-full-frame images can be displayed. This information will be
used whenever the application subsequently needs to insert a background
layer, unless another BACK chunk provides new background information
before that happens.
The BACK chunk contains 6, 7, 9, or 10 bytes. If any field is
omitted, all remaining fields must also be omitted.
The first layer displayed by a viewer is always a
background layer that fills the entire frame.
The BACK chunk provides a background that the viewer can use
for this purpose (or must use, if it is mandatory). If it is
not "mandatory"
the viewer can choose another background if it wishes. If the
BACK chunk is not present,
or if the background is not fully opaque or has been clipped to less than
full frame,
the viewer must provide
or complete
its own background layer for the first frame. Each layer
after the first must be composited over the layers that precede
it, until a FRAM chunk with framing mode 3 or 4 causes another
background layer to be generated.
Viewers are expected, however, to composite every foreground layer
against a fresh copy of the background, when the framing
mode given in the FRAM chunk is 3, and to composite the first
foreground layer of each subframe against a fresh copy of the background,
when the framing mode is 4. Also, when the framing mode is 3 or 4 and no
foreground layer appears between consecutive FRAM chunks,
a background layer alone is displayed as a separate frame.
The images and
the background are both clipped to the subframe boundaries given in the
FRAM chunk. Anything outside these boundaries is inherited
from the previous subframe.
If the background layer is transparent and the subsequent foreground layers
do not cover the transparent area with opaque pixels, the application's
background becomes re-exposed in any uncovered pixels within the subframe
boundaries.
The background image (or tiled assembly) is also
clipped to its own boundaries and located like any other image,
and is only displayed if it is potentially visible. When
the background image is used for tiling, the upper left tile is located
according to the background image's location attributes and the entire
assembly is clipped according to its clipping attributes. Viewers might
actually follow some other procedure, but the final appearance of each
frame must be the same as if they had filled the area within the subframe
boundaries with the background color, then displayed the background
image, and then displayed the foreground image (or images), without
delay.
Note that any background layer, including the one that begins the
first frame of the datastream, must be inserted at the latest
possible moment, in case the background image is
replaced or is modified
by a Delta-PNG datastream or its location or clipping boundaries are
changed by a MOVE or CLIP chunk,
or in case a new BACK chunk appears
before that moment.
It is an error to specify a It is not an error to specify a The three BACK components are always written as though
for an RGBA PNG with 16-bit sample depth. For example, a mid-level
gray background could be specified with (0x9999, 0x9999, 0x9999).
The background color is interpreted in
the current color space as defined by any top-level gAMA,
cHRM, iCCP, sRGB chunks that have appeared
prior to the BACK chunk in the MNG datastream. If no such
chunks appear, the color space is unknown.
The color space of the background image, if one is used, is determined
in the same manner as the color space of any other image.
Multiple instances of the BACK chunk are permitted in a MNG
datastream.
The BACK chunk can be omitted. If a background is needed
and the BACK chunk is omitted, then the viewer must supply its
own background. For the purpose of counting layers, such a viewer-supplied
background layer is counted the same as a background supplied by the
BACK chunk.
In practice, most applications that use MNG as part of a
larger composition should ignore the BACK data if
The FRAM chunk provides information that a decoder needs for
generating frames and interframe delays. The FRAM parameters
govern how the decoder is to behave when it encounters a FRAM
chunk, an embedded image, or a SHOW chunk.
The FRAM chunk also delimits subframes.
Bit 1 of the MHDR simplicity profile can be used to promise that
the FRAM chunk is not present.
An empty FRAM chunk is just a subframe delimiter. A
nonempty one is a subframe delimiter, and it also changes FRAM
parameters, either for the upcoming subframe or until reset
("upcoming subframe" refers to the subframe immediately following the
FRAM chunk). When the
FRAM chunk is not empty, it contains a framing-mode byte, an
optional name string, a zero-byte separator, plus four 1-byte fields
plus a variable number of optional fields.
When the FRAM parameters are changed, the new parameters
affect the subframe that is about to be defined, not the one that is
being terminated by the FRAM chunk.
Framing modes:
The framing mode provides information to the decoder that it uses
whenever it is about to display an image, and when it is processing
the next FRAM chunk.
Any of these events generates a layer, even
if no pixels are actually changed:
When a decoder is ready to perform a display update, it must check
the framing mode, to decide whether it should
restore the background (framing modes 3 and 4) or not (framing modes 1 and 2),
and whether it needs to wait for the interframe delay to elapse before
continuing (framing modes 1 and 3) or not (framing modes 2 and 4).
When the interframe delay is zero, viewers are not required actually
to update the display but can continue to process the remainder of the
frame and composite the next image over the existing frame before displaying
anything. The final result must appear the same as if each image had been
displayed in turn with no delay.
Regardless of the framing mode, encoders must insert a
background layer, with a zero delay, ahead of the first image layer in the
datastream,
even when the BACK chunk is not present or has been clipped
to less than full-frame. This layer
must be included in the layer count but not in the frame count.
Also, viewers that jump to a segment must insert a
background layer, with a zero delay, ahead of the segment,
even when the BACK chunk is not present in the prologue segment,
if they jumped from the interior of a segment.
Such layers are not included in either the layer count
or the frame count.
In the usual case, the interframe delay is nonzero, and
multiple layers are present, so each
frame is a single subframe composed of several layers. When the interframe
delay is zero, the subframe is combined with subsequent subframes until one with
a nonzero interframe delay is encountered, to make up a single frame.
The decoder must wait until the interframe delay for the previous
frame has elapsed before displaying the frame.
When
When the background layer is transparent or does not fill the clipping
boundaries of the image layer, the application
is responsible for supplying a background color or image against which
the image layer is composited, and if the MNG is being displayed
against a changing scene, the application should
refresh the entire MNG frame against a new copy of the background layer
whenever the application's background scene changes (see the
"background transparency" bit of the simplicity profile).
A transparent or clipped background layer is handled as in framing mode 3.
The subframe name must conform to the same formatting rules as
those for a PNG tEXt keyword: It must consist only of printable
Latin-1 characters and must not have leading or trailing blanks, but
can have single embedded blanks. There must be at least one (unless
the subframe name is omitted) and no more than 79 characters in the
keyword. Keywords are case-sensitive. There is no null byte within
the keyword.
No specific use for the subframe name is specified in
this document, except that it can be included in the optional index
that can appear in the SAVE chunk.
Applications can use this
field for such purposes as constructing an external list of subframe
in the datastream. The subframe name only applies to the upcoming
subframe; subsequent subframes are unnamed unless they also have their
own The interframe delay value is the desired minimum time to elapse from
the beginning of displaying one frame until the beginning of displaying
the next frame. When the interframe delay is nonzero, which will
probably be the usual case, layers are frames. When it is zero, a
frame consists of any number of consecutive subframes, until
a nonzero delay subframe is encountered and completed. Decoders are not
obligated or encouraged to display such subframes individually; they can
composite them offscreen and only display the complete frame.
There is no interframe delay before the first layer (the implicit
background layer) in the datastream nor after the final frame, regardless
of the framing mode.
The timeout field can be a number or <infinity>. Infinity
can be represented by 0x7fffffff. Under certain termination conditions,
the application can adjust the interframe delay, provided that it is
not less than the lesser of the timeout and the specified interframe delay,
and not greater than the greater of the specified interframe delay and
the timeout.
The termination condition given in the
The When the The initial values of the FRAM parameters are:
The layer clipping boundaries from
the FRAM chunk are only
used for clipping, not for placement.
The DEFI
or MOVE
chunk
can be used to specify the placement of each
image within the layer. The DEFI
or CLIP
chunk can be used to specify
clipping boundaries for each image.
Even when the left and top subframe
boundaries are nonzero, the image locations are measured with respect to
the {0,0} position in the display area.
The left and top subframe boundaries are inclusive, while the right and bottom
boundaries are exclusive.
If the layers
do not cover the entire area defined by the layer clipping
boundaries with opaque pixels, they are composited against
whatever already occupies the area, when the framing mode is 1 or 2.
When the framing mode is 3 or 4, they are composited against
the background defined by the
BACK chunk, or against an application-defined background, if
the BACK chunk is not present or does not define a mandatory
background. The images, as well as the background, are clipped to the
layer clipping boundaries for the subframe. Any pixels outside the
layer clipping boundaries remain unchanged from the previous layer.
The A viewer does not actually have to follow the procedure of erasing
the screen, redisplaying the background, and recompositing the images
against it, but what is displayed when the frame is complete must be the
same as if it had. It is sufficient to redraw the parts of the display
that change from one frame to the next.
The Note that the synchronization point does not occur immediately, but
the first frame that follows the FRAM chunk, and, if the.
The identifier New location of an existing object or objects (replacing or
incrementing the location given in the DEFI chunk).
The MOVE chunk gives the position, measured downward and
to the right of the frame origin, in pixels, where the named object or
group of objects is to be located.
The chunk's contents are:
The new location applies to a single object, if
It is not an error for the MOVE chunk to name an object that
has not previously been defined. In such cases, nothing is done to the
nonexistent object. It is permitted to move "frozen" objects
provided that the encoder includes chunks to move them back to their
original positions prior to then end of the segment.
When an object is discarded, its object attribute set, which includes
the MOVE data, is also discarded.
This chunk gives the new boundaries (replacing or incrementing those
from the DEFI chunk) to which an existing object or group of
objects must be clipped for display. It contains the following 21
bytes:
The new clipping boundaries apply to a single object, if
The clipping boundaries are expressed in pixels, measured rightward
and downward from the frame origin.
The left and top clipping boundaries are inclusive and the right and
bottom clipping boundaries are exclusive, i.e., the pixel located at
{x,y} is only displayed if the pixel falls within the physical limits of
the display hardware and all of the following are true:
It is not an error for the CLIP chunk to name an object that
has not previously been defined. In such cases, nothing is done to the
nonexistent object. It is permitted to clip "frozen" objects
provided that another CLIP chunk resets them to their original
values prior to the end of the segment.
When an object is discarded, its object attribute set, which includes
the CLIP data, is also discarded.
The SHOW chunk is used to change the potential visibility
of one or more previously-defined objects and to direct that they be
displayed. It contains 2, 4, or 5 bytes, or it can be empty. When any
field is omitted, all remaining fields must also be omitted.
The decoder processes the objects (or images) named in the
SHOW chunk in the order When the SHOW chunk is empty, the decoder displays
all existing potentially visible images, without changing their
If When When
When When Assuming a nonzero interframe delay, any of the following sequences
would cause the image identified by It is not necessary to follow an IHDR-IEND,
JHDR-IEND, BASI-IEND, or DHDR-IEND
sequence or PAST chunk with a SHOW chunk to
display the resulting image, if it was already caused to appear by
It is not an error for the SHOW chunk to name
a nonviewable object or an object that
has not previously been defined. In such cases, nothing is done to the
nonexistent object.
It is permitted to change the potential visibility of "frozen"
objects provided that another SHOW chunk resets them to their original
values prior to the end of the segment.
The TERM chunk suggests how the end of the MNG datastream
should be handled, when a MEND chunk is found. It contains
either a single byte or ten bytes:
The loop created by processing a TERM must always be treated
by the decoder as if it were a cacheable <user-discretion> loop, with
Applications must not depend on anything that has been drawn on the output
buffer or device during the previous iteration. Its contents become
undefined when the TERM loop restarts.
MNG editors that extract a series of PNG or JNG files from a MNG datastream
are expected to execute the TERM loop only once, rather than emitting
the files repeatedly.
The TERM chunk, if present, must appear either immediately
after the MHDR chunk or immediately prior to a SEEK
chunk.
The TERM chunk is not considered to be a part of any
segment for the purpose of determining the copy-safe status of any
chunk.
Only one TERM chunk is permitted in a MNG datastream.
Simple viewers and single-frame viewers can ignore the TERM
chunk. It has been made critical only so MNG editors will not
inadvertently relocate it.
The SAVE chunk marks a point in the datastream at which
objects are "frozen" and other chunk information
is "saved". The
SEEK chunk marks positions in the MNG datastream where a
restart is possible, and where the decoder must restore the "saved"
information, if they have jumped or skipped to a SEEK point from
the interior of a segment.
They only need to restore information that they will use, e.g., a viewer
that processes gAMA and global PLTE and tRNS,
but ignores
iCCP and sPLT, need only restore the value of gamma
and the global PLTE and tRNS data from the prologue
segment but not the values of the iCCP and sPLT data.
Simple decoders that only read MNG datastreams sequentially can
safely ignore the SAVE and SEEK
chunks, although it
is recommended that, for efficient use of memory, they at least mark
existing objects as "frozen" when the SAVE chunk is
processed and discard all "unfrozen" objects whenever
the SEEK or empty DISC chunk is processed.
The SAVE chunk marks a point in the datastream at which
objects are "frozen" and other
chunk information is "saved"; a decoder skipping
or jumping to a SEEK chunk from the interior of a segment
must restore the "saved"
chunk information if it has been redefined or discarded. In addition,
the SAVE chunk can contain an optional index to the MNG
datastream.
The SAVE chunk can be empty, or it can contain an index
consisting of the following:
plus zero or more of the following index entries:
The SAVE chunk must be present when the SEEK
chunk is present. It appears after the set of chunks that define
information that must be retained for the remainder of the datastream.
These chunks, collectively referred to as the prologue segment, are no
different from chunks in other segments. They can be chunks
that define objects, or they can be chunks that define other
information such as
gAMA, cHRM, and sPLT. If any chunks appear
between the SAVE chunk and the first SEEK chunk, these
chunks also form a part of the prologue segment, but their contents become
undefined when the SEEK chunk appears.
Only one instance of the SAVE chunk is permitted in a MNG
datastream. It is not allowed anywhere after the first SEEK
chunk.
It is not permitted, at any point beyond the SAVE chunk, to
modify or discard any object that was defined ahead of the SAVE
chunk.
An object appearing ahead of the SAVE chunk can be the
subject of a CLON chunk. If the clone is a partial clone,
modifying it is not permitted, because this would also modify the object
buffer that the original object points to.
A chunk like gAMA that overwrites a single current value is
permitted after the SAVE chunk, even if the chunk has appeared
ahead of the SAVE chunk. Decoders are responsible for saving a
copy of the chunk data (in any convenient form) when the SAVE
chunk is encountered and restoring it when skipping or jumping to a
SEEK chunk from the interior of a segment. If no instance of
the chunk appeared ahead of the
SAVE chunk, the decoder must restore the chunk data to its
original "unknown" condition when it skips or jumps to
a SEEK chunk from the interior of a segment.
It is the encoder's responsibility, if it changes or
discards any "saved" data, to restore it to
its "saved" condition (or
to nullify it, if it was unknown) prior to the end of the segment. This
makes it safe for simple decoders to ignore the SAVE/SEEK
mechanism.
Known chunks in this category include DEFI, FRAM,
BACK, PLTE, cHRM, tRNS,
fPRI, gAMA, iCCP, bKGD,
sBIT, pHYg, pHYs, and sRGB.
In addition, it is the responsibility of the encoder to include chunks
that restore the
potential visibility, location, and clipping boundaries of any
"frozen" objects to their "saved" condition.
In the case of chunks like sPLT that can occur multiple
times, with different "purpose" fields, additional instances of the
chunk are permitted after the SAVE chunk, but not with the
same keyword as any instances that occurred ahead of the SAVE
chunk. The decoder is required to forget such additional instances when
it skips or jumps to a SEEK chunk from the interior of a segment,
but it must retain those instances that were defined prior to the SAVE
chunk. Encoders are required to nullify such additional instances prior to the
end of the segment. Known chunks in this category include only sPLT.
If an entry for a segment (entry type 0 or 1) appears in the optional
index, there must also be an entry for every segment, whether named or
not, except for the prologue segment, that precedes it.
All entries must appear in the index in the same order that they appear in
the MNG datastream.
There must never be a segment entry (type 0 or 1) for the prologue segment, but
there can be entries for named images or subframes in the prologue, placed
ahead of the first segment entry.
Only named images or subframes are permitted, and it is not
an error to omit any or all named images or subframes. Nor is it an error to
omit a contiguous set of segments at the end of the datastream from the index.
Offsets are calculated from the first byte of the MNG 8-byte
signature, which has offset=0. This is true even if the MNG datastream
happens to be embedded in some other file and the signature bytes are
not actually present.
Applications with direct access to the datastream can use the index
to find segments, subframes, and exported images quickly. After
processing the prologue segment, they can jump directly to any segment
and then process the remaining datastream until the desired subframe,
image, or time is found. Applications that have only streaming access
to the datastream can still use the index to decide whether to decode
the chunks in a segment or to skip over them.
Only one instance of the SAVE chunk is permitted in a MNG
datastream. If the SEEK chunk is present, the SAVE
chunk must be present, prior to the first SEEK chunk. The
only chunks not allowed ahead of the SAVE chunk are the
SEEK chunk and the MEND chunk. The SAVE
chunk must not appear inside a LOOP-ENDL pair.
The SEEK chunk marks positions ("seek points")
in the MNG
datastream where a restart is possible, and where the decoder must restore
certain information to the condition that existed when the SAVE chunk
was processed, if it has skipped or jumped to the SEEK chunk from
the interior of a segment.
The SEEK chunk can be empty, or it can contain a segment name.
The segment name is optional. It must follow the format of a
tEXt keyword: It must consist only of printable Latin-1
characters and must not have leading or trailing blanks, but can have
single embedded blanks. There must be at least one and no more than 79
characters in the keyword. There is no null byte terminator within the
segment name, nor is there a separate null byte terminator. Segment
names are case-sensitive. Use caution when printing or displaying
keywords (Refer to Security
considerations, below, Chapter 16).
No specific use for the
segment name is specified in this
document, but applications can use the segment name for such purposes
as constructing a menu of seek points for a slide-show viewer.
It can be included in the optional index that can appear in the SAVE
chunk.
It is recommended that the same name not appear in any other
SEEK chunk or in any FRAM or eXPI chunk.
Segment names should not begin with the case-insensitive strings
"CLOCK(", "FRAME(", or
"FRAMES(", which are reserved for use in URI
queries and fragments
(see Uniform Resource Identifier below).
Applications must not use any information preceding the SEEK
chunk, except for:
When the SEEK chunk is encountered, the decoder can discard
any objects appearing after the SAVE chunk, as though an empty
DISC chunk were present.
In addition to providing a mechanism for skipping frames or
backspacing over frames, the SEEK chunk provides a means
of dealing with a corrupted datastream. The viewer would abandon
processing and simply look for the next SEEK chunk before
resuming. Note that looking for a PNG IHDR chunk would not
be sufficient because the PNG datastream might be inside a loop or a
Delta-PNG datastream, or it might need data from preceding MOVE
or CLIP chunks.
When a decoder jumps to a seek point from the interior of a segment, it must
restore the information that it saved when it processed the SAVE
chunk, and it must reset the object attributes and magnification factors for
object 0 to their default values.
When it encounters a SEEK chunk during normal sequential
processing of a MNG datastream, it need not restore anything,
because the encoder will have written chunks that restore all saved information.
Multiple instances of the SEEK chunk are permitted. The
SEEK chunk must not appear prior to the SAVE
chunk. The SAVE chunk must also be present if the SEEK
chunk is present. The SEEK must not appear between a
LOOP chunk and its ENDL chunk.
This section describes ancillary MNG chunks. MNG-compliant decoders
are not required to recognize and process them.
The eXPI chunk takes a snapshot of
a viewable object (either concrete or abstract),
associates the name with that snapshot, and makes
the name available to the "outside world" (like a scripting
language).
The chunk contains an object identifier (snapshot id) and a name:
When the snapshot_id is zero, the snapshot is the first instance
of an embedded image
with object_id=0
following the eXPI chunk.
When the
snapshot_id is nonzero, the snapshot is an already-defined object with that
object_id as it already exists when the eXPI chunk is encountered.
Note that the Names beginning with the word "thumbnail" are reserved for snapshot
images that are intended to make good icons for the MNG. Thumbnail
images are regular PNG
or Delta-PNG
images, but they would normally have
smaller dimensions and fewer colors than the MNG frames.
They can be defined with the potential visibility field set
to "invisible" if they
are not intended to be shown as a part of the regular display.
The Multiple instances of the eXPI chunk are permitted
in a MNG datastream, and they need not have different values of
The fPRI chunk allows authors to assign a priority to a
portion of the MNG datastream. Decoders can decide whether or not to
decode and process that part of the datastream based on its "priority"
compared to some measure of "cost."
The fPRI chunk contains two bytes:
While 256 distinct values of If a viewer has established a nonzero "cost," it must skip any
portion of the datastream whose priority is less than that "cost."
The "cost" must be established prior to processing the proloque
segment. If the decoder changes its "cost" it must process again
according to the new "cost", unless it knows that there were
no fPRI chunks in the prologue segment.
The SAVE, SEEK, and MEND chunks always
have It is not permissible for a portion of the datastream to depend on
any portion of the datastream having a lower value, because a decoder
might have skipped the lower value portion. Use of the fPRI
chunk is illustrated in Example 5
and Example 9.
Viewers that care about the priority must assume
Multiple instances of the fPRI chunk are permitted.
The nEED chunk can be used to specify needed resources, to
provide a quick exit path for viewers that are not capable of displaying
the MNG datastream.
The nEED chunk contains a list of keywords that the decoder
must recognize. Keywords are typically private critical chunk names.
The nEED chunk should be placed early in the MNG datastream,
preferably very shortly after the MHDR chunk.
The keywords are typically 4-character private critical chunk
names, but they could be any string that a decoder is required to
recognize. No critical chunks defined in this specification or in the
PNG specification should be named in a nEED chunk, because
MNG-compliant decoders are required to recognize all of them, whether
they appear in a nEED chunk or not. The purpose of the
nEED chunk is only to identify requirements that are above and
beyond the requirements of this document and of the PNG specification.
Each keyword string must follow the format of a tEXt
keyword: It must consist only of printable Latin-1 characters and
must not have leading or trailing blanks, but can have single embedded
blanks. There must be at least one and no more than 79 characters
in the keyword. Keywords are case-sensitive. There is no null byte
terminator within the keyword. A null separator byte must appear after
each keyword in the nEED chunk except for the last one.
Decoders that do not recognize a chunk name or keyword in the
list should abandon the MNG datastream or request user intervention.
The normal security precautions should be taken when displaying the
keywords.
The MNG pHYg chunk is identical in syntax to the PNG
pHYs chunk. It applies to complete
full-frame MNG layers and not to the individual images within them.
Conceptually, a MNG viewer that processes the pHYg chunk will
first composite each image into a full-frame layer, then apply
the pHYg scaling to the layer, and finally composite the scaled
layer against the frame.
MNG datastreams can include both the pHYs chunk (either at the
MNG top level or within the PNG and JNG datastreams) and the
pHYg chunk (only at the MNG top level), to ensure that the images
are properly displayed either when displayed by a MNG viewer or
when extracted into a series of individual PNG or JNG datastreams
and then displayed by a PNG or JNG application. The pHYs and
pHYg chunks would normally contain the same values, but this is
not necessary.
The MNG top-level pHYg chunk can be nullified by a
subsequent empty pHYg chunk appearing in the MNG top level.
The namespace for MNG chunk names is separate from that of PNG. Only
those PNG chunks named in this paragraph are also defined at the MNG top
level. They have exactly the same syntax and semantics as when they
appear in a PNG datastream:
A MNG editor that writes PNG datastreams should not include
the top-level iTXt, tEXt, tIME,
and zTXt chunks in the generated PNG datastreams.
These PNG chunks are also defined at the MNG top level. They
provide default values to be used in case they are not provided in
subsequent PNG datastreams. Any of these chunks can be nullified by the
appearance of a subsequent empty chunk with the same chunk name. Such
empty chunks are not legal PNG or JNG chunks and must only appear in the
MNG top level.
In the MNG top level, all of these chunks are written as though
for 16-bit RGBA PNG datastreams. Decoders are responsible for
reformatting the chunk data to suit the actual bit depth and color
type of the datastream that inherits them.
A MNG editor that writes PNG or JNG datastreams is expected to
include the top-level cHRM, gAMA, iCCP,
and sRGB chunks in the generated PNG or JNG datastreams, if
the embedded image does not contain its own chunks that define the
color space. When it writes the sRGB chunk, it should write
the gAMA chunk (and perhaps the cHRM chunk), in
accordance with the PNG specification, even though no gAMA or
cHRM chunk is present in the MNG datastream.
It is also expected to write the pHYs chunk and the reformatted
top-level bKGD chunk in the generated PNG or JNG datastreams, and
the reformatted sBIT chunk only in generated PNG datastreams, when
the datastream does not have its own bKGD, pHYs,
or sBIT chunks.
The top-level sRGB chunk nullifies the preceding
top-level gAMA and cHRM chunks, if any, and
either the top-level gAMA or the top-level cHRM chunk
nullifies the preceding top-level sRGB chunk, if any.
This PNG chunk is also defined at the MNG top
level. It
provides a value that takes precedence over those that might be provided
in subsequent PNG or JNG datastreams and provides a value to be used
when it is not provided in subsequent PNG or JNG datastreams.
It also takes precedence over the
PLTE chunk in a subsequent PNG datastream when the
PLTE and hIST chunks are being used as a suggested
palette (i.e., When a decoder needs to choose between a suggested palette
defined at the MNG level and a suggested palette defined in the PNG datastream
(either with the sPLT chunk, or with the PLTE/hIST
chunks for grayscale or truecolor images), it should give precedence to
the palette from the MNG level, to avoid spurious layer-to-layer color
changes.
MNG editors that write PNG datastreams should ignore the
sPLT data from the MNG level and simply copy
any sPLT chunks appearing within the embedded PNG
datastreams.
JNG (JPEG Network Graphics) is the lossy sub-format for MNG objects.
MNG-LC and MNG-VLC
applications can choose to support JNG or not. Those that
do not can check bit 4 (JNG is present/absent) of the MHDR
simplicity profile to decide whether they can process the datastream.
Note: This specification depends on the PNG Portable Network Graphics
specification [PNG].
The PNG specification is available at the PNG home page,
A JNG datastream consists of a header chunk (JHDR),
JDAT chunks that contain a complete JPEG datastream,
optional IDAT chunks that contain a PNG-encoded grayscale
image that is to be used as an alpha mask, and an IEND chunk.
The alpha mask, if present, must have the same dimensions as the image
itself. The JDAT and IDAT chunks can be interleaved.
Some of the PNG ancillary chunks are also recognized in JNG datastreams.
While JNG is primarily intended for use as a sub-format within MNG, a
single-image JNG datastream can be written in a standalone file. If so,
the JNG datastream begins with an 8-byte signature containing
(decimal) which is similar to the PNG signature
with "\213 J N G"
instead of "\211 P N G" in bytes 0-3.
We may at some future time register an Internet Media
Type for JNG files. Until then, the interim media type
JNG is pronounced "Jing."
This section specifies the critical chunks that are defined in the
JNG format.
The format of the JHDR chunk introduces a JNG datastream.
It contains:
The width, height, image_sample_depth, image_compression_method,
and image_interlace_method fields are redundant because equivalent
information is also embedded in the JDAT datastream. They
appear in the JHDR chunk for convenience. Their values must be
identical to their equivalents embedded in the JDAT chunk. We
use four bytes in the width and height fields for similarity to MNG and
PNG, and to leave room for future expansion, even though two bytes would
have been sufficient.
When the A JNG datastream must contain one or more JDAT chunks, whose
data, when concatenated, forms a complete JNG JPEG datastream.
JNG decoders are required to read all baseline JNG JPEG and
eight-bit progressive JNG JPEG datastreams. Twelve-bit capability is
not required.
JDAT chunks are like PNG IDAT chunks in that
there may be multiple JDAT chunks, the data from which
are concatenated to form a single datastream that can be sent to
the decompressor. No chunks are permitted among the sequence of
JDAT chunks, except for interleaved IDAT chunks.
The ordering requirements of other ancillary chunks are the same
with respect to JDAT as they are in PNG with respect to the
IDAT chunk.
A JNG JPEG is a baseline, extended-sequential, or progressive JPEG as
defined by JPEG Part 1
[ISO/IEC-10918-1].
JNG uses only JFIF-compatible
[JFIF]
component interpretations, and imposes a few additional restrictions
that reflect limitations of many existing JPEG implementations. In
particular, only Huffman entropy coding is permitted.
Actually, a JNG may contain two separate JNG JPEG datastreams
(one eight-bit and one twelve-bit), each contained in a series
of JDAT
chunks, and separated by a JSEP chunk
(see the JSEP chunk specification below, Paragraph 5.1.6).
Decoders that are unable to (or do not wish to) handle twelve-bit datastreams
are allowed to display the eight-bit datastream instead, if one is present.
The core of the JNG JPEG definition is baseline JNG JPEG, which
is JPEG Part 1's definition of baseline JPEG further restricted by
JFIF restrictions and JNG-specific restrictions.
JPEG, which is also defined in JPEG Part 1 and has JNG-specific
restrictions.
A baseline JPEG according to JPEG Part 1 is DCT-based
(lossy) sequential
JPEG, using 8-bit sample precision and Huffman entropy coding, with the
following further restrictions:
The SOF marker type for baseline JPEG is SOF0.
JDAT datastreams must always follow "interchange JPEG" rules: all
necessary quantization and Huffman tables must be included in the
datastream; no tables can be omitted.
The image data is always stored left-to-right, top-to-bottom.
The encoded data shall have one of the two color space
interpretations allowed by the JFIF specification:
By convention, the luminance coefficients are always those
defined by CCIR Recommendation 601-1:
The constant Half_scale is 128 when dealing with eight-bit
data, 2048
for twelve-bit data. With these equations, Y, Cb, and Cr all have the same
range as R, G, and B: 0 to 255 for eight-bit data, 0 to 4095 for twelve-bit
data.
The JFIF convention for YCbCr differs from typical digital
television practice in that no headroom/footroom is reserved: the coefficient
values range over the full available 8 or 12 bits.
Intercomponent sample alignment shall be such that the
first (upper
leftmost) samples of each component share a common upper left corner
position. This again differs from common digital TV practice, in which
the first samples share a common center position. The JFIF convention
is simpler to visualize: subsampled chroma samples always cover an
integral number of luminance sample positions, whereas with co-centered
alignment, chroma samples only partially overlap some luminance samples.
JNG imposes three additional restrictions not found in the
text of either JPEG Part 1 or the JFIF specification:
In other words, the chroma components may be downsampled 2:1
or 1:2 horizontally or vertically relative to luminance, or they may be left
full size. These four sampling ratios are the only ones supported by a
wide spectrum of implementations (1x2 is relatively uncommon, and is usually
the result of a lossless rotation of a 2x1 sampling).
For grayscale images, the sampling factors are irrelevant
according to a strict reading of JPEG Part 1. Hence decoder authors should
accept any sampling factors for grayscale. However, we recommend that encoders
always emit sampling factors 1h1v for grayscale, since some decoders
have been observed to malfunction when presented with other sampling
factors.
For JNG progressive JPEG datastreams, the JPEG process is
progressive Huffman coding (SOF marker type SOF2) rather than baseline (SOF0).
All JNG-compliant decoders must support full progression, including both
spectral-selection and successive-approximation modes, with any sequence
of scan progression parameters allowed by the JPEG Part 1 standard.
Otherwise, all the restrictions listed above apply, except
these:
We require full progression support since relatively little
code savings can be achieved by subsetting the JPEG progression features.
In particular, successive approximation offers significant gains in
the visual quality of early scans. Omitting successive-approximation
support from a decoder does not save nearly enough code to justify
restricting JNG progressive encoders to spectral selection only.
No particular progressive scan sequence is specified or
recommended by this specification. Not enough experience has been gained with
progressive JPEG to warrant making such a recommendation. To allow
for future experimentation with scan sequences, decoders are expected
to handle any JPEG-legal sequence. Again, the code savings that might
be had by making restrictive assumptions are too small to justify a
limitation.
When the JSEP chunk is present, both images must be
progressive if one of them is progressive.
JNG JPEGs may optionally use 12-bit sample precision as
defined in JPEG Part 1.
For a sequential image, the SOF marker type must be SOF1
(extended sequential) not SOF0, and the baseline restriction of two Huffman
tables is removed. Also, the encoder may use either 8-bit or 16-bit
quantization tables. All other JNG baseline restrictions still apply.
It is recommended that JNG encoders not use extended-sequential mode
except to encode 12-bit data.
For a progressive image, the only difference between
8-bit and 12-bit
modes is that the sample precision is 12 bits and the encoder may use
either 8-bit or 16-bit quantization tables. All other JNG restrictions
still apply.
This chunk is exactly like the IDAT chunk in a PNG grayscale
image, except that it is interpreted as an alpha mask to be applied to
the image data from the JDAT chunks,
when The filter method can be any filter method that is defined for PNG
datastreams that are embedded in MNG datastreams.
The IDAT
chunks can be interleaved with the JDAT chunks
(see Recommendations for Encoders:
JNG interleaving below).
No other chunk
type can appear among the sequence of IDAT and JDAT
chunks. No other chunk type can appear between the sequences of
IDAT and JDAT chunks when they are not interleaved.
The samples in the IDAT must be presented in noninterlaced order, left
to right, top to bottom. As in PNG, zero means fully transparent and
The IDAT chunks must precede the JSEP chunk, if the
JSEP chunk is present. Minimal viewers that ignore the twelve-bit
JDAT chunks must read the IDAT chunks and apply the
alpha samples to the eight-bit image that is contained in the JDAT
chunks that precede the JSEP chunk.
Viewers that skip the eight-bit
JDAT chunks must decode the IDAT
chunks that precede the JSEP chunk and apply the
alpha samples to the twelve-bit image that is contained in the JDAT
chunks that follow the JSEP chunk.
This chunk is exactly like the JDAT chunk in a non-progressive
JNG 8-bit grayscale image, except that it is interpreted as an alpha mask to
be applied to the image data from the JDAT chunks,
when Like IDAT chunks, the JDAA chunks must precede
the JSEP chunk, if the JSEP chunk is present, and are
handled similarly.
The JNG IEND chunk is identical to its counterpart in
PNG. Its data length is zero, and it serves to mark the end of the JNG
datastream.
The JSEP chunk is empty.
A JSEP chunk must appear between the JDAT
chunks of an eight-bit datastream and those of a twelve-bit datastream, when
Some PNG ancillary chunks can also appear in JNG datastreams, and are
used for the same purposes as described in the PNG specification
[PNG]
and the Extensions to the PNG Specification document [PNG-EXT].
If the bKGD chunk is present, it must be written as if it
were written for a PNG datastream with sample_depth=8. It has one
2-byte entry for grayscale JNGs and three 2-byte entries for color JNGs.
The first (most significant) byte of each entry must be 0.
The following chunks have exactly the same meaning and have the same
syntax as given in the PNG specification: cHRM, gAMA,
iCCP, sRGB, pHYs, oFFs, sCAL,
iTXt, tEXt, tIME, and zTXt.
The PNG PLTE, hIST, pCAL, sBIT,
sPLT, tRNS, fRAc, and gIF* chunks are
not defined in JNG.
When cHRM, gAMA, iCCP, or sRGB
are present, they provide information about the color space of the
decoded JDAT image, and they have no effect on the decoded
alpha samples from the IDAT or JDAA chunks. Any viewer that
processes the gAMA chunk must also recognize and process the
sRGB chunk. It can treat it as if it were a gAMA
chunk containing the value .45455 and it can ignore
its "intent" field.
The chunk copying and ordering rules for JNG are the same as those in
PNG, except for the fact that the JDAT chunks and IDAT
or JDAA chunks can be interleaved.
A Delta-PNG datastream describes a single image, by giving the
changes from a previous PNG (Portable Network Graphics) image or
nonviewable PNG-like object, a JNG (JPEG Network Graphics) image, or
another Delta-PNG image.
No provision is made in this specification for storing a Delta-PNG
datastream as a standalone file. A Delta-PNG datastream will normally
be found as a component of a MNG datastream. Applications that need
to store a Delta-PNG datastream separately should use a different
file signature and filename extension, or they can wrap it in a MNG
datastream consisting of the MNG signature, the MHDR chunk, a
BASI chunk with the appropriate dimensions and an IEND
chunk, the Delta-PNG datastream, and the MEND chunk.
The decoder must have available a parent (decoded) object
that has an object buffer from which
the original chunk data is known. The parent object can be the result
of decoding a PNG, another Delta-PNG datastream, or it could have been
generated by a PNG-like datastream introduced by a BASI chunk.
The child image is always of the same basic type (at present only
PNG and JNG are defined) as the parent object. The child is always a
viewable image even if the parent is not.
The decoder must not have modified the pixel data in the parent
object by applying output transformations such as gAMA or
cHRM, or by compositing the image against a background.
Instead, the decoder must make available to the Delta-PNG decoder the
unmodified pixel data along with the values for the gAMA,
cHRM, and any other recognized chunks from the parent object
datastream.
A Delta-PNG datastream consists of a DHDR and IEND
enclosing other optional chunks (if there are no other chunks, the
decoder simply copies the parent image, and displays it if its
Chunk structure (length, name, CRC) and the chunk-naming system
are identical to those defined in the PNG specification. Definitions
of This section describes critical Delta-PNG chunks. MNG-compliant
decoders must recognize and process them.
The DHDR chunk introduces a Delta-PNG datastream.
Subsequent chunks, through the next IEND chunk, are interpreted
according to the Delta-PNG format.
The DHDR chunk can contain 4, 12, or 20 bytes:
The The When For all other values of When
The IDAT chunk data contains a filtered and perhaps
interlaced set of delta pixel samples. The delta samples are presented
in the order specified by An encoder calculates the delta sample values from the samples
in the
parent object and those in the child image by subtracting the parent
object samples from the child image samples, modulo Only the pixels
in the block defined by the block location and dimensions given in the
DHDR chunk are changed. The size of the IDAT data
must correspond exactly to this rectangle.
When the parent object has The color type must match that of the parent,
except that
when the parent has PNG If the
When the IHDR chunk is present, the compression
method, filter method, and interlace method
need not be the same as those of the parent object. The new
values are used in decoding the IDAT data, and the new values are
inherited by the child object.
Whenever the sample depth differs from that of
the parent object, the resulting object inherits the original value from
the parent. The value from the IHDR chunk is
only used for decoding the IDAT data in this and subsequent
Delta-PNGs. Implicit in this is the requirement for decoders to remember
in the object buffer
not only the sample depth of the object but (separately) the
"pixel sample depth"
for use in decoding the IDAT chunks
of subsequent Delta-PNG datastreams that do not contain their own
IHDR chunk.
The parent object cannot have alpha samples that were carried in JPEG-encoded
JDAA chunks.
The
The If the
If the The color type must match that of the parent,
except for the cases mentioned for delta type 1, above.
It is permitted to use JPEG-encoded JDAA chunks to convey
the new alpha data. If this is done, then the alpha channel of the object
can no longer be used as the parent for block-pixel-addition or
block-alpha-addition.
End of Delta-PNG datastream. An IEND chunk must be present
for each DHDR chunk in a MNG datastream.
The IEND chunk is empty.
This chunk is used to "promote" a parent object to a higher bit depth
or to add an alpha channel, before making changes to it.
When a decoder encounters the PROM chunk, it must promote
the pixel data. The cases are:
If the sample depth has been changed, the sample values must be
widened. The decoder must use left-bit-replication or zero-fill
according to the specified If the basis object contains data from the PNG bKGD chunk,
this data must be promoted as well. If a grayscale object is being
promoted to a truecolor object, the background RGB samples are set equal
to the grayscale background sample. If the bit depth has been changed,
the background samples are widened in accordance with the specified
If the basis object contains data from the PNG sBIT chunk,
this data must also be promoted. If a grayscale object is being
promoted to a truecolor object, the new RGB bytes are set equal to the
grayscale byte. When an alpha channel is added, the alpha byte is set
equal to the sample depth of the basis image. If the sample depth has
been changed, the sBIT bytes do not change.
The PROM chunk is not permitted to "demote" a parent object
to an object with a lesser bit depth or from one with an alpha channel
to one without an alpha channel.
The PROM chunk must appear ahead of the IHDR chunk,
if IHDR is present, and ahead of any chunks that would have
followed IHDR, if IHDR is omitted.
Inside a Delta-PNG datastream, the IHDR chunk introduces
an incomplete PNG (Portable Network Graphics) datastream. The parent
object must be a PNG or PNG-based Delta-PNG. The datastream can be
introduced by a complete PNG IHDR chunk or by an IPNG
chunk, which is empty.
If the IHDR chunk is present, its The See the PNG specification
and the Extensions to the PNG Specification document [PNG-EXT]
for the format of the PNG chunks.
The The IPNG chunk is empty.
The IPNG chunk can be used instead of the IHDR
chunk if the IHDR chunk is not needed for resetting the
value of The IHDR chunk can also be omitted when
If the PLTE chunk is present, it need not have the same
length as that inherited from the parent object, but it must contain
the complete palette needed in the child image. If it is shorter than
the palette of the parent object, decoders can discard the remaining
entries and the child image must not refer to them. Decoders can also
truncate any tRNS data inherited from an indexed-color parent
object. If the new palette is longer than the parent palette, and
a new tRNS chunk is not present in an indexed-color image,
the tRNS data must be extended with opaque entries. The new
palette must not be longer than the object's When processing the tRNS chunk, if If it is desired only to overwrite or add palette entries,
the PPLT chunk can be used. This might be useful for
palette-animation applications. This chunk can also be used to
overwrite or add entries to the transparency (alpha) data from the
parent's tRNS chunk.
The PPLT chunk contains a The When When If the new entry is beyond the range of the original palette,
the values are simply appended, regardless of the contents of
Inside a Delta-PNG datastream, the JHDR chunk introduces an
incomplete JNG (JPEG Network Graphics) datastream. The parent object
must be a JNG or JNG-based Delta-PNG. The datastream is introduced by a
complete JNG JHDR chunk.
If the JHDR chunk is present, its The See the JNG specification above for the format of the JNG
chunks. The PNG datastream must contain at least JHDR and
IEND, but can inherit other chunk data from the parent
object. Except for IDAT, any chunks appearing between IHDR and
IEND are always treated as replacements or additions and not as
deltas.
The IJNG chunk is empty.
The IJNG chunk can be used instead of the JHDR
chunk if the JHDR chunk is not needed for resetting the value
of any of the JHDR fields. The purpose of this chunk is to
identify the beginning of the JNG datastream, so decoders can start
interpreting JNG chunks instead of Delta-PNG chunks. The decoder must
treat this datastream as though the JHDR chunk were present in
the location occupied by the IJNG chunk.
The JHDR chunk can also be omitted when
All chunks in the parent object with the specified name are inhibited
from being copied into the child image.
If multiple names appear in the DROP chunk, it is shorthand
for multiple DROP chunks.
The chunk name must be the name of a chunk whose data begins with
a null-terminated text string. Some parent object chunks with the
specified chunk name are inhibited from being copied into the child
image. If polarity is <only>, then any parent chunk whose
keyword appears in the keywords list is inhibited. If polarity is
<all-but>, then any parent object chunk whose keyword does not
appear in the keywords list is inhibited.
The format of the keyword is the same as that specified for the
parent chunk. Comparisons of keywords in the parent chunk and the
DBYK chunk are case sensitive.
Use caution when printing or displaying keywords (Refer to Security
considerations, below, Chapter 16).
The ORDR chunk informs the applier of the Delta-PNG of the
ordering restrictions for ancillary chunks. It contains one or more
5-byte sequences:
Critical chunk names must not appear in the ORDR chunk. The
applier needs to know everything about them anyway.
If a chunk name appears in the ORDR chunk, it is a promise
that any chunk of that name appearing in the parent object which is not
inhibited by DROP/DBYK will not be broken by this Delta-PNG,
and therefore the applier must copy it into the child image at a
location compatible with its ordering restrictions.
If any ancillary chunk appears in the parent object, and it is not
inhibited, and its name does not appear in the ORDR chunk, then
the applier should copy it into the child only if it knows the chunk
well enough to be sure that it is consistent with the changes made by
the Delta-PNG, and knows where it can be placed in the child. Those
conditions are always true of safe-to-copy chunks.
If any critical chunk defined in neither this specification nor the
PNG specification appears in the parent object or in the Delta-PNG,
it is a fatal error unless the applier knows how to handle it. The
specification of the critical chunk can include provisions for this
scenario.
This section describes ancillary Delta-PNG chunks. MNG-compliant
decoders should recognize and process them, but are not required to.
A gAMA, cHRM, iCCP, sRGB or
similar chunk existing in the parent object would not affect the pixel
data in a concrete object inherited by this Delta-PNG datastream
because they are not used in decoding the pixel data. Applications
are responsible for ensuring that the pixel values that are inherited
from the parent object are the raw pixel data that existed prior to any
transformations that were applied while displaying the parent image.
These color transformations are applied to the resulting pixel data for
display purposes.
MNG viewers must ignore oFFs and pHYs chunks that
appear inside a PNG or JNG datastream or are inherited from the MNG top
level. MNG editors are expected to treat them as if they were unknown
copy-safe chunks.
Any other ancillary PNG chunks that the decoder recognizes
when processing
a PNG datastream should also be recognized and handled when processing
a delta-PNG datastream. Any chunks that it does not recognize should
be processed as instructed by the ORDR, DROP,
and DBYK chunks. MNG
viewers are free to ignore any ancillary chunks, while MNG editors should
handle them in accordance with the copying rules.
The PNG specification places ordering requirements on many chunks
with respect to the PLTE and IDAT chunks. If neither
of these two chunks is present, and the ORDR chunk is not
present, known chunks (always including all standard chunks described in
the PNG specification) are considered to have appeared in their proper
order with respect to the critical chunks. Unknown chunks are ordered
as described in above (Paragraph 6.1.12).
The chunk copying rules for MNG are the same as those in PNG, except
that a MNG editor is not permitted to move unknown chunks across any of
the following chunks:
The copy-safe status of an unknown chunk is determined from the chunk
name, just as in PNG. If bit 5 of the first byte of the name is 0
(Normally corresponding to an uppercase ASCII letter), the unknown chunk
is critical and cannot be processed or copied. If it is 1 (usually
corresponding to a lowercase ASCII letter), the unknown chunk is
ancillary and its copy-safe status is determined by bit 5 of the fourth
byte of the name, 0 meaning copy-unsafe and 1 meaning copy-safe.
If an editor makes changes to the MNG datastream that render unknown
chunks unsafe-to-copy, this does not affect the copy-safe status of any
chunks beyond the next SEEK chunk or prior to the previous
one. However, if it makes such changes prior the SAVE chunk,
this affects the copy-safe status of all top-level unknown chunks in the
entire MNG datastream.
Changes to the MHDR chunk do not affect the copy-safe status of any
other chunk.
The SAVE, SEEK, and TERM chunks are not
considered to be a part of any segment. Changes to the data in the
SAVE or SEEK chunks do not affect the copy-safe
status of any other chunks. Adding or removing a SEEK chunk
affects the copy-safe status of unknown chunks in the newly-merged
or newly-separated segments. Adding, removing, or changing the
TERM chunk has no effect on the copy-safe status of any chunk.
As in PNG, unsafe-to-copy ancillary chunks in the top-level MNG
datastream can have ordering rules only with respect to critical chunks.
Safe-to-copy ancillary chunks in the top-level MNG datastream can have
ordering rules only with respect to the
SAVE, SEEK,
SHOW, and PAST chunks, DHDR-IEND, BASI-IEND,
IHDR-IEND, JHDR-IEND
sequences, or with respect to any other
critical "header-end" sequence
that might be defined in the future that could contain IDAT or
similar chunks.
The copying rules for unknown chunks inside IHDR-IEND,
BASI-IEND, DHDR-IEND,
and JHDR-IEND sequences
are governed by the PNG and JNG specifications, and any changes inside
such sequences have no effect on the copy-safe status of any top-level
MNG chunks.
The copy-safe status of chunks inside a DHDR-IEND sequence
depends on the copy-safe status of the chunks in its parent object.
This section specifies the minimum level of support that is expected of
MNG, MNG-LC, or MNG-VLC-compliant
decoders, and provides recomendations for viewers that
will support slightly more than the minimum requirements. All critical
chunks must be recognized, but some of them can be ignored after they
have been read and recognized. Ancillary chunks can be ignored, and do
not even have to be recognized.
Anything less than this level of support requires subsetting.
Applications that provide less than minimal MNG
support should check the MHDR "simplicity profile" for the
presence of features that they are unable to support or do not wish to
support. A specific subset, in which
"complex MNG features"
and JNG are absent,
is called "MNG-LC".
In MNG-LC datastreams, bit 0
of the simplicity profile must be 1 and bits 2 and 4
must be 0.
Another subset is called "MNG-VLC".
In MNG-VLC datastreams, "simple MNG features"
are also absent, and bit 1 must therefore also be 0.
Subsets are useable when the set of MNG datastreams to be processed
is known to be (or is very likely to be) limited to the feature set in
MNG-LC or MNG-VLC.
Limiting the feature set in a widely-deployed WWW browser to anything
less than MNG with 8-bit JNG support would be highly inappropriate.
Some subsets of MNG
support are listed in the following table, more
or less in increasing order of complexity.
One reasonable path for an application developer to follow might
be to develop and test the application at each of the following
levels of support in turn:
An equally reasonable development path might be
On the other hand, a developer
working on an application for storing multi-page fax documents might
have no need for more than "MNG-VLC without transparency".
We are allowing conformant decoders to skip twelve-bit JNGs
because those are likely to be rarely encountered and used only for
special purposes. There is no profile flag to indicate the presence
or absence of 12-bit JNGs.
Bit 4 of the simplicity profile can be used to promise that
JNG chunks are not present. Viewers that choose not to support
JNG can check this bit before deciding to proceed.
MNG-compliant decoders must support JNG, but
MNG-LC and MNG-VLC
decoders are not required to support JNG.
MNG-compliant decoders are required to support Delta-PNG, but
MNG-LC and MNG-VLC decoders are not.
The following recommendations do not form a part of the
specification.
It is a good idea to use a single color space for all of the layers
in an animation, where speed and fluidity are more important than
exact color rendition. This is best accomplished by defining a
single color space at the top level of MNG, using
and either an sRGB chunk or the gAMA and
cHRM chunks and perhaps the iCCP
chunk, and removing any color space chunks from the individual images
after converting them to the common color space.
When the encoder converts all images to a single color space before
putting them in the MNG datastream, this will allow decoders to improve
the speed and consistency of the display.
For single-frame MNG datastreams, however, where decoding speed is
less important and exact color rendition might be more important, it is
best to leave the images in their original color space, as recommended in
the PNG specification, to avoid any loss of data due to conversion, and
to retain the individual color space chunks if the images have different
color spaces.
Always use framing mode 1 or 2 when all of the images are opaque.
This avoids unnecessary screen clearing, which can cause flickering.
If it
is necessary to establish a synchronization point immediately, this can
be done by using two consecutive FRAM chunks, the first setting
a temporary Embedded images should not be enclosed in loops unless absolutely
necessary. It is better to store them ahead of time and then use
SHOW chunks inside the loops. Otherwise, decoders will be
forced to repeatedly decode them. See Examples 2, 8, 11, and 12,
below (Chapter 17).
Authors of MNG files that are intended for transmission over a
network should consider whether it is more economical for the client to
rebuild the index from scratch than it is to transmit it. Web pages
that are likely to be downloaded over slow lines, and whose clients
are unlikely to use the index anyway, generally should have empty
SAVE chunks. No information is lost by deleting the index,
because the MNG datastream contains all of the information needed to
build the index. If an application does build an index, and the file
is going to be kept as a local file, the application should replace
the empty SAVE chunk with one containing the index. See above (Paragraph 4.4.1).
When a JNG datastream contains an alpha channel, and the file is
intended for transmission over a network, it is useful to interleave
the IDAT or JDAA and the JDAT chunks.
In the case of sequential
JPEG, the interleaving should be arranged so that the alpha data
arrives more or less in sync with the color data for the scanlines.
In the case of progressive JPEG, the alpha data should be interleaved
with the first JPEG pass, so that all of the alpha data has
arrived before the beginning of the second JPEG pass.
It is recommended that the JDAA chunk be used only to convey
smoothly varying alpha channels and not to convey binary transparency
which is more precisely and efficiently conveyed in IDAT chunks.
The simplicity profile in the MHDR chunk can be ignored or
it can be used for
If a decoder reads an ENDL chunk for which the matching
LOOP chunk is missing, or has been skipped for some reason, any
active loops with a higher The PNG specification gives a good explanation of how to composite a
partially transparent image over an opaque image, but things get more
complicated when both images are partially transparent.
Pixels in PNG and JNG images are represented using gamma-encoded RGB
(or gray) samples along with a linear alpha value. Alpha processing
can only be performed on linear samples. This chapter assumes that R,
G, B, and A values have all been converted to real numbers in the range
[0..1], and that any gamma encoding has been undone.
For a top pixel {Rt,Gt,Bt,At} and a bottom pixel {Rb,Gb,Bb,Ab}, the
composite pixel {Rc,Gc,Bc,Ac} is given by:
When the bottom pixel is fully opaque (Ab = 1.0), the function
reduces to:
When the bottom pixel is not fully opaque, the function is
much simpler if premultiplied alpha is used. A pixel that uses
non-premultiplied alpha can be converted to premultiplied alpha by
multiplying R, G, and B by A.
For a premultiplied top pixel {Rt,Gt,Bt,At} and a premultiplied
bottom pixel {Rb,Gb,Bb,Ab}, the premultiplied composite pixel
{Rc,Gc,Bc,Ac} is given by:
As mentioned in the PNG specification, the equations become much
simpler when no pixel has an alpha value other than 0.0 or 1.0, and the
RGB samples need not be linear in that case.
The decoder must retain information about each object (except for
objects with The following information must be retained, for each nonzero object
that is defined and not subsequently discarded:
When the encoder knows that data in the object buffer will not be
needed by subsequent frames, it can make life easier for decoders by
using When a fatal error is encountered, such as a bad CRC or an unknown
critical MNG chunk, minimal viewers
that do not implement the
SAVE/SEEK mechanism
should simply abandon the MNG datastream.
More capable MNG viewers should attempt to recover gracefully by
abandoning processing of the segment and searching for a SEEK
chunk. If such errors occur before the SAVE chunk is reached,
the viewer should abandon the MNG datastream.
When an error occurs within a image datastream, such as an unknown
critical PNG chunk or a missing parent object where one was required,
only that image should be abandoned and the associated object should be
discarded. If a bad CRC is found, indicating a corrupted datastream,
the entire segment should be abandoned, as above.
MNG editors, on the other hand, should be more strict and reject any
datastream with errors unless the user intervenes.
Decoders are required to be able to interpret datastreams that
contain interlaced PNG images, but are only required to display the
completed frames; they are not required to display the images as they
evolve. Viewers that are decoding datastreams coming in over a slow
communication link might want to do that, but MNG authors should not
assume that the frames will be displayed in other than their final form.
When a PLTE chunk is received, it only affects the display
of the PNG datastream that includes
or inherits
it. Decoders must
take care that it does not retroactively affect anything that has already
been decoded.
If PLTE or PPLT is present in a Delta-PNG
datastream, the new palette is used in displaying the image defined by
the Delta-PNG; if no IDAT chunk is present and the image type
is PNG indexed-color, then the resulting image is displayed using the
old pixel samples as indices into the new palette, which provides a
"palette animation" capability.
If a frame contains two or more images, the PLTE chunk in
one image does not affect the display of the
other, unless one image
is a subsequent Delta-PNG without a PLTE chunk, that has been
declared by the DHDR A composite frame consisting only of indexed-color images should not
be assumed to contain 256 or fewer colors, since the individual palettes
do not necessarily contain the same set of colors.
Encoders can supply
a top-level sPLT chunk with a suggested reduced global palette,
to help decoders build an appropriate palette when necessary.
Viewers that can only display a single frame must display the first
frame that they encounter.
It is strongly recommended that such viewers
understand the fPRI chunk above (Paragraph 4.5.2),
which
will enable them to find and display the best representative frame, if
the encoder has identified one.
MNG provides four
types of clipping, in addition to any clipping that
might be required due to the physical limitations of the display device.
This is the only type of clipping available in MNG-VLC
datastreams.
Decoders can use these parameters to establish the size of
a window in which to display the MNG frames. When the In the case of a MNG datastream that consists of a PNG or JNG
datastream, with the PNG or JNG signature, the
The clipping boundaries are expressed in pixels, measured rightward
and downward from the frame origin.
The left and top clipping boundaries are inclusive and the right and
bottom clipping boundaries are exclusive, i.e., the pixel located at
{x,y} is only displayed if the pixel falls within the physical limits of
the display hardware and all of the following are true:
Editors must recreate or delete the optional SAVE chunk
index whenever they make any change that affects the offsets of chunks
following the portion of the datastream that is changed. If the changes
do not involve the addition, deletion, or relocation of segments,
frames, and images, then it is sufficient to zero out the offsets.
The SAVE chunk is not considered to be in any MNG segment,
so changing it has no effect on the copy-safe status of unknown chunks
in any other part of the MNG datastream.
When the SAVE chunk is expanded to include an index, all
chunks that follow will have their offsets changed by an amount equal
to the change in the length of the data segment of the SAVE
chunk, so the offset table will have to be adjusted accordingly. If a
SAVE chunk is already present with zero offsets, the correct
offsets can be written without adjustment.
Editors that create a series of PNG or JNG datastreams from a MNG datastream
should check the termination condition of any LOOP chunks and execute
loops only On systems where file names customarily include an extension
signifying file type, the extension When and if the MNG format becomes finalized, the
MNG authors intend to register Although we define a standalone JNG format, we recommend that such
files be used only temporarily while compiling or disassembling MNG
datastreams. We may at some future time register an Internet Media
Type for JNG files. Until then, the interim media type
Segments, subframes, and objects are externally accessible via named
SEEK, eXPI, and FRAM chunk names. They can be
referred to by URI, as in
When the URI specializer ("#" or "?")
is "#", and the fragment
identifier (the string following the specializer) is the name of a
segment, i.e., a named SEEK chunk, the viewer should display
the sequence from the beginning of the named segment up to the next
segment. When it refers to a subframe or an image, i.e., a named
FRAM or eXPI chunk, it should display the single frame
(as it exists when the next FRAM chunk is encountered) or image
that is identified by the fragment identifier. If the SAVE
chunk is present and contains the optional index, this can help the
client find the needed segment quickly.
When the URI specializer is "?" (server side query),
the "query
component" is the string following the "?" specializer
and up to but not including the "#" if
the "#" specializer is also present. The
server should find the segment that is named in the query component
or the segment that contains the subframe or image named in the query
component, and it should return a datastream consisting of:
If no SAVE chunk is present, the server must simply return
the entire MNG datastream. Servers that are unwilling to parse the MNG
datastream and are unconcerned about bandwidth can return the entire MNG
datastream even when the SAVE chunk is present. Authors should defend
against this behavior by including both a query and a fragment in the
URI even when a segment is being requested.
The client can process this as a complete MNG datastream, either
displaying the entire segment, if no fragment identifier is present, or
extracting the segment, frame or image that is named in a fragment
identifier and displaying it, if a fragment identifier is present
(a fragment identifier must be present if a frame or image is being
requested). To "extract a frame" means to decode
the returned datastream through the end of the frame that contains the
named subframe and to display the result as a single still image. If
the layers of the named subframe do not cover the entire frame, pixels
from the background and from earlier subframes must be included in the
resulting composition.
A part of the MNG datastream can also be requested by timecode, as in
or by frame number, as in
The timecode must consist of starting and ending clock values, as
defined in the W3C SMIL recommendation, separated by a hyphen (ASCII
code 45).
When the URI specializer is "#", the viewer should play
that part of
the sequence beginning and ending at the requested times, measuring
from zero time at the beginning of the MNG datastream, or beginning and
ending with the specified frame numbers. To do this it must start with
the segment containing the requested time and decode any part of the
segment up to that time, composing but not displaying the frames; this
will provide the background against which the desired frames are
displayed.
When the URI specializer is "?", the server can send the
entire MNG datastream, or, preferably, it should construct a complete MNG file
containing:
If the server does not send the entire MNG datastream, and the first
segment after the SAVE chunk is not sent but a later segment
is sent, the optional index
must be written even if it does not exist in the source file. The index
must contain at least one "type 0" entry that gives the nominal
start
time and frame number for the first segment that is sent after the
SAVE chunk. The offset field can be set to zero and the segment
name can be omitted.
The query component should always be repeated as a fragment
identifier, so clients can find the requested item in case the server
sends more than what was requested.
MNG datastreams should not contain segment, subframe, or image
names that begin with the case-insensitive strings
"CLOCK(", "FRAME(", or "FRAMES(",
which are reserved for use in URI queries and
fragments (see Uniform Resource Identifier below).
See [RFC-2396]
and the W3C SMIL recommendation
at http://www.w3.org/TR/.
This (incomplete as of version 0.97) section does not form a part of
the specification. It provides
the rationale behind some of the design decisions in MNG.
Explain why the interframe delay has to be provided before
the subframes of layers are defined, instead of having a simpler
DELA chunk that occurs in the stream where the delay is
wanted.
Some delta types are not allowed when the parent object is a JNG
image. Explain why types 4 and 6 (pixel replacement and color channel
replacement) are not allowed under these circumstances.
Filter method 64 could have been implemented as a new critical chunk in
embedded PNG datastreams.
The FILT chunk would turn on this type of filtering.
The choice of using a new filter method instead of a new critical chunk was
made based on simplicity of implementation and possible eventual inclusion
of this method in PNG. Also, using the filter-method byte helps implementors
avoid confusion about whether this is a color transform (which could affect
the implementation of tRNS and other color-related chunks) or part
of the filtering mechanism (which would not conceivably affect color-related
chunks).
We considered using an ancillary chunk (e.g., fILt
or fILT) to turn on the new
filtering method. This would have an advantage that existing applications
could manipulate the files, but viewers that ignore the chunk would display
the image in unacceptably wrong colors, and editors could mistakenly discard
the chunk.
Q. Why not just use a BASI chunk to encode solid-color rectangles?
A. The MAGN chunk also allows encoding of gradient-filled rectangles.
Q. Why not just use PNG to encode gradient-filled rectangles?
A. While PNG can encode vertical and horizontal gradients fairly
efficiently, it cannot do diagonal ones efficiently, and none
are as efficient as a 30-byte MAGN chunk plus a 4-pixel PNG.
Q. Why not use full-scale low-quality JPEG/JNG?
A. Low-quality JPEG with reduced dimensions can be much smaller than even
the lowest-quality full-sized JPEG. For example, color lena converted
to a 512x512 JPEG using ImageMagick software, quality 1, is 2944 bytes,
while converted to a 64x64 JPEG, quality 50, it's only 1171 bytes[*] and
looks less posterized when magnified 8 times using the linear interpolation
method. This is a compression ratio of 671:1. Similar results were
obtained with the greyscale lena (2217, 941 bytes[*], and 279:1,
respectively). [Show PSNR numbers] [*]including 21 bytes for a
"MAGN 00 00 2 08 07" chunk.
Such images can then be magnified to full scale with the MAGN
chunk, for
use as preview ("LOWSRC") images. For natural images, this has been
demonstrated to be about 40 to 50 times as efficient as using Adam7
interlacing. For example, the size increase of the color lena image due
to interlacing is 45460 bytes, compared to the 1171 bytes needed for a
reduced JNG in front of a noninterlaced color lena.
It appears that in general, usable preview images of truecolor
photographic images can be made at compression ratios from M*800:1 to
M*2500:1, where M is the number of megapixels in the original image, by
reducing the original image spatially to width and height in the range
64 to 200 pixels and then compressing the result to a medium-quality JNG.
Q. Why not use the pHYg chunk?
A. It is not mandatory for decoders to process the pHYg chunk
and it does not apply to individual images; it
is used to scale the entire MNG frame. The pHYs chunk cannot be
used either because MNG decoders are required to ignore it.
Q. Why not 4-byte magnification factors instead of 2-byte ones?
A. Encoders can start with a larger object or, except for
object 0, magnify it twice.
Q. Why not 1-byte magnification factors, then?
A. With typical screen widths currently 1280 or 1620 pixels and film and
printer pages currently about 3000 pixels wide, magnifying a 1x1 image
to a width of more than 255 pixels would not be uncommon.
Q. I want to magnify a "frozen" object.
A. You can make a full clone and magnify that.
Q. Why define Methods 4 and 5?
A. Method 4 is useful for magnifying an alpha-encoded image while maintaining
binary transparency. Method 5 is useful for making an alpha-gradient
while preserving sharp edges in the main image.
It has been suggested that a new global MNG chunk, JTAB,
be defined to hold global JPEG quantization and huffman tables that
could be inherited by JNG datastreams from which these have been
omitted. This has not been tested, and we are reluctant to add new
critical chunks to the MNG specification now.
Proposed 9 December 2000
Proposed 5 December 2000
Removed filter methods 32, 96 and FILT chunk, which
failed to be approved. Revised Filter method 64,
which was approved on December 4, 2000.
Revised and clarified the specifications for the BACK and
FRAM chunks.
Proposed 01 October 2000
Added filter method 64 (intrapixel differencing).
Added filter methods 32 and 96 (leveling)
Added FILT chunk.
Added new proposed MAGN methods 3, 4, and 5.
Deleted "or can be ignored" from the background transparency
bit of the simplicity profile.
Released 01 October 2000
Added the MAGN chunk. This was approved by a formal vote.
Caution: there were errors in the interpolation formula
for MAGN (unbalanced parentheses, "+m" was
"+1") in the proposal that was voted upon; those errors have
been fixed in this public release.
Added JPEG-encoded alpha channel in JNG and Delta-PNG datastreams,
stored in a new JDAA chunk. This was approved by a formal vote.
Added a "stored objects" flag to promise that even
when "complex MNG features" are present, it is not necessary
to create object buffers. This proposal was approved by a formal vote.
Separated the "transparency" profile bit
into "transparency", "semitransparency",
and "background transparency", and adds discussion
of "background
transparency" to the BACK
and FRAM chunk specifications.
This
proposal was approved by a formal vote.
Added a "validity" flag to maintain backward compatibility
of the simplicity profile. If it is zero, then the "background
transparency", "semitransparency",
and "stored objects" flags do not
make any promises.
Global sRGB nullifies global gAMA and cHRM,
and vice versa.
It is permitted to change the potential visibility,
location, and clipping boundaries of "frozen" objects, provided
that the encoder writes chunks to restore them to their "frozen"
values prior to the end of the segment.
Added a note that top-level color-space chunks do not have any effect
on already-decoded objects.
Made editorial changes to the BASI chunk specification.
Made editorial changes in the LOOP chunk specification.
Made editorial changes in the PAST chunk specification. Changed
the names of left_pb, etc., to left_past_cb, etc.
Mentioned a fifth type of clipping: clipping the result of PAST
operations to the dimensions of the PAST destination object.
Changed "ASCII" to "character codes" in the
specification of the the sync field of the LOOP chunk.
Fixed typo in JNG signature bytes, to be the decimal equivalent
of "JNG" instead of "PNJ".
Disallowed the JSEP chunk when Relocated misplaced TERM chunk in Example 14.
Clarified some wording in the SEEK chunk specification, and
added a cross reference to the existing requirement to insert a background
layer when making random access to a segment.
Added terminology entries for "animation",
"framing rate",
"interpolation",
"iteration",
"replication",
and "nullify".
Clarified treatment of the alpha sample in the BASI chunk
when the color type is 0, 2, or 3, and clarified that the BASI
chunk inherits default DEFI values if no DEFI chunk
is present.
URLs were changed to "libpng.org".
Instances of "mpng-list" were updated to "mng-list".
Changed "Repeat count" to "Iteration count" in
the LOOP chunk specification, and "times to
repeat" to "times to execute" in the description of
the "Iteration max" field in the TERM chunk, and
added a statement about representing infinity.
Revised the
MNG-LC and MNG-VLC definitions
to include the possibility of
the new transparency flags and the new validity flag being set.
Added two examples related to the MAGN chunk.
Released 28 February 2000.
Minor editorial changes only.
A new example was added.
Released 18 July 1999.
The changes that are not simple editorial
changes were approved by votes of the
PNG Development group that closed 16 July 1999 (pHYg
and change to treatment of the pHYs chunk), 14 July 1999 (global
bKGD and sBIT) and 25 June 1999 (change to LOOP
chunk and treatment of the DEFI chunk and nonviewable objects).
Initial public release, approved by the PNG Development Group
on 11 May 1999.
See also
Pennebaker, William B., and Joan L. Mitchell,
"JPEG : Still Image Data Compression Standard"
Van Nostrand Reinhold, ISBN:0442012721, September 1992
See also the PNG-1.2 specification:
Security considerations are addressed in the PNG specification.
An infinite or just overly long loop could give the appearance
of having locked up the machine, as could an unreasonably long
interframe delay or a misplaced Some people may experience epileptic seizures when they are exposed
to certain kinds of flashing lights or patterns that are common in
everyday life. This can happen even if the person has never had any
epileptic seizures. All graphics software and file formats that
support animation and/or color cycling make it possible to encode
effects that may induce an epileptic seizure in these individuals.
It is the responsibility of authors and software publishers to issue
appropriate warnings to the public in general and to animation creators
in particular.
No known additional security concerns are raised by this format.
We use the "#" character to denote commentary in these
examples; such comments are not present in actual MNG datastreams.
The simplest MNG datastream is a single-image PNG datastream. The
simplest way to create a MNG from a PNG is:
The resulting MNG file looks like:
If file.png contains an sRGB chunk and
also gAMA and cHRM chunks that are recommended in the
PNG specification for "fallback" purposes, you can remove
those gAMA and cHRM chunks from file.mng
because any MNG viewer that processes the gAMA chunk is also
required to recognize and process the sRGB chunk, so those
chunks will always be ignored. Any MNG editor that converts the MNG file
back to a PNG file is supposed to insert the recommended gAMA
and cHRM chunks.
This example demonstrates a very simple movie, such as might result
from directly converting an animated GIF that contains a simple series
of full-frame images:
This slideshow gives exactly the same output as Example 3, but the
storage in the datastream is more efficient (the IDAT chunks will be
smaller) while the memory requirements in the decoder are larger. Image
ID 1 is used to store the ornate logos and frame design that appear on
every slide. The DHDR-IEND datastreams only contain deltas due to the
text and other information that is unique to each slide.
This movie is still fairly simple, but it capitalizes on
frame-to-frame similarities by use of Delta-PNG datastreams, and also
demonstrates the use of the fPRI chunk.
Here is an example single-composite-frame MNG, with thumbnails, which
takes a grayscale image and draws it side-by-side with a false-color
version of the same image:
Here is another movie, illustrating the use of Delta-PNG datastreams
as sprites:
This movie illustrates the use of several abstract images with
Show_mode=6 to describe an animated sprite, and the PAST chunk to turn
it around. The sprite runs back and forth across the background ten
times. The FRAM clipping boundaries restrict the screen updates to the
small region that changes, with a little "wiggle room" to make sure the
disturbed part of the background gets updated.
The opaque parts of this image will "fade in" gradually. This
example also illustrates the use of the PPLT and fPRI
chunks.
In this example, we store a series of twenty-four 150 x 150 x 150
blocks of eight-bit voxels. Each block is stored as a composite frame with
the first image being a PNG whose pixels represent the top layer of
voxels, which is followed by 149 Delta-PNG images representing the rest
of the layers of voxels. Only one image is defined, through which the
parent image is passed along from PNG to Delta-PNG to Delta-PNG. This
example also illustrates the use of unregistered ancillary chunks that
describe the x, y, and z scales and pixel calibration.
Here is another composite frame, illustrating the use of the LOOP
syntax to tile a large (1024 by 768) image area with a small (128 by 64)
image.
Here is a better approach, which creates a reusable tiled image by
means of the PAST chunk.
Here is an example of scrolling a 3000-line-high image (perhaps an
image of some text, but could be anything) through a 256-line-high
window with an alpha-blended border.
Alternatively, we can declare the scrolling object to be the
background and use framing-mode 3:
This demonstrates the use of the SHOW chunk with
This will cycle through the ten objects in the forward direction,
100 times, unless terminated sooner by the user or the decoder.
This will cycle through the ten objects, back and forth,
50 times, unless terminated sooner by the user or the decoder.
Outline of a program to convert GIF animations to MNG format:
Where "<image>" represents a PNG or Delta-PNG containing a GIF
frame converted to PNG format.
Caution: if you write such a program, you might have to pay royalties
in order to convey it to anyone else.
Outline of a program to convert simple GIF animations that do not
use the "restore-to-previous" disposal method to "simple" MNG
(or "MNG-LC") format:
Where "<image>" represents a PNG datastream containing a GIF
frame that has been converted to PNG format.
Caution: if you write such a program, you might have to pay royalties
in order to convey it to anyone else.
This demonstrates the determination of the layer-count and frame-count
that should be written in the MHDR chunk.
For framing_modes 1 and 2, the FRAM chunks themselves
don't generate layers. For framing_modes 3 and 4, they do
generate layers ("B" for background), and also generate frames if
there is no embedded image with which to combine the background layer.
Note that every framing_mode creates a "B" layer at the beginning.
Given the following chunk stream:
in which
This table shows the layer count and frame count for each of
the four possible values of framing-mode:
Here is an example of storing a library of icons in a
MNG-LC datastream.
All of the icons use the same palette, transparency, and colorspace,
so these are put in global chunks at the beginning. Empty PLTE
chunks in the embedded images are used to import the global palette and
transparency data.
This is similar, but it uses Delta PNG datastreams to create modified
versions by replacing the palette. This can be more storage-efficient,
but requires a full MNG decoder because of the presence of Delta PNG
datastreams.
This demonstrates the methods used in the MAGN chunk.
Original 3x2 object or embedded image:
Magnification method 1, XM = 5, YM = 3.
Replicates each pixel 4 additional times in
the X direction and 2 additional times in the
Y direction; new size is 15x6:
Magnification method 3, XM = 8, YM = 4
Fills the X intervals with 7 new pixels
and the Y interval with 3 new pixels
replicating the closest pixel to get
pixel values; new size is 17x5:
This example demonstrates the use of MNG to display a region of interest
(ROI) at a higher quality than the rest of the frame, and the MAGN
chunk to convey a highly-compressed but very lossy image, a drop shadow,
and a diagonal gradient background.
For the particular image used in this example (the 512x512 color Lena
from Bragzone [URL]), the resulting 600x600 frame occupies about 2.6
times the file size when written as a simple JNG and about 26 times the
file size when written as a simple PNG.
Contributors' names are presented in alphabetical order:
This document was built from the file mng-master-20001209
on 09 December 2000.
Copyright © 1998, 1999, 2000 by: Glenn Randers-Pehrson
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specification and any associated documentation will at all times remain
with copyright holder.
4.2.10. DISC Discard objects
Discard_id: 2 bytes (nonzero unsigned integer).
...etc...
object_id in the
discard_id list, when no such object has been stored, or when
the object has already been discarded.
4.3. Critical MNG image displaying chunks
4.3.1. BACK Background
Red_background: 2 bytes (unsigned integer).
Green_background: 2 bytes (unsigned integer).
Blue_background: 2 bytes (unsigned integer).
Mandatory
background: 1 byte (unsigned integer).
0: Background color and background
image are advisory. Applications
can use them if they choose to.
1: Background color is mandatory.
Applications must use it.
Background image is advisory.
2: Background image is mandatory.
Applications must use it.
Background color is advisory.
3: Background color and background
image are both mandatory.
Applications must use them.
This byte can be omitted if the
remaining fields are also omitted.
If so, the background color is
advisory.
Background
image_id: 2 bytes (unsigned nonzero integer).
Object_id of an image that is to be
used as the background layer or part
of it. If the image does not cover
the area defined by the layer clipping
boundaries with opaque pixels, the
remainder of this area is filled with the
background color or application background
and the background image is composited
against it. This field can be omitted if
the background_tiling byte is also omitted;
if so, no background image is defined, and
the background image_id from any previous
BACK chunk becomes undefined. This byte
must be omitted in MNG-LC and MNG-VLC
datastreams, and when the "stored objects"
flag in the simplicity profile is valid
and is zero.
Background
tiling: 1 byte (unsigned integer).
0: Do not tile the background.
1: Tile the background with the
background image. This field
can be omitted; if so, do not
tile the background.
This byte must be omitted in
MNG-LC and MNG-VLC datastreams.
background_image_id when the
"stored objects" flag in the simplicity profile is valid and
zero.
background_image_id
when such an image is not viewable and potentially visible or
does not yet exist or ceases to exist for some reason,
or to fail to specify a background_image_id even when
the mandatory_background flag is 2 or 3.
Viewers must be prepared to fall back temporarily to using the background
color or application background in this event, and to resume using the
background image whenever a potentially visible viewable object with the
background_image_id becomes available.
They also must be prepared for the contents, viewability, location, potential
visibility, and clipping boundaries of the background image to change, just
like any other object, if it has not been "frozen". The
background image is allowed to have transparency, subject to any promises
made in the simplicity profile.
mandatory_background=0 and the application already has
its own background definition. This will frequently be the case in
World Wide Web pages, to achieve nonrectangular transparent animations
displayed against the background of the page.
4.3.2. FRAM Frame definitions
Framing mode: 1 byte.
0: Don't change framing mode.
1: No background layer is generated,
except for one ahead of the very
first foreground layer in the
datastream. The interframe delay
is associated with each foreground
layer in the subframe.
2: No background layer is generated,
except for one ahead of the very
first image in the datastream.
The interframe delay is associated
only with the final layer in the
subframe. A zero interframe delay
is associated with the other layers
in the subframe.
3: A background layer is generated
ahead of each foreground layer. The
interframe delay is associated with
each foreground layer, and a zero delay
is associated with each background
layer.
4: The background layer is generated
only ahead of the first foreground
layer in the subframe. The
interframe delay is associated only
with the final foreground layer in
the subframe. A zero interframe
delay is associated with the background
layers, except when there is no
foreground layer in the subframe, in
which case the interframe delay is
associated with the sole background
layer.
Subframe name: 0 or more bytes (Latin-1 Text). Can be
omitted; if so, the subframe is nameless.
Separator: 1 byte: (null). Must be omitted if all
remaining fields are also omitted.
Change interframe
delay: 1 byte.
0: No.
1: Yes, for the upcoming subframe only.
2: Yes, also reset default.
This field and all remaining fields can be
omitted as a group if no frame parameters
other than the framing mode or the subframe
name are changed.
Change timeout and
termination: 1 byte
0: No.
1: Deterministic, for the upcoming subframe
only.
2: Deterministic, also reset default.
3: Decoder-discretion, for the upcoming
subframe only.
4: Decoder-discretion, also reset
default.
5: User-discretion, for the upcoming subframe
only.
6: User-discretion, also reset default.
7: External-signal, for the upcoming subframe
only.
8: External-signal, also reset default.
This field can be omitted only if the previous
field is also omitted.
Change layer clipping
boundaries: 1 byte.
0: No.
1: Yes, for the upcoming subframe only.
2: Yes, also reset default.
This field can be omitted only if the previous
field is also omitted.
Change sync id
list: 1 byte.
0: No.
1: Yes, for the upcoming subframe only.
2: Yes, also reset default list.
This field can be omitted only if the previous
field is also omitted.
Interframe
delay: 4 bytes (unsigned integer). This field must
be omitted if the change_interframe_delay
field is zero or is omitted. The range is
[0..2^31-1] ticks.
Timeout: 4 bytes (unsigned integer). This field must
be omitted if the change_timeout_and_termination
field is zero or is omitted. The range is
[0..2^31-1]. The value 2^31-1 (0x7fffffff)
ticks represents an infinite timeout period.
Layer clipping boundary
delta type: 1 byte (unsigned integer).
0: Layer clipping boundary values are
given directly.
1: Layer clipping boundaries
are determined by adding the FRAM
data to the values from the previous subframe.
This and the following four
fields must be omitted if the
change_layer_clipping_boundaries field
is zero or is omitted.
Left layer cb or Delta left
layer cb: 4 bytes (signed integer).
Right layer cb or Delta right
layer cb: 4 bytes (signed integer).
Top layer cb or Delta
top layer cb: 4 bytes (signed integer).
Bottom layer cb or Delta bottom
layer cb: 4 bytes (signed integer).
Sync id: 4 bytes (unsigned integer). Omit if
change_sync_id_list=0 or if the new
list is empty; repeat until all
sync_ids have been listed. The
range is [0..2^31-1].
framing_mode=1, the decoder must wait until the
interframe delay for the previous frame has elapsed before displaying
each image. Each foreground layer is a separate subframe and frame.
framing_mode=2, viewers are
expected to display all of the images in a frame at once, if possible, or as
fast as can be managed, without clearing the display or restoring the
background.
framing_mode=3, a background layer is generated and
displayed immediately before each image layer is displayed. Otherwise,
framing_mode=3 is identical to framing_mode=1.
Each foreground layer together with its background layer make up a single
subframe and frame.
framing_mode=4, the background layer is generated
and displayed immediately
before each frame, i.e., after each FRAM chunk, with no interframe
delay before each image. The decoder must wait until the
interframe delay for the previous frame has elapsed before displaying the
background layer.
Otherwise, framing_mode=4 is identical to framing_mode=2.
All of the foreground layers
between consecutive FRAM chunks, together with one background
layer, make up a single subframe.
frame_name field. It is recommended that the same name
not appear in any other FRAM chunk or in any SEEK
or eXPI
chunk. Subframe names should not begin with the
case-insensitive
strings "CLOCK(", "FRAME(", or "FRAMES(",
which are reserved for use in URI queries and
fragments (see Uniform Resource Identifier below).
change_timeout_and_termination field specifies whether and
over what range the normal interframe delay can be lengthened or
shortened. It can take the following values:
sync_id, but
must wait no less than the smaller of the timeout and the interframe
delay nor no longer than the greater of the timeout and the interframe
delay. If the decoder cannot receive signals, this condition degenerates
into "decoder-discretion".
sync_id list can be omitted if the termination
condition is not "external-signal".
sync_id list is changed, the number of
sync_id entries is determined by the remaining length of the
chunk data, divided by four. This number can be zero, which either
inactivates the existing sync_id list for one frame or
deletes it.
Framing mode = 1
Subframe name = <empty string>
Interframe delay = 1
Left subframe boundary = 0
Right subframe boundary = frame width
Top subframe boundary = 0
Bottom subframe boundary = frame height
Termination = deterministic
Timeout = 0x7fffffff (infinite)
Sync id = <empty list>
interframe_delay field gives the duration of
display, which is the minimum time that must elapse from the
beginning of displaying one layer until the beginning of displaying
the next (unless the termination condition and timeout permit this
time to be shortened). It
is measured in "ticks" using the tick length determined from
ticks_per_second defined in the MHDR chunk.
When the interframe delay is zero, it indicates that
the layer is to be combined with the subsequent layer or layers into a
single frame, until a nonzero interframe delay is specified or
the MEND chunk is reached.
sync_id list provides a point at which the processor
must wait for all pending processes to reach the synchronization
point having the same sync_id before resuming, perhaps
because of a need to synchronize a sound datastream (not defined
in this specification) with the display, to synchronize stereo
images, and the like. When the period defined by the sum of the
interframe_delay and the timeout fields
elapses, processing can resume even though the processor has not
received an indication that other processes have reached the
synchronization point.
sync_id=0 is reserved to represent
synchronization with a user input from a keyboard or pointing device.
The sync_id values 1-255 are reserved to represent the
corresponding ASCII letter, received from the keyboard (or a simulated
keyboard), and values 256-1023 are reserved for future definition
by this specification. If multiple channels (not defined in this
specification) are not present, viewers can ignore other values
appearing in the sync_id list.
4.3.3. MOVE New image location
First object: 2 bytes (unsigned integer).
Last object: 2 bytes (unsigned integer).
Location
delta type: 1 byte (unsigned integer).
0: MOVE data gives X_location and
Y_location directly.
1: New locations are determined
by adding the MOVE data to the
location of the parent object.
X_location or
delta X_location: 4 bytes (signed integer).
Y_location or
delta Y_location: 4 bytes (signed integer).
first_object=last_object, or to a group of consecutive
object_ids, if they are different. Last_object
must not be less than first_object. Negative values are
permitted for the X and Y location. The positive directions are
downward and rightward from the frame origin. The MOVE chunk
can specify an image placement that is partially or wholly outside the
display boundaries. In such cases, the resulting image must be clipped
to fit within its clipping boundaries, or not displayed at all if it
falls entirely outside its clipping boundaries. The clipping boundaries
are determined as described in the specification for the CLIP
chunk below (Paragraph 4.3.4).
The left and top boundaries are inclusive, while the right and bottom
boundaries are exclusive.
4.3.4. CLIP Object clipping boundaries
First object: 2 bytes (unsigned integer).
Last object: 2 bytes (unsigned integer).
Clip delta type: 1 byte (unsigned integer).
0: CLIP data gives boundary values
directly.
1: CLIP boundaries are determined
by adding the CLIP data to their
previous values for this object.
Left_cb or
delta_left_cb: 4 bytes (signed integer).
Right_cb or
delta_right_cb: 4 bytes (signed integer).
Top_cb or
delta_top_cb: 4 bytes (signed integer).
Bottom_cb or
delta_bottom_cb: 4 bytes (signed integer).
first_object=last_object, or to a group of consecutive
objects, if they are different. the last_object must not be less
than first_object.
0 <= x < frame_width (from the MHDR chunk)
0 <= y < frame_height
Left_lcb <= x < right_lcb (from the FRAM chunk)
Top_lcb <= y < bottom_lcb
Left_cb <= x < right_cb (from the CLIP chunk)
Top_cb <= y < bottom_cb
4.3.5. SHOW Show images
First image: 2 bytes (nonzero unsigned integer).
Last image: 2 bytes (nonzero unsigned integer).
This field can be omitted if the show_mode
byte is also omitted. If so, decoders must
assume the default values, show_mode=0
and last_image=first_image.
Show_mode: 1 byte (unsigned integer).
0: Make the images potentially visible.
and display them (set do_not_show=0).
1: Make the images invisible (set
do_not_show=1).
2: Don't change do_not_show flag;
display those that are potentially
visible.
3: Mark images "potentially visible"
(do_not_show=0), but do not display
them.
4: Toggle do_not_show flag; display
any that are potentially visible after
toggling.
5: Toggle do_not_show flag, but do
not display even if potentially visible
after toggling.
6: Step through the images in the given
range, making the next image potentially
visible (set do_not_show=0) and display
it. Set do_not_show=1 for all other
images in the range. Jump to the
beginning of the range when reaching the
end of the range. Perform one step for
each SHOW chunk (in reverse order if
last_image < first_image).
7. Make the next image in the range
(cycle) potentially visible (do_not_show=0),
but do not display it. Set
do_not_show=1 for the rest of the
images in the range.
This field can be omitted. If so, decoders
must assume the default, show_mode=0.
first_image through
last_image, and resets the do_not_show flag for
each of the objects. If show_mode is even-valued, it also
displays the images if they are potentially visible and are viewable
images.
do_not_show status. The empty SHOW chunk is
equivalent to
SHOW 1 65535 2
last_image < first_image the images are processed in
reverse order.
show_mode is odd-valued, nothing is displayed
unless a subsequent SHOW chunk with an even-valued
show_mode appears.
show_mode is even-valued, each visible image that
is displayed generates a separate layer, even
if it is offscreen and no pixels are actually displayed. In such cases,
the layer is totally transparent. When show_mode is odd, or
when show_mode is 2 or 4 or is empty and no image is visible, no
layer is generated.
show_mode is 1, 4, 5, 6, or 7, images can
be made invisible. This is not permitted when the framing mode is 2 or 4
in the FRAM chunk and the images have already appeared in the
frame, because simple viewers will have already drawn them and have no
way to make them invisible again without redrawing the entire frame.
show_mode is 6 or 7, a single layer is generated.
The decoder must make the
next image in the "cycle" visible. To do this, it must examine
the do_not_show flag for each image in the range
first_image through last_image, and make the next
one (the one with the next higher value of image_id that
exists and is "viewable") after the first visible one it finds visible
and the rest invisible. When first_image > last_image,
the cycle is reversed, and the "next" image is the one with the next
lower value of image_id. In either case, if the first
visible one found was last_id, or none were visible, it must make
first_image visible. These modes are useful for manipulating
a group of sequential images that represent different views of an
animated icon. See Example 8, below (Chapter 17).
If no "viewable" object is in the specified range, an empty layer
must be generated.
show_mode is 0, 2, 4, or 6, separate layers will be
generated, each containing an instance of one visible image at the
location specified by the DEFI, CLON, or MOVE
chunk and clipped according to the boundaries specified by the
CLIP and FRAM chunks. When the MOVE or
CLON chunk is used in the delta form, which will frequently be
the case, each image must be displaced from its previous position by the
values given in the MOVE or CLON chunk.
object_id=6 in a
composite frame to blink:
LOOP 0 0 10
FRAM 4 # Show background
SHOW 1 10 # Show images 1 thru 10.
FRAM # Show background
SHOW 1 5 # Show images 1 thru 5.
SHOW 7 10 # Show images 7 thru 10.
ENDL
FRAM 4 # Show background
LOOP 0 0 10
SHOW 1 5 # Show images 1 thru 5.
SHOW 6 6 4 # Toggle potential visibility of image 6
SHOW 7 10 # and show it; show images 7 thru 10.
FRAM
ENDL
FRAM 4 # Show background
LOOP 0 0 10
SHOW 6 6 5 # Toggle potential visibility of image 6.
SHOW 1 10 2 # Show potentially visible images in 1
FRAM # through 10.
ENDL
do_not_show=0 in the DEFI chunk that introduced the
image. Similarly, the CLON chunk need not be followed by a
SHOW chunk, if do_not_show=0 in the CLON
chunk.
4.3.6. TERM Termination action
Termination
action: 1 byte (unsigned integer)
0: Show the last frame indefinitely.
1: Cease displaying anything.
2: Show the first frame after the TERM
chunk.
If processing the fPRI chunk,
use a "cost" of 255.
3: Repeat the sequence starting
immediately after the TERM chunk
and ending with the MEND chunk.
Action after
iterations: 1 byte
0: Show the last frame indefinitely after
iteration_max iterations have been
done.
1: Cease displaying anything.
2: Show the first frame after the TERM
chunk.
If processing the fPRI chunk,
use a "cost" of 255.
This and the remaining fields must be
present if termination_action is 3, and
must be omitted otherwise.
Delay: 4 bytes (unsigned integer). Delay, in
ticks, before repeating the sequence.
Iteration max: 4 bytes (unsigned integer). Maximum
number of times to execute the sequence.
Infinity is represented by 0x7fffffff.
iteration_min=1.
4.4. SAVE and SEEK chunks
4.4.1. SAVE Save information
Offset size: 1 byte (unsigned integer).
4: Offsets and nominal start times are
expressed as 32-bit integers.
8: Offsets and nominal start times are
expressed as 64-bit integers.
Entry type: 1 byte (unsigned integer).
0: Segment with nominal start time,
nominal layer number, and nominal
frame number.
1: Segment.
2: Subframe.
3: Exported image.
Offset: 4 or 8 bytes (unsigned integer). Omit if
entry_type > 1, set equal to zero
if the offset is unknown.
Nominal start
time: 4 or 8 bytes: (unsigned integer). Start
time of the segment, measured in ticks
from the beginning of the sequence,
assuming that all prior segments were
played as intended on an ideal player,
ignoring any fPRI chunks. Omit if
entry_type > 0.
Nominal layer
number: 4 bytes (unsigned integer). Sequence
number of the first layer in the segment,
assuming that all prior segments were
played as intended on an ideal player,
ignoring any fPRI chunks; the first
layer of the first segment being layer 0.
Omit if entry_type > 0.
Nominal frame
number: 4 bytes (unsigned integer). Sequence
number of the first frame in the segment,
assuming that all prior segments were
played as intended on an ideal player,
ignoring any fPRI chunks; the first frame
of the first segment being frame 0. Omit
if entry_type > 0.
Name: 1-79 bytes (Latin-1 text). Omit for
unnamed segments.
Separator: 1 byte (null) (must be omitted after
the final entry).
4.4.2. SEEK Seek point
Segment name: 1-79 bytes (Latin-1 string).
They also must not depend on anything that has been drawn on the output
buffer or device. Its contents become undefined when the SEEK
chunk is encountered. Decoders that make random access to a seek
point from the interior of a segment must insert a background layer before
processing the segment. Encoders must ensure that simple viewers do not
need to do this.
4.5. Ancillary MNG chunks
4.5.1. eXPI Export image
Snapshot id: 2 bytes (unsigned integer).
(must be zero in MNG-LC and
MNG-VLC datastreams)
Snapshot name: 1-79 bytes (Latin-1 text).
snapshot_name is associated with the
snapshot, not with the snapshot_id nor its future contents;
changing the image identified by snapshot_id will not
affect the snapshot.
The snapshot_name means nothing inside the scope of the
MNG specification, except that it can be included in
the optional index that can appear in the SAVE chunk.
If two eXPI chunks use the same name, it is the outside world's
problem (and the outside world's prerogative to regard it as an error).
It is recommended, however, that the snapshot_name not be
the same as that appearing in any other eXPI
chunk or in any FRAM
or SEEK
chunk. A decoder that knows of no
"outside world" can simply ignore the eXPI chunk. This
chunk could be used in MNG datastreams that define libraries of related
images, rather than animations, to allow applications to extract
images by their snapshot_id.
snapshot_name string must follow the format of a
tEXt keyword: It must consist only of printable Latin-1
characters and must not have leading or trailing blanks, but can have
single embedded blanks. There must be at least one and no more than
79 characters in the keyword. Keywords are case-sensitive. There is
no null byte terminator within the snapshot_name string,
nor is there a separate null byte terminator. Snapshot names should
not begin with the case-insensitive strings
"CLOCK(", "FRAME(", or "FRAMES("
which are reserved for use in URI queries and
fragments (see Uniform Resource Identifier below).
snapshot_id.
4.5.2. fPRI Frame priority
fPRI delta type: 1 byte (unsigned integer).
0: Priority is given directly.
1: Priority is determined by adding
the fPRI data to the previous
value, modulo 256.
Priority or
delta priority: 1 byte (signed integer). Value to be
assigned to subsequent chunks until
another fPRI chunk is reached.
priority are possible, it
is recommended that only the values 0 (low priority), 128 (medium
priority), and 255 (high priority) be used. Viewers that can only
display a single image can look for one with priority=255 and
stop after displaying it. If the datastream contains a large number
of frames and includes periodic "initial" frames that do not contain
Delta-PNG datastreams, each "initial" frame could be preceded by a
fPRI with priority=128 and followed by one with
priority=0, and the best representative initial frame could
be preceded by a fPRI chunk with priority=255. Then
single-image viewers would just display the representative frame, slow
viewers would display just the "initial" frames, and fast viewers
would display everything.
priority=255; decoders must look for these chunks in
addition to the fPRI chunk while skipping a low-priority
portion of the datastream.
priority=255 for any portion of the MNG datastream that is
processed prior to the first fPRI chunk.
4.5.3. nEED Resources needed
Keyword: 1-79 bytes.
Separator: 1 byte (null).
...etc...
4.5.4. pHYg Physical pixel size (global)
4.6. Ancillary PNG chunks
color_type != 3). This chunk can appear for
any color type. There can be multiple sPLT chunks in a MNG
datastream. If a palette_name is repeated, the previous
palette having the same palette_name is replaced. It is
not permitted, at the MNG top level, to redefine a palette after the
SAVE chunk with the same palette_name as one that
appears ahead of the SAVE chunk. It is permitted, however, to
define and redefine other palettes with other palette_name
fields. A single empty sPLT chunk can be used to nullify all
sPLT chunks that have been previously defined in the MNG top
level, except for those that appeared ahead of the SAVE chunk,
when the SAVE chunk has been read.
5. The JPEG Network Graphics (JNG) Format
http://www.libpng.org/pub/png/
139 74 78 71 13 10 26 10
image/x-jng can be used. It is recommended that the
file extension ".jng" (lower case preferred) be used.
5.1. Critical JNG chunks
5.1.1. JHDR JNG header
Width: 4 bytes (unsigned integer, range 0..65535).
Height: 4 bytes (unsigned integer, range 0..65535).
Color type: 1 byte
8: Gray (Y).
10: Color (YCbCr).
12: Gray-alpha (Y-alpha).
14: Color-alpha (YCbCr-alpha).
Image sample
depth: 1 byte
8: 8-bit samples and quantization tables.
12: 12-bit samples and quantization
tables.
20: 8-bit image followed by a 12-bit
image.
Image compression
method: 1 byte
8: ISO-10918-1 Huffman-coded baseline JPEG.
Image interlace
method: 1 byte.
0: Sequential JPEG, single scan.
8: Progressive JPEG.
Alpha sample
depth: 1 byte.
0, 1, 2, 4, 8, or 16, if the Alpha
compression method is 0 (PNG)
8, if the Alpha compression method
is 8 (JNG).
Alpha compression
method: 1 byte.
0: PNG grayscale IDAT format.
8: JNG 8-bit grayscale JDAA format.
Alpha filter
method: 1 byte.
0: Adaptive PNG (see PNG spec) or
not applicable (JPEG).
Alpha interlace
method: 1 byte.
0: Noninterlaced PNG or sequential
single-scan JPEG.
color_type is 8 or 10 (no alpha channel), the
last four bytes, which describe the IDAT or JDAA data,
must be set to zero. The alpha_sample_depth must be nonzero
when the alpha channel is present.
5.1.2. JDAT JNG image data
Y = Luma_red*R + Luma_green*G + Luma_blue*B
Cb = (B - Y) / (2 - 2*Luma_blue) + Half_scale
Cr = (R - Y) / (2 - 2*Luma_red) + Half_scale
Luma_red = 0.299
Luma_green = 0.587
Luma_blue = 0.114
5.1.3. IDAT JNG PNG-encoded alpha data
alpha_compression_method=0. The alpha channel, if
present, can have sample depths 1, 2, 4, 8, or 16.
2alpha_sample_depth-1 means fully opaque.
5.1.4. JDAA JNG JPEG-encoded alpha data
alpha_compression_method=8. The alpha channel, if
present, can have only sample depth 8. The JDAA
chunks can be interleaved with the JDAT chunks
(see Recommendations for Encoders:
JNG interleaving below).
5.1.5. IEND End of JNG datastream
5.1.6. JSEP 8-bit/12-bit image separator
image_sample_depth=20 in the JHDR chunk. When
image_sample_depth != 20, the JSEP chunk must not
be present. The eight-bit
datastream must appear first. Both images must have the same width,
height, color type, compression method, and interlace method. Viewers can
choose to display one or the other image, but not both.
5.2. Ancillary JNG chunks
6. The Delta-PNG Format
do_not_show=0).
compression_method and interlace_method are
also the same as defined in the PNG specification. The definition
of filter_method is the same as for PNG datastreams that
are embedded in MNG datastreams
(see the IHDR chunk specification, above, Paragraph 4.2.3).
6.1. Delta-PNG critical chunks
6.1.1. DHDR Delta-PNG datastream header
Object id: 2 bytes (nonzero unsigned integer).
Identifies the parent object from which
changes will be made. This is also the
object_id of the child image, which can
be used as the parent image for a
subsequent Delta-PNG.
Image type: 1 byte.
0: Image type is unspecified. An IHDR,
JHDR, IPNG, or IJNG chunk must be
present. If JHDR or IJNG is present,
delta_type must not be 1, 3, 4, or 6.
1: Image type is PNG. IHDR and IPNG can
be omitted under certain conditions.
2: Image type is JNG. JHDR and IJNG can
be omitted under certain conditions.
Delta_type must not be 1, 3, 4, or 6.
Delta type: 1 byte.
0: Entire image replacement.
1: Block pixel addition, by samples,
modulo 2^sample_depth.
2: Block alpha addition, by samples,
modulo 2^sample_depth. Regardless of
the color type of the parent image,
the IDAT data are written as a
grayscale image (color type 0), but
the decoded samples are used as deltas
to the alpha samples in the parent
image. The parent image must have
(or be promoted to via the PROM chunk)
a color type that has an alpha channel.
3: Block color addition. Similar to
delta type 1 except that only the
color channels are updated even when
the parent has an alpha channel.
4: Block pixel replacement.
5: Block alpha replacement.
6: Block color replacement.
7: No change to pixel data.
Block width: 4 bytes (unsigned integer). This field
must be omitted when delta_type=7.
Block height: 4 bytes (unsigned integer). This field
must be omitted when delta_type=7.
Block
X_location: 4 bytes (unsigned integer),
measured in pixels from the left edge
of the parent object. This field must
be omitted when delta_type=0 or when
delta_type=7.
Block
Y_location: 4 bytes (unsigned integer),
measured in pixels from the top edge
of the parent object. This field must
be omitted when delta_type=0 or when
delta_type=7.
object_id must identify an existing object, and the
object must be a "concrete" object, i.e., it must have the property
concrete_flag=1.
image_type, whether given explicitly as 1 or 2
or implied by the presence of an IHDR, IPNG,
JHDR, or IJNG chunk, must be the same as that of the
parent object.
delta_type=0, the width and height of the child image
are given by the block_width and block_height
fields.
delta_type, the width and height
of the child image are inherited from the parent object.
delta_type=1-6, the block_width
and block_height fields give the size of the block of
pixels to be modified or replaced, and block_X_location and
block_Y_location give its location with respect to the top
left corner of the parent object. The block must fall entirely within
the parent object.
delta_type=0 in the DHDR chunk, the
pixel data in the IDAT chunks represent a completely new
image, with dimensions given by the block_width and
block_height fields of the DHDR chunk. Data from
chunks other than IDAT or JDAT can be inherited from
the parent object. If the IHDR or JHDR chunk is present,
all of its fields except width and height
(which must be ignored by decoders) provide
new values that are inherited by subsequent objects. The "pixel
sample depth" and "alpha sample depth" are also reset
equal to the
IHDR sample_depth value (in the case of a JNG object,
the new "alpha sample depth" is taken from the JHDR
alpha_sample_depth field). If the IHDR or JHDR
chunk is not present,
the IDAT chunks are decoded according to the parent object's
sample depth, and not according to the "pixel
sample depth" or "alpha sample depth" which are used for
decoding the IDAT chunks in subsequent Delta-PNG datastreams
when delta_type is nonzero.
delta_type=1 in the DHDR chunk, the pixel
data in the IDAT chunks represent deltas from the pixel data in
a parent object known to the decoder, including the alpha channel, if the
parent object has an alpha channel.
interlace_method, filtered
according to the filter_method and compressed according to
the compression_method given in the IHDR chunk.
The pixel data includes alpha
samples, if the parent object has an alpha channel.
2sample_depth.
When decoding the IDAT chunk, the child image bytes are
obtained by adding the delta bytes to the parent object bytes, modulo
2sample_depth. This is similar in operation to the PNG SUB filter,
except that it works by samples instead of by bytes.
color_type=3, the
deltas are differences between index values, not between color samples.
color_type=3, the delta can have
color_type=0, and vice versa, since the contents of
the IDAT chunks of either color type are indistinguishable.
pixel_sample_depth
does not match the
object_sample_depth, the delta must be scaled to the
object_sample_depth using the zero-fill or right-shift method
described in the PNG specification, before performing the pixel addition.
delta_type=2 in the DHDR chunk, the pixel
data in the IDAT chunks represent deltas from the alpha
data in a parent object known to the decoder. The color samples
are not changed, and the updated alpha samples are calculated in
the same manner as the updated pixel samples are calculated when
delta_type=1.
color_type is 0 (grayscale), regardless of the
color_type of the parent object. The parent object must have
an alpha channel or must have been promoted to a type that has an alpha
channel. The compression method, filter method, and interlace method
need not be the same. If they are different, the child object inherits
the new values, and the new values will be used in decoding the data in
any subsequent IDAT chunks. Neither the parent object nor the
delta object can have alpha samples that were carried in JPEG-encoded
JDAA chunks.
sample_depth value from the IHDR chunk is
interpreted as a new value of alpha_sample_depth and is
only used for decoding the IDAT data in this and subsequent
Delta-PNGs. Implicit in this is the requirement for decoders to remember
in the object buffer not only the sample depth of the object
but (separately) the alpha_sample_depth
for use in decoding the IDAT chunks in any subsequent Delta-PNG
datastreams.
alpha_sample_depth
does not match the
object_sample_depth, the delta must be scaled to the
object_sample_depth,
using the zero-fill or right-shift method described in the PNG specification,
before performing the pixel addition.
delta_type=3 is similar to delta_type=1
except that the alpha channel is not included in the IDAT
pixels; the alpha channel is inherited from the parent object.
The color type of the parent must be one that has an alpha channel
(4 or 6) and the color type of the delta must be the corresponding color type
(0 or 2) that does not have an alpha channel.
delta_type=4 in the DHDR chunk, the pixel
data in the IDAT chunks represent replacement values for the
pixel samples in the rectangle given by the block location and dimension
fields in the DHDR chunk, including the alpha channel, if the
parent object has an alpha channel.
pixel_sample_depth does not match
the object_sample_depth,
the pixel data must be scaled to the object_sample_depth
before making the replacements,
using the left bit replication method described in the PNG specification,
or by the right shift method in the unlikely event that the
pixel_sample_depth is larger than
the object_sample_depth.
delta_type=5 in the DHDR chunk, the pixel
data in the IDAT chunks represent replacement values of the
alpha samples in the rectangle given by the block location and dimension
fields in the DHDR chunk. The sample depth of the
data (i.e. the "alpha sample depth") need not match the sample depth
of the parent object, and color_type is
0 (grayscale), regardless of the color_type of the parent
object.
If the sample depths differ, the samples must be scaled
to the object_sample_depth, using
the left bit replication method or right shift method described
in the PNG specification, depending on whether
the alpha_sample_depth is larger or smaller than
the object_sample_depth.
The parent object must have an alpha channel or must have been
promoted to a type that has an alpha channel. The compression method,
filter method, and interlace method need not be the same. If they
differ, the child object inherits the new values.
delta_type=6 is similar to delta_type=4
except that the alpha channel is not included in the IDAT
pixels; the alpha channel is inherited from the parent object.
The color type of the parent must be one that has an alpha channel
(4 or 6) and the color type of the delta must be the corresponding color type
(0 or 2) that does not have an alpha channel.
delta_type=7 in the DHDR chunk, there is
no change to the pixel data, and it is an error for
IDAT, JDAT, or JDAA to appear. If the IHDR
or JHDR chunk appears, the width, height, and color_type fields are
ignored, the PNG sample depth
(or JNG alpha_sample_depth) is used to update the pixel_sample_depth
and alpha_sample_depth, and the data in the remaining fields
are inherited by the child object.
delta_type != 0. The decoder needs to remember the
pixel_sample_depth and alpha_sample_depth to use with each
object. They are initialized to the sample_depth value
from the IHDR chunk that appears when the object is first
created but can be changed by the appearance of the IHDR chunk in
a Delta-PNG datastream that has delta_type != 0.
If the object is a JNG image, they are initialized from the value
of alpha_sample_depth from
the original JHDR chunk, and can be changed by the appearance
of the JHDR chunk in a Delta-PNG datastream that has
delta_type != 0.
6.1.2. IEND End of Delta-PNG datastream
6.1.3. PROM Promotion of parent object
New color type: 1 byte.
New sample depth: 1 byte.
Fill method: 1 byte.
0: Left-bit-replication
1: Zero fill
color_type 0 -> 4)
color_type 2 -> 6)
color_type 0 -> 2)
color_type 4 -> 6)
color_type 0 -> 6)
color_type 3 -> 2)
color_type 3 -> 6)
JNG color_type 8 -> 10)
JNG color_type 8 -> 12)
JNG color_type 8 -> 14)
JNG color_type 10 -> 14)
JNG color_type 12 -> 14)
color_type
fill_method to fill the additional
bits of each sample. If cheap transparency information is present in a
grayscale or truecolor object, its sample values must also be widened in
the same manner. If the image type is JNG, then the new sample depth
refers to the alpha_sample_depth and only the alpha channel
is affected, if one is present. If the color_type has been
promoted from indexed-color, the original bit depth is always considered to be
8. See the PNG specification [PNG]
for
further information on these filling methods. Any alpha channel added
in this manner is eligible to be updated by block-alpha-addition in this
or a subsequent Delta-PNG.
fill_method. If the basis object is a JNG, the bKGD
chunk is not affected.
6.1.4. IHDR PNG image header
width,
and height
fields are ignored. The values for these parameters are inherited from
the parent object or from the PROM chunk.
sample_depth, color_type,
compression_method, interlace_method, and
filter_method fields, if different from those of the parent
object, are used in decoding any subsequent IDAT chunks, and
the new values will be inherited by any subsequent image that uses this
object as its parent. These do not change the sample_depth
and color_type of the object itself; those can only be
changed by using the PROM chunk or by using delta_type=0.
filter_method can be any filter_method that is
allowed in PNG datastreams that are embedded in a MNG datastream.
The
PNG datastream must contain at least IHDR and IEND
(whether actually present in the datastream or omitted and included
by implication, as described below), but can inherit other chunk data
from the parent object. Except for IDAT and PPLT,
any chunks appearing between IHDR and IEND are always
treated as replacements or additions and not as deltas.
6.1.5. IPNG Incomplete PNG
compression_method, filter_method, or
interlace_method. The purpose of this chunk is to identify
the beginning of the PNG datastream, so decoders can start interpreting
PNG chunks instead of Delta-PNG chunks. The decoder must treat this
datastream as though the IHDR chunk were present in the
location occupied by the IPNG chunk.
image_type=1 and the PNG datastream begins with a
PLTE chunk, a PPLT chunk, or an IDAT chunk.
In this case, no IPNG chunk is required, either. The decoder
must treat this datastream as though the IHDR chunk were
present, immediately preceding the first PNG chunk. If the first PNG
chunk is neither a PLTE chunk, a PPLT chunk, nor an
IDAT chunk, then either the IPNG or IHDR must
be present to introduce the PNG datastream.
6.1.6. PLTE and tRNS
sample_depth
would allow, and must not have more than 256 entries.
color_type=3
and PLTE is not supplied, then the number of allowable entries
is determined from the number of PLTE entries in the parent
object. A tRNS chunk appearing in a Delta-PNG datastream is
always treated as a complete replacement for the tRNS chunk
data in the parent object. All entries beyond those actually supplied
are overwritten with the "opaque" value (255).
6.1.7. PPLT Partial palette
delta_type byte and one
or more groups of palette entries:
PPLT delta type: 1 byte.
0: Values are replacement RGB samples.
1: Values are delta RGB samples.
2: Values are replacement alpha
samples.
3: Values are delta alpha samples.
4: Values are replacement RGBA
samples.
5: Values are delta RGBA samples.
First index,
first group: 1 byte.
Last index,
first group: 1 byte.
First set of
samples: 1, 3, or 4 bytes.
...etc...
Last set of
samples: 1, 3, or 4 bytes.
First index,
second group: 1 byte.
...etc...
last_index must be equal to or greater than
first_index. The groups are not required to appear in
ascending order. If any index of any group is beyond the end of the
parent object's palette, the palette and tRNS data must be
extended just as if a longer complete PLTE chunk had appeared.
If there are gaps in the resulting extended palette, the colors must be
filled with {0,0,0} and the alphas filled with 255. If alpha samples
are supplied (PPLT_delta_type > 1) and no tRNS
data is present in the parent object, a tRNS chunk must be
created in the child object as though a complete tRNS chunk
had appeared. The new palette must not be longer than the object's
sample_depth would allow.
PPLT_delta_type=0, the values are replacements for
the existing samples in the palette.
PPLT_delta_type=1, the values are added to the
existing samples (modulo 256) to obtain the new samples.
PPLT_delta_type.
6.1.8. JHDR JNG image header
width,
height, image_sample_depth,
image_color_type, image_filter_method, and
image_interlace_method fields are ignored. The values for these
parameters are inherited from the parent object.
alpha_compression_method,
alpha_interlace_method, and alpha_filter_method fields,
if different from those of the parent object, are used in decoding any
subsequent IDAT chunks, and the new values will be inherited by
any subsequent image that uses this object as its parent. If the
alpha_sample_depth differs, it will be used in decoding the
IDAT chunk data of the Delta-PNG and subsequent Delta-PNG
datastreams; but the child object itself will retain the original sample depth,
and must also retain the "alpha sample depth" for use in decoding
subsequent Delta-PNG datastreams. The decoded alpha samples must be
scaled to the object's sample depth before the replacements or delta
calculations are done.
6.1.9. IJNG Incomplete JNG
image_type=2 and the JNG datastream begins with a
JDAT or JDAA chunk. Note: Be sure that the first
JDAT or JDAA chunk
precedes the first IDAT chunk. In this case, no IJNG
chunk is required, either. The decoder must treat this datastream as
though the JHDR chunk were present, immediately preceding
the first JDAT chunk. If the first JNG chunk is not a
JDAT or JDAA chunk, then either the IJNG
or JHDR must be present to introduce the JNG datastream.
6.1.10. DROP Drop chunks
Chunk name: 4 bytes (ASCII text).
etc.
6.1.11. DBYK Drop chunks by keyword
Chunk name: 4 bytes (ASCII text).
Polarity: 1 byte (unsigned integer).
0: Only.
1: All-but.
Keywords (null-separated Latin-1 text strings).
6.1.12. ORDR Ordering restrictions
Chunk name: 4 bytes (ASCII text).
Order type: 1 byte.
0: Anywhere.
1: After IDAT and/or JDAT or JDAA.
2: Before IDAT and/or JDAT or JDAA.
3: Before IDAT but not before PLTE.
4: Before IDAT but not after PLTE.
etc.
6.2. Ancillary Delta-PNG chunks
6.2.1. gAMA, cHRM, iCCP, sRGB Color space chunks
6.2.2. oFFs and pHYs
6.2.3. Other ancillary PNG chunks
6.3. Chunk ordering requirements
7. Chunk Copying Rules
8. Minimum Requirements for MNG-Compliant Viewers
MHDR Profile bits Profile Level of support
31-10 9 8 7 6 5 4 3 2 1 0 value
0 0 0 0 1 0 0 0 0 0 1 65 MNG-VLC without transparency
0 0 1 1 1 0 0 1 0 0 1 457 MNG-VLC
0 0 1 1 1 0 1 1 0 0 1 473 MNG-VLC with JNG
0 0 1 1 1 0 0 1 0 1 1 459 MNG-LC
0 0 1 1 1 0 1 1 0 1 1 475 MNG-LC with JNG
0 0 1 1 1 0 1 1 1 1 1 479 MNG without stored objects
0 1 1 1 1 0 0 1 1 1 1 975 MNG without JNG or Delta-PNG
0 1 1 1 1 0 1 1 1 1 1 991 MNG without Delta-PNG
0 1 1 1 1 1 0 1 1 1 1 1007 MNG without JNG
0 1 1 1 1 1 1 1 1 1 1 1023 or 0 MNG
0 1 1 1 1 1 1 1 1 1 1 1023 or 0 MNG with 12-bit JNG support
| | | | | | | | | |
| | | | | | | | | +- Validity
| | | | | | | | +--- Simple MNG features
| | | | | | | +----- Complex MNG features
| | | | | | +------- Transparency
| | | | | +--------- JNG
| | | | +----------- Delta-PNG
| | | +------------- Validity of bits 7,8, and 9
| | +--------------- Semitransparency
| +----------------- Background transparency
+------------------- Stored objects
8.1. Required MNG chunk support
ticks_per_second must be supported by animation viewers.
The simplicity profile, frame count, layer count, and nominal play time
can be ignored. Decoders that provide less than minimal support can use
the simplicity profile to identify datastreams that they are incapable
of processing.
iteration_count must be supported. The
nest_level should be used as a sanity check but is not
required. When iteration_min=1 either explicitly or
when it is omitted and termination_condition != 0, the
LOOP chunk
and its ENDL chunk can be ignored (bit 2 of the simplicity profile
can be used to promise that this is true for all loops).
framing_mode and clipping parameters must be
supported. The interframe_delay must be supported
except by single-frame viewers. The sync_id and
timeout data can be ignored.
8.2. Required PNG chunk support
color_type, bit_depth,
compression_method, filter_method and
interlace_method must be supported (interlacing, as in PNG,
need not necessarily be displayed on-the-fly; the image can be displayed
after it is fully decoded). The alpha-channel must be supported, at
least to the degree that fully opaque pixels are opaque and fully
transparent ones are transparent. It is recommended that alpha be fully
supported. Alpha is not present (or can be ignored because it has no
effect on the appearance of any frame) if bit 3 of the simplicity profile is 0.
8.3. Required JNG chunk support
color_type, bit_depth,
compression_method, filter_method and
interlace_method must be supported (interlacing, as in PNG,
need not necessarily be displayed on-the-fly; the image can be displayed
after it is fully decoded). The alpha-channel must be supported, at
least to the degree that fully opaque pixels are opaque and fully
transparent ones are transparent. It is recommended that alpha be fully
supported.
image_sample_depth=8 must be supported. The JSEP
chunk must be recognized and must be used by minimal decoders to select
the eight-bit version of the image, when both eight-bit and twelve-bit versions
are present, as indicated by image_sample_depth=20 in the
JHDR chunk. When image_sample_depth=12, minimal
decoders are not obligated to display anything. Such decoders can
choose to display nothing or an empty rectangle of the width and height
specified in the JHDR chunk.
This can be done by processing the JNG as though a
viewable transparent BASI object had appeared:
BASI width height 1 4 0 0 0 0 00 00 00 00 1
IEND
8.4. Required Delta-PNG chunk support
9. Recommendations for Encoders
9.1. Use a common color space
9.2. Use the right framing mode
9.3. Immediate frame sync point
interframe_delay=0, timeout, and
sync_id, and the second establishing the synchronization
point:
FRAM 2 0 1 1 0 1 0000 timeout sync_id
FRAM 0 name
9.4. Embedded images in LOOPs
9.5. Including optional index in SAVE chunk
9.6. Interleaving JDAT, JDAA, and IDAT chunks
9.7. Use of the JDAA chunk
10. Recommendations for Decoders
10.1. Using the simplicity profile
10.2. ENDL without matching LOOP
nest_level should be terminated,
and processing can resume after the next SEEK chunk. Simple
viewers that do not process the SAVE chunk should abandon the
MNG datastream. See above.
10.3. Note on compositing
Ac = 1 - (1 - At)(1 - Ab)
if (Ac != 0) then
s = At / Ac
t = (1 - At) Ab / Ac
else
s = 0.0
t = 1.0
endif
Rc = s Rt + t Rb
Gc = s Gt + t Gb
Bc = s Bt + t Bb
Ac = 1
Rc = At Rt + (1 - At) Rb
Gc = At Gt + (1 - At) Gb
Bc = At Bt + (1 - At) Bb
Ac = 1 - (1 - At)(1 - Ab)
Rc = Rt + (1 - At) Rb
Gc = Gt + (1 - At) Gb
Bc = Bt + (1 - At) Bb
10.4. Retaining object data
object_id=0) for possible redisplay with the
SHOW chunk or for possible use as the parent object for a
subsequent Delta-PNG datastream.
target_x and
target_y, if the object was the destination of a
PAST chunk.
object_id=0 or by using the DISC or the
SEEK chunk. Abstract images rather than concrete objects
should be used if the encoder knows that the data will not later be used
as the parent object for a Delta-PNG.
10.5. Decoder handling of fatal errors
10.6. Decoder handling of interlaced images
10.7. Decoder handling of palettes
object_id field to depend on the
other.
10.8. Behavior of single-frame viewers
10.9. Clipping
frame_width and frame_height are defined in
the MHDR chunk and cannot be changed by any other MNG chunk.
frame_width
or frame_height exceeds the physical dimensions of the
display hardware, the contents of the area outside those dimensions is
undefined. If a viewer chooses, it can create "scroll bars" or the
like, to enable persons to pan and scroll to the offscreen portion
of the frame. If this is done, then the viewer is responsible for
maintaining and updating the offscreen portion of the frame.
frame_width
and frame_height are defined by the width and
height fields of the IHDR (or JHDR) chunk.
frame_width and frame_height)
will remain on the display from frame to frame without being explicitly
redisplayed.
See Example 8,
which displays a
large background image once, and then, in each frame, only redisplays
the portion of the background surrounding the moving sprite.
0 <= x < frame_width (from the MHDR chunk)
0 <= y < frame_height
Left_lcb <= x < right_lcb (from the FRAM chunk)
Top_lcb <= y < bottom_lcb
Left_cb <= x < right_cb (from the DEFI or CLIP chunk)
Top_cb <= y < bottom_cb
11. Recommendations for Editors
11.1. Editing datastreams with optional index
11.2. Handling LOOP and TERM chunks
iteration_min times. The loop created by the TERM
chunk should be executed only once.
12. Miscellaneous Topics
12.1. File name extension
.mng is recommended for
MNG (including MNG-LC and MNG-VLC)
files. Lowercase .mng is
preferred if file names are case-sensitive. The extension .jng is
recommended for JNG files.
12.2. Internet media type
video/mng
as the Internet Media Type for
MNG [RFC-2045],
[RFC-2048].
At the date of this document, the media
type registration process had not been started. It is recommended
that implementations also recognize the interim media type
video/x-mng.
image/x-jng can be used.
12.3. Uniform Resource Identifier (URI)
SRC=file.mng#segment_name
SRC=file.mng#subframe_name
SRC=file.mng#snapshot_name
SRC=file.mng?segment_name#segment_name
SRC=file.mng?snapshot_name#snapshot_name
SRC=file.mng#clock(10s-20s)
SRC=file.mng#clock(0:00-0:15)
SRC=file.mng?clock(0:00-0:15)#clock(0:00-0:15)
SRC=file.mng#frame(10)
SRC=file.mng#frames(30-60)
SRC=file.mng?frames(30-60)#frames(30-60)
13. Rationale
Interframe delay
DHDR delta types
Additional filter methods
FILT
method (1 byte)
64: intrapixel differencing
data (variable, depends on method)
method 64 requires no data
MAGN chunk rationale
Global JPEG tables
14. Revision History
14.1. Version 0.98c
14.2. Version 0.98b
14.3. Version 0.98a
14.4. Version 0.98
image_sample_depth != 20
14.5. Version 0.97
14.6. Version 0.96
JDAT_sample_depth" to
"image_sample_depth" and
"IDAT_sample_depth" to
"alpha_sample_depth", etc.
14.7. Version 0.95
15. References
ftp://ftp.isi.edu/in-notes/rfc2083.txt
also available at
ftp://swrinde.nde.swri.edu/pub/png/documents/.
This
specification has also been published as a W3C Recommendation, which is
available at
http://www.w3.org/TR/REC-png.html.
Randers-Pehrson, G., et. al., "PNG (Portable Network Graphics
Format) Version 1.2", which is available at
ftp://swrinde.nde.swri.edu/pub/png/documents/.
ftp://swrinde.nde.swri.edu/pub/png/documents/pngext-*.
ftp://ftp.isi.edu/in-notes/rfc2045.txt
ftp://ftp.isi.edu/in-notes/rfc2048.txt
ftp://ftp.isi.edu/in-notes/rfc2396.txt
16. Security Considerations
sync_id with a long
timeout value. Therefore a decoder should always
provide a simple method for users to escape out of a loop or delay,
either by abandoning the MNG entirely or just proceeding to the next
SEEK chunk. Decoders should check for user input after
each loop iteration (not just after each frame) in case of infinite loops
that are empty or that generate layers with zero interframe delay.
(The SEEK chunk makes it safe for a
viewer to resume processing after it encounters a corrupted portion of a
MNG datastream or jumps out of the interior of a segment for any reason.)
17. Appendix: Examples
17.1. Example 1: A single image
copy file.png file.mng
\211 P N G \r \n ^z \n # PNG signature.
IHDR 720 468 8 0 0 0 0 # Width and Height, etc.
sRGB 2
gAMA 45455
IDAT ...
IEND
17.2. Example 2: A very simple movie
\212 M N G \r \n ^z \n # MNG signature.
MHDR 256 300 # Width and height.
1 # 1 tick per second.
5 4 4 # Layers, frames, play time
7 # Simplicity profile
DEFI 1 0 0 IHDR ... IDAT ... IEND # Four PNG datastreams
DEFI 2 0 0 IHDR ... IDAT ... IEND # are read and stored
DEFI 3 0 0 IHDR ... IDAT ... IEND # and are displayed as
DEFI 4 0 0 IHDR ... IDAT ... IEND # they are read.
SAVE # This is needed so we can place TERM before SEEK.
TERM 3 0 120 10 # When done, repeat from TERM 10 times.
SEEK
SHOW
MEND
17.3. Example 3: A simple slideshow
\212 M N G \r \n ^z \n # MNG signature.
MHDR 720 468 1 # Width and height, 1 tick per second.
6 5 5 # Layers, frames, play time.
1 # Simplicity profile (MNG-VLC)
FRAM 1 0 2 2 0 2 1 600 0 # Set interframe_delay to 1,
# timeout to 600 sec, and sync_id list to {0}.
SAVE
SEEK "Briefing to the Workforce"
IHDR ... IDAT ... IEND # DEFI 0, visible, abstract
SEEK "Outline" # is implied.
IHDR ... IDAT ... IEND
SEEK "Our Vision" IHDR ... IDAT ... IEND
SEEK "Our Mission" IHDR ... IDAT ... IEND
SEEK "Downsizing Plans" IHDR ... IDAT ... IEND
MEND
17.4. Example 4: A more storage-efficient slideshow
\212 M N G \r \n ^z \n # MNG signature.
MHDR 720 468 # Width and height.
1 6 5 5 15 # 1 tick per second, complex, no JNG.
DEFI 1 1 1 # Define image 1, invisible, concrete.
IHDR ... IDAT ... IEND
FRAM 1 0 2 2 0 2 1 600 0 # set interframe_delay to 1,
# timeout to 600 sec and sync_id list to {0}.
SAVE
SEEK "Briefing to the Workforce"
CLON 1 2 DHDR 2 ... IDAT ... IEND SHOW 2
SEEK "Outline"
CLON 1 2 DHDR 2 ... IDAT ... IEND SHOW 2
SEEK "Our Vision"
CLON 1 2 DHDR 2 ... IDAT ... IEND SHOW 2
SEEK "Our Mission"
CLON 1 2 DHDR 2 ... IDAT ... IEND SHOW 2
SEEK "Downsizing Plans"
CLON 1 2 DHDR 2 ... IDAT ... IEND SHOW 2
MEND
17.5. Example 5: A simple movie
\212 M N G \r \n ^z \n # MNG signature.
MHDR 720 468 # Width and height.
30 6 5 15 # 30 ticks per second.
47 # Delta-PNG, transparent, complex
tEXtTitle\0Sample Movie
fPRI 0 128 # Default frame priority is "medium".
FRAM 1 0 2 0 0 0 3 # Set interframe_delay to 1/10 sec.
DEFI 1 0 1 # Set default image to 1 (concrete).
SAVE
SEEK "start"
IHDR 720 468 8 2 0 0 0 # DEFI 1 is implied.
IDAT ...
IEND
DHDR 1 1 1 20 30 100 220 # A PNG-delta frame.
IDAT ... # The IDAT gives the 20x30 block
IEND # of deltas.
DHDR 1 1 1 20 30 102 222 # Another PNG-delta frame.
IDAT ... # This time the deltas are in a 20 x 30
IEND # block at a slightly different location.
SEEK "frame 3" # OK to restart here because a
# complete PNG frame follows.
fPRI 0 255 # This is the representative frame that
IHDR 720 468 ...# will be displayed by single-frame
IDAT ... # viewers.
IEND
fPRI 0 128 # Return to medium frame priority.
DHDR 1 1 1 720 468 0 0 # Another PNG-delta frame.
IDAT ... # The entire 720x468 rectangle changes
IEND # this time.
SEEK "end"
MEND # End of MNG datastream.
17.6. Example 6: A single composite frame
\212 M N G \r \n ^z \n # MNG signature.
MHDR 1024 512 0 # Width, height, ticks per second
4 1 0 47 # Layers, frames, time, simplicity
BACK 16448 16448 52800 1 # Must use sky blue background.
PLTE ... # Define global PLTE
gAMA 50000 # Define global gAMA
DEFI 1 1 # Define invisible abstract thumbnail image.
IHDR 64 64 4 3 0 0 0 PLTE IDAT ... IEND # use global PLTE
eXPI 1 "thumbnail 1"
DEFI 1 1 # Also define a larger thumbnail.
IHDR 96 96 4 3 0 0 0 PLTE IDAT ... IEND # use global PLTE
eXPI 1 "thumbnail 2"
DISC # Discard the thumbnail image.
FRAM 4 "Two views of the data"
DEFI 1 0 1 6 6 # Define first (bottom) image.
IHDR 500 500 16 0 .. # A 16-bit graylevel image.
IDAT ...
IEND # End of image.
CLON 1 2 0 1 0 0 518 6 # Make full invisible concrete clone.
SHOW 2 2 3 # Mark it for immediate display during
# the upcoming delta-PNG operation.
DHDR 2 1 7 # Modify it (no change to pixels).
ORDR faLT 2 # Establish chunk placement.
gAMA 100000 # Local gamma value is 100000 (gamma=1.0).
tEXtComment\0The faLT chunk is described in ftp://swrinde...
faLT ... # Apply pseudocolor to parent image.
IEND # End of image.
DEFI 3 0 0 900 400 # Overlay near lower right-hand corner.
IHDR 101 101 2 3 ...
PLTE ... # Use a local PLTE and global gAMA.
tRNS ... # It is transparent (maybe a logo).
IDAT ... # Note that the color type can differ
IDAT ... # from that of the other images.
IEND # End of image.
MEND # End of MNG datastream.
17.7. Example 7: A movie with sprites
\212 M N G \r \n ^z \n # MNG signature.
MHDR 512 512 30 0 0 0 47 # Start of MNG datastream.
FRAM 2 "frame 1" 0 2 0 0 0 3 # First frame
# sets interframe_delay=3 ticks.
DEFI 1 # Define image 1 (abstract, LOCA 0 0).
IHDR 512 512 ... # It is a full-display PNG image.
etc # Chunks according to PNG spec.
IEND # SHOW 1 is implied by DEFI 1.
DEFI 2 0 1 300 200 # Define image 2, concrete.
IHDR 32 32 ... # It is a small PNG.
gAMA 50000
IDAT ...
IEND
FRAM 0 "frame 2" # Start new frame.
# New location for image 1 is still 0,0.
SHOW 1 # Display image 1 from previous frame.
MOVE 2 2 1 10 5 # New (delta) location for image 2.
SHOW 2 # Retrieve image 2 from previous frame,
CLON 2 3 0 1 0 # make a full clone of it as image 3.
0 400 500 # Location for image 3.
DHDR 3 1 7 0 0 0 0 # Modify image 3 (no change to pixels).
tRNS ... # Make it semitransparent.
IEND # SHOW 3 is implied by CLON visibility.
FRAM 0 "frame 3" # Next frame (repeat this FRAM-SHOW 1 3
# sequence with different locations to
# move the images around).
# New location for image 1 is still 0,0.
MOVE 2 2 1 10 5 # New (delta) location for image 2.
MOVE 3 3 1 5 -2 # New location for image 3.
SHOW 1 3 # Show images 1 through 3.
FRAM 0 "frame 4" # Another frame.
etc.
FRAM 0 "frame 99"
etc. # More frames.
MEND # End of MNG datastream.
17.8. Example 8: A movie with an animated sprite
\212 M N G \r \n ^z \n # MNG signature.
MHDR 512 512 30 0 0 0 15 # Start of MNG datastream.
FRAM 2 "frame 1" 0 2 0 0 0 3 # First frame.
DEFI 1 IHDR 512 512 ... # Background PNG image.
etc ... IEND # Chunks according to PNG spec.
DEFI 10 1 0 x0 y0 # Static part of sprite.
IHDR 64 64 ... IDAT ... IEND
DEFI 11 1 0 x0 y1 # View 1 of animated part.
IHDR 64 32 ... IDAT ... IEND # (y1=y0+64)
DEFI 12 1 0 x0 y1 # View 2 of animated part.
IHDR 64 32 ... IDAT ... IEND
DEFI 13 1 0 x0 y1 # View 3 of animated part.
IHDR 64 32 ... IDAT ... IEND
FRAM 0 0 0 0 2 0 0 x0-dx x0+64+dx y0-dy y1+32+dy
LOOP 0 0 10
LOOP 1 0 150
FRAM 0 "left-to-right" 0 0 2 0 1 dx dx dy dy
MOVE 10 13 1 dx dy # Move animated icon {dx, dy}.
SHOW 1 SHOW 10 # Show background and static part.
SHOW 11 13 6 # Select the next view of the
ENDL 1 # animated part and show it.
FRAM SHOW 1
PAST 10 0 0 0 10 1 4 0 0 0 0 0 64 64
PAST 11 0 0 0 11 1 4 0 0 0 0 0 64 32
PAST 12 0 0 0 12 1 4 0 0 0 0 0 64 32
PAST 13 0 0 0 13 1 4 0 0 0 0 0 64 32
LOOP 1 0 150
FRAM 0 "right-to-left" 0 0 2 0 1 -dx -dx -dy -dy
MOVE 10 13 1 -dx -dy # Move animated icon {-dx, -dy}.
SHOW 1 SHOW 10 # Show background and static part.
SHOW 11 13 6 # Select the next view of the
ENDL 1 # animated part and show it.
ENDL 0 FRAM
MEND
17.9. Example 9: "Fading in" a transparent image
\212 M N G \r \n ^z \n # MNG signature.
MHDR 64 64 30 0 0 0 47 # Width, height, ticklength, ....
BACK 52800 52800 52800 # "Browser gray" default background.
FRAM 3 0 2 0 0 0 3 # Set interframe_delay=3 ticks. Use
# framing mode 3 so background gets restored.
DEFI 1 1 1 # Invisible and "concrete".
IHDR ... # PNG header.
PLTE ...
tRNS 0 # Entries are zero for the transparent (0)
# color and 255 for the nontransparent ones.
IDAT ...
IEND
fPRI 0 0 # Give the fade-in sequence a low priority.
CLON 1 2 # Make a working concrete copy of the image
# that will be modified during the low-priority
# part of the datastream. It is a full clone.
DHDR 2 1 7 # No change to pixel data.
tRNS 0 0 0 0 0 0 ... # Make all pixels fully transparent.
IEND
SHOW 2 2 3 # Make it visible but don't show it now.
LOOP 0 0 15
DHDR 2 1 7 # A Delta-PNG.
# Delta-type 7 means no change to pixels.
PPLT 1 10 3 16 16 16 16 ... # Increment all alphas except
IEND # for entry 0 by 16.
SHOW 2
ENDL 0 # Nontransparent pixel alpha=15, 31, ... 240.
DISC 2 # Discard the working copy.
fPRI 0 255 # Give the final frame the highest value
FRAM 0 0 1 0 0 0 60 # Hold the last frame for at least
# 60 ticks (2 sec). Applications might show it longer.
SHOW 1 # This copy still has alpha=255 for the
# opaque pixels and alpha=0 for the others.
MEND # End of MNG.
17.10. Example 10: Storing three-dimensional images
\212 M N G \r \n ^z \n # MNG signature.
MHDR 150 150 1 # Width, height, ticklength.
0 0 0 47 # Layers, frames, time, simplicity.
tEXtTitle\0Weather modeling results
tEXtComment\0The xxSC, yySC, zzSC, and ttSC chunks
in this file are written according to the Proposed
chunk specifications version 19970203 and Sci-Vis
chunks specification version 19970203 available at
ftp://swrinde.nde.swri.edu/pub/png-group/documents/
xxSC kch\0 [sig\0] kilometers\0 0\0 150
yySC kch\0 [sig\0] kilometers\0 0\0 150
zzSC kch\0 [sig\0] Height (kilometers)\0 0\0 15
ttSC kch\0 [sig\0] Time (hours)\0 0\0 24
pCAL kch\0 0 255 0 2 Degrees Celsius\0 0\0 45
DEFI 1 0 1 # All images will have image = 1
SAVE # and be visible and "concrete".
SEEK
FRAM 2 # Initial composite image.
IHDR 150 150 16 # Width, height, bit depth for top layer.
0 0 0 0 # Color, comp, filter, interlace.
IDAT ...
IEND # No DEFI chunk, so it is image 0.
DHDR 1 1 0 # Source=0, PNG, pixel addition,
150 150 0 0 # Block is entire image.
IDAT ... # IHDR is omitted; everything matches top.
IEND # IEND is also omitted.
etc. # Repeat DHDR through IEND 148 more times.
SEEK
FRAM # End of first block.
etc. # Repeat FRAM through SEEK 19 more times.
SEEK
MEND # End of MNG.
17.11. Example 11: Tiling
\212 M N G \r \n ^z \n # MNG signature.
MHDR 1024 768 0 # Start of MNG datastream.
98 1 0 15 # Layers, frames, time, simplicity.
FRAM 2
DEFI 1 1 0 0 -64 # Set up an offscreen "abstract" copy
IHDR 128 64 ... PLTE ... IDAT ... IEND # of the tile.
LOOP 0 0 12 # Y loop -- make 12 rows of tiles.
MOVE 1 1 1 0 64 # Move the first copy down 64 rows.
SHOW 1 # Display it.
CLON 1 2 1 # Create a partial clone of the tile.
LOOP 1 0 7 # X loop - 7 additional columns.
MOVE 2 2 1 0 128 # Move it to the right 128 columns.
SHOW 2 # Use the second copy.
ENDL 1
ENDL 0
MEND
\212 M N G \r \n ^z \n # MNG signature.
MHDR 1024 768 0 # Start of MNG datastream.
3 1 0 15 # Layers, frames, time, simplicity.
DEFI 1 1 # Set up an offscreen "abstract" copy
IHDR 128 64 ... PLTE ... IDAT ... IEND # of the tile.
DEFI 2 # The abstract, visible, viewable image to
BASI 1024 768 8 2 0 0 0 0 0 0 0 1 # be tiled. Initially
IEND # all pixels are zero.
PAST 2 0 0 0 # Destination and target location.
# src mod orient offset clipping
1 0 8 0 0 512 0 0 1024 0 768
# End of PAST chunk data.
MEND
17.12. Example 12: Scrolling
\212 M N G \r \n ^z \n # MNG signature.
MHDR 512 256 30 # Width, height, ticks per second
6513 3257 3257 15 # Layers, frames, time, simplicity.
BACK 50000 50000 50000 0 # advisory gray background
DEFI 1 1 0 0 256 # Define image 1 but don't display now.
# Initially it is offscreen, just
# below the 512 by 256 window.
IHDR 512 3000 1 0 ... # A PNG datastream containing the
PLTE ... # text (or whatever) to be scrolled.
IDAT ...
IEND
DEFI 2
IHDR 512 256 8 6 ... # A PNG datastream containing some kind
PLTE ... # of alpha-blended border that is
tRNS ... # transparent in the center.
IDAT ...
IEND
LOOP 0 0 3256
MOVE 1 1 1 0 -1 # Jack image 1 up one scanline, 3256 times.
# It ends up just above the 512 by 256 window.
# The border does not move.
FRAM 1 0 2 0 0 0 0 # Interframe_delay = 0 ticks.
# We use Framing_mode=1 to avoid unnecessary
# screen clearing between frames.
SHOW 1 # Show first image and continue without delay.
FRAM 1 0 2 0 0 0 1 # Interframe_delay = 1 tick.
SHOW 2 # Composite second image over first, wait 1 tick.
ENDL 0
MEND
(Same as above down to the LOOP chunk.)
BACK 50000 50000 50000 2 1 # Advisory gray background.
# Mandatory image background.
FRAM 3 0 2 0 0 0 1 # Interframe_delay = 1 tick.
LOOP 0 0 3256
MOVE 1 1 1 0 -1 # Jack background up one scanline, 3256 times.
SHOW 2 # Composite the second image over it, wait 1 tick.
ENDL 0
MEND
17.13. Example 13: Cycling animations
show_mode=6 to create animations that cycle through
a series of ten objects.
\212 M N G \r \n ^z \n # MNG signature.
MHDR 400 88 30 # Width, height, ticks per second
11 1001 1001 7 # Layers, frames, time, simplicity.
DEFI 1 ...
etc. # Define 10 objects.
DEFI 10 ...
LOOP 0 100 6 # 100 iterations, user-discretion, cacheable
SHOW 1 10 6
ENDL 0
MEND
\212 M N G \r \n ^z \n # MNG signature.
MHDR 400 88 6 # Width, height, ticks per second
11 901 901 7 # Layers, frames, time, simplicity.
DEFI 1 ...
etc. # Define 10 objects.
DEFI 10 ...
CLON 11 9 1 # Make partial clones of objects 2-9
etc. # in reverse order, as objects 11-18.
CLON 18 2 1
LOOP 0 50 6 # 50 iterations, user-discretion, cacheable
SHOW 1 18 6
ENDL 0
MEND
17.14. Example 14: Converting a GIF animation
begin
write "MHDR" chunk
saved_images := 0; Interframe_delay := 0
First_frame := TRUE
if(loops>1) "write TERM 3 0 0 loops" chunk
write "BACK" chunk
for subimage in gif89a file do
if(interframe_delay != gif_duration) then
interframe_delay := gif_duration
write "FRAM 4 0 2 2 0 2 0 interframe_delay 0" chunk
First_frame := FALSE
else if(First_frame == TRUE)then
write "FRAM 4" chunk
First_frame := FALSE
else
write "FRAM" chunk
endif
if(X_loc == 0 AND Y_loc == 0) then
write "DEFI saved_images 1 1" chunk
else
write "DEFI saved_images 1 1 X_loc Y_loc" chunk
write "<image>"
write "SHOW 0 saved_images" chunk
if (gif_disposal_method == 0
OR gif_disposal_method == 2) then
/* (undefined or restore background) */
write "DISC" chunk
saved_images := 0
else if (gif_disposal_method == 1) then
/* (keep) */
saved_images := saved_images + 1
else if (gif_disposal_method == 3) then
/* (restore previous) */
write "DISC saved_images" chunk
endif
endfor
write "FRAM" and "MEND" chunks
end
17.15. Example 15: Converting a simple GIF animation
begin
write "MHDR" chunk
Interframe_delay := 0; Previous_mode := 1
Framing_mode := 1
if(loops>1) "write TERM 3 0 0 loops"
write "mandatory BACK" chunk
for subimage in gif89a file do
if(interframe_delay != gif_duration) then
interframe_delay := gif_duration
write "FRAM 0 0 2 2 0 2 0 interframe_delay 0"
endif
if(X_loc != 0 OR Y_loc != 0) then
write "DEFI 0 0 0 X_loc Y_loc" chunk
endif
write "<image>"
if (gif_disposal_method < 1) then
/* (none or keep) */
Framing_mode := 1
else if (gif_disposal_method == 2) then
/* (restore background) */
write "FRAM 4 0 1 0 1 0 0 L R T B"
Previous_mode := 4; Framing_mode := 1
else if (gif_disposal_method == 3) then
/* (restore previous) */
error ("can't do gif_disposal method = previous.")
endif
if(Framing_mode != Previous_mode) then
write "FRAM Framing_mode" chunk
Previous_mode := Framing_mode
endif
end
write "MEND" chunk
end
17.16. Example 16: Counting layers and frames
MHDR sRGB Fn F I I I F F I I I F F I I I MEND
Fn represents a FRAM chunk with framing_mode n
F represents an empty FRAM chunk;
I represents an embedded image
Framing Layer count Frame count
mode
1 B,I,I,I, I,I,I, I,I,I = 10 BI,I,I, I,I,I, I,I,I = 9
2 B,I,I,I, I,I,I, I,I,I = 10 BIII,III,III = 3
3 3*(B, B,I, B,I, B,I) = 21 3*(B,BI,BI,BI) = 12
4 3*(B,B,I,I,I) = 15 B,BIII,B,BIII,B,BIII = 6
17.17. Example 17: Storing an icon library
MHDR 96 96 1 6 5 5 11 # Profile 11 is MNG-LC
sRGB 2 # Global sRGB
PLTE ... # Global PLTE
tRNS 0 # Global tRNS
eXPI 0 "thumbnail"
IHDR 32 32 ... PLTE IDAT ... IEND
eXPI 0 "left arrow"
IHDR 96 96 ... PLTE IDAT ... IEND
eXPI 0 "right arrow"
IHDR 96 96 ... PLTE IDAT ... IEND
eXPI 0 "up arrow"
IHDR 96 96 ... PLTE IDAT ... IEND
eXPI 0 "down arrow"
IHDR 96 96 ... PLTE IDAT ... IEND
MEND
MHDR 96 96 1 6 5 5 47 # Profile 47 is MNG without JNG
sRGB 2 # Global sRGB
PLTE ... # Global PLTE
tRNS 0 # Global tRNS
eXPI 0 "thumbnail"
IHDR 32 32 ... PLTE IDAT ... IEND
SAVE
<index>
SEEK "left arrows"
DEFI 1
IHDR 96 96 ... PLTE IDAT ... IEND
eXPI 1 "red left arrow"
DHDR 1 1 7 PPLT ... IEND # Change some palette entries.
eXPI 1 "blue left arrow"
SEEK "right arrows"
IHDR 96 96 ... PLTE IDAT ... IEND
eXPI 1 "red right arrow"
DHDR 1 1 7 PPLT ... IEND
eXPI 1 "blue right arrow"
MEND
17.18. Example 18: MAGN methods
1 9 1
9 17 9
1 1 1 1 1 9 9 9 9 9 1 1 1 1 1
1 1 1 1 1 9 9 9 9 9 1 1 1 1 1
1 1 1 1 1 9 9 9 9 9 1 1 1 1 1
9 9 9 9 9 17 17 17 17 17 9 9 9 9 9
9 9 9 9 9 17 17 17 17 17 9 9 9 9 9
9 9 9 9 9 17 17 17 17 17 9 9 9 9 9
Magnification method 2, XM = 8, YM = 4.
Fills the X intervals with 7 new pixels
and the Y interval with 3 new pixels
and interpolates to get pixel values;
new size is 17x5:
1 2 3 4 5 6 7 8 9 8 7 6 5 4 3 2 1
3 4 5 6 7 8 9 10 11 10 9 8 7 6 5 4 3
5 6 7 8 9 10 11 12 13 12 11 10 9 8 7 6 5
7 8 9 10 11 12 13 14 15 14 13 12 11 10 9 8 7
9 10 11 12 13 14 15 16 17 16 15 14 13 12 11 10 9
1 1 1 1 1 9 9 9 9 9 9 9 9 1 1 1 1
1 1 1 1 1 9 9 9 9 9 9 9 9 1 1 1 1
1 1 1 1 1 9 9 9 9 9 9 9 9 1 1 1 1
9 9 9 9 9 17 17 17 17 17 17 17 17 9 9 9 9
9 9 9 9 9 17 17 17 17 17 17 17 17 9 9 9 9
17.19. Example 19: MAGN chunks and ROI
MHDR 600 600 0 5 1 0 19
# Gradient background
MAGN 00 00 2 599
sRGB 1
IHDR IDAT IEND <dblue2x2.png> # 93 bytes
# Drop shadow
DEFI 0 0 0 52 52
BASI 512 512 1 4 0 0 0 51 51 51 153 1
IEND # Grey-Alpha object, 46 bytes
# Main image, with most of the region of interest
# replaced with a solid rectangle, and reduced to
# 128x128 dimensions, low quality JPEG compression.
DEFI 0 0 0 36 36
MAGN 00 00 2 04 04 06 05 06 05
JHDR 128 128 10 8 8 0 0 0 0 0
JDAT <lena_q25_fourth.jpg> # 2514 bytes
IEND
# Region of interest, full scale, cropped to
# dimensions 200x313 at location 192,200,
# high quality JPEG compression.
MAGN # Turn off magnification of future object 0
DEFI 0 0 0 228 236
JHDR 200 312 10 8 8 0 0 0 0 0
JDAT <lena_face_q65.jpg> # 8001 bytes
IEND
MEND
18. Credits
Editor
Contributors
Trademarks
Document source
Copyright Notice
End of MNG Specification.