It is a proposal, and the format is subject to change.
Comments on this document can be sent to the PNG 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
<URL:ftp://swrinde.nde.swri.edu/pub/mng/documents/>.
The MNG format provides a mechanism for reusing image data without having to retransmit it. Multiple images can be composed into a "frame," and an image can be used as a "sprite" that moves from one location to another in subsequent frames.
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 PND datastream.
A PND datastream defines an image in terms of a parent PNG or PND 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.
The MNG format uses the same chunk structure that is defined in the PNG specification, and shares other features of the PNG format. Any valid PNG datastream is also a valid MNG datastream.
This document includes examples that demonstrate various capabilities of MNG including simple movies, composite frames, loops, fades, tiling, scrolling, storage of voxel data, converting GIF animations to MNG format.
Note: This [proposed] specification depends on the PNG Portable Network Graphics specification [PNG]. The PNG specification is available at the PNG home page,
http://www.wco.com/~png/A MNG datastream describes a sequence of single frames, each of which can be composed of one or more visible images.
An image is the result of decoding
A typical MNG datastream consists of
Images can be "concrete" or "abstract". The distinction allows decoders to use more efficient manipulation of 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 first eight bytes of a MNG datastream are
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 1-4.
MNG does not yet accommodate sound or complex sequencing information,
but these capabilities might be added
at a later date, in a backwards-compatible manner. These issues are being
discussed in the mpng-list@dworkin.wustl.edu mailing list.
At some future date, support
for a lossy image format such as the proposed PNP (Portable Network Photo)
format might be added. PNP is under discussion
by pnp-list@dworkin.wustl.edu.
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 in Chapter 5, below. 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 image is an independent entity, i.e., no chunk is ever enclosed in the data segment of another chunk.
An independent PNG datastream, with a PNG signature, is also a valid MNG datastream that must be recognized and decoded by MNG-compliant decoders. This kind of MNG datastream will contain only a single embedded image.
Because the embedded images making up a MNG are in PNG format, MNG shares the good features of PNG:
4 bytes: max_frame_width (unsigned integer).
Maximum width of any frame to be
displayed.
4 bytes: max_frame_height (unsigned integer).
Maximum width of any frame to be
displayed.
4 bytes: ticks_per_second (unsigned integer).
This is the unit used by the FRAM chunk to
specify frame_duration and sync_timeout.
The "max_frame_width" and "max_frame_height" fields
give the intended display size (measured in pixels) and provide default
clipping boundaries (see Paragraph 3.2.9, below). These can
be set to zero if the MNG datastream contains no visible images.
The "ticks_per_second" field must be nonzero if the datastream contains more than one frame. It should be set to zero if the datastream contains exactly one frame, so simple applications that only examine file headers can use this field to identify single-frame MNGs.
The "SAVE" chunk marks a point in the datastream at which images and other chunk information are "saved"; decoders encountering a "SEEK" must restore this information if it has redefined or discarded it.
The "SAVE" chunk is empty.
It appears after the set of chunks that define information that must be retained for the remainder of the datastream. The "SAVE" chunk must be present when the "SEEK" chunk is present. These can be chunks that define images, or they can be chunks that define other information such as "gAMA", "cHRM", and "sPLT".
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 image that was defined ahead of the "SAVE" chunk.
An image 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 image buffer that the original image 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 the "SEEK" chunk is encountered. If no instance of the chunk appeared ahead of the "SEEK" chunk, the decoder must restore the chunk data to its original "unknown" condition when it encounters a "SEEK" chunk. Known chunks in this category include "DEFI", "FRAM", "BACK", "cHRM", "eXPI", "fPRI", "gAMA", "pHYs", "sRGB", and "tERm".
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 encounters a "SEEK" chunk, but it must retain those instances that were defined prior to the "SAVE" chunk. Known chunks in this category include only "sPLT".
Only one instance of the "SAVE" chunk is permitted in a MNG datastream, and, 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 "SEEK" can be empty, or it can contain a keyword.
n bytes: keyword (Latin-1 string).The keyword is optional. It need not be terminated by a null byte; if it is, the null byte will be ignored. The keyword 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. Use caution when printing or displaying keywords (Refer to Security considerations, Chapter 11, below). No specific use for the keyword is specified in this document, but applications can use the keyword for such purposes as constructing a menu of SEEK points for a slide-show viewer.
Applications must not use any information preceding the "SEEK" chunk, except for
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 PND datastream, or it might need data from preceding "MOVE" or "CLIP" chunks.
When the "SEEK" chunk is encountered, a decoder must restore the information that it saved when it processed the "SAVE" chunk.
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.
1 byte: start_loop_level (unsigned integer).
1 byte: must_run_loop (unsigned integer).
0: The decoder must run the loop exactly
repeat_count times, because execution of
the loop modifies the properties of
images that might be needed later.
1: It is safe for a decoder to run the loop
fewer than repeat_count times, provided
that it runs the loop at least once. Every
iteration of the loop generates an
identical sequence of frames.
4 bytes: repeat_count (unsigned integer), range
[0..2^31-1].
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 images for any PND datastreams in the loop are the
images in existence prior to entering the "LOOP" chunk, but in
subsequent iterations these parent images might have been modified.
The "must_run_loop" field can be used to inform decoders that
it is safe to reduce the number of loop iterations.
When the "LOOP" chunk is present, an "ENDL" chunk with the same "loop_level" must be present later in the MNG datastream. Loops can be nested. Each inner loop must have a higher value of "start_loop_level" than the loop that encloses it.
If "repeat_count" is zero, the loop is done zero times. Upon encountering a "LOOP" chunk with "repeat_count=0", decoders simply skip chunks until the matching "ENDL" chunk is found, and resume processing with the chunk immediately following it.
It is the responsibility of the encoder to make sure that the assertions made by the "must_run_loop" field are true. When "must_run_loop=1", all iterations of the loop are identical, and a viewer could choose to actually execute the loop just once, while storing copies of the composited frames for redisplay, and then replaying the stored frames for the remaining "repeat_count" iterations. Also, when "must_run_loop=1", it is safe to escape from the loop upon reaching the "ENDL" chunk even though the loop has not yet been executed for "repeat_count" iterations. It is unsafe, however, to escape from the interior of a loop while it is being executed; in this event, the decoder must proceed to the next "SEEK" chunk before resuming.
The "ENDL" chunk ends a loop that begins with the "LOOP" chunk. It contains a single one-byte field:
1 byte: end_loop_level (unsigned integer),
range [0..255].
When the "ENDL" chunk is encountered,
the loop "repeat_count" is decremented. 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 "loop_level" should be present earlier in the MNG datastream. If the matching "LOOP" chunk is missing, or if it has been skipped for some reason, the "ENDL" chunk must be ignored.
The "SEEK" chunk is permitted inside a "LOOP-ENDL" pair, but only when "must_run_loop=1".
An empty "FRAM" chunk is just a frame delimiter. A nonempty one is a frame delimiter, and it also changes "FRAM" parameters, either for the upcoming frame or until reset. When the "FRAM" chunk is not empty, it contains a "framing_mode" byte, an optional name string, plus four 1-byte fields plus a variable number of optional fields.
1 byte: framing_mode.
0: Don't change framing_mode.
1: Each image is an individual frame.
2: The group of images appearing prior to
the next "FRAM" chunk form a composite
frame. All images are initially invisible
and are made visible with explicit or
implied "SHOW" chunks. The level, or
stacking order, of each image is given by
its order of appearance in the datastream.
3: The group of "n" images appearing prior to
the next "FRAM" chunk define "n" frames as
they accumulate, and a frame boundary
occurs after each image is defined or
appears in a "SHOW" chunk. All images are
initially made invisible but remain
visible after being explicitly made
visible. The level, or stacking order,
of each image is given by its order of
appearance in the datastream.
n bytes: frame_name (Latin-1 Text). Can be
omitted if all other fields are also omitted.
1 byte: zero-byte separator. Can be omitted
if all remaining fields are also omitted.
1 byte: change_frame_duration.
0: no.
1: yes, for the next frame only.
2: yes, also reset default.
This field and the next three can be omitted
as a group if no frame parameters other than
the framing_mode are changed.
1 byte: change_sync_timeout.
0: no.
1: yes, for the next frame only.
2: yes, also reset default.
1 byte: change_frame_boundaries.
0: no.
1: yes, for the next frame only.
2: yes, also reset default.
1 byte: change_sync_id_list.
0: no.
1: yes, for this frame only.
2: yes, also reset default list.
1 byte: fb_delta_type (unsigned integer).
(omit this and the following four fields
if change_frame_boundaries=0).
0: frame_boundary values are given directly.
1: frame_boundaries are determined by adding
the FRAM data to their previous values.
4 bytes: left_fb or delta_left_fb (signed integer).
4 bytes: right_fb or delta_right_fb (signed integer).
4 bytes: top_fb or delta_top_fb (signed integer).
4 bytes: bottom_fb or delta_bottom_fb (signed
integer).
4 bytes: frame_duration (unsigned integer) (omit
if change_frame_duration=0). The range
is [0..2^31-1] ticks.
4 bytes: sync_timeout (unsigned integer) (omit if
change_sync_timeout=0). The range is
[0..2^31-1]. The value 2^31-1 (0x7fffffff)
ticks represents an infinite timeout period.
4 bytes: sync_id (unsigned integer) (omit if
change_sync_id_list=0 or if the new list is
empty; repeat until all sync_id's have been
listed). The range is [0..2^31-1].
When the "FRAM" parameters are changed, the new parameters
affect the frame that is about to be defined, not the one that is
being terminated by the "FRAM" chunk.
The "frame_name" must conform to the same formatting rules as those for a "SEEK" keyword. No specific use for the "frame_name" is specified in this document, but applications can use this field for such purposes as constructing an external list of frames in the datastream. The "frame_name" only applies to the upcoming frame; subsequent frames are unnamed unless they also have their own "frame_name" field.
When the "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.
The initial values of the "FRAM" parameters are
framing_mode := 1 frame_name := <empty string> frame_duration := 0 left_fb := 0 right_fb := max_frame_width top_fb := 0 bottom_fb := max_frame_height sync_timeout := 0x7fffffff (infinite) sync_id := <empty list>The "MOVE" chunk can be used to specify the placement of each image within the frame. The "CLIP" chunk can be used to specify clipping boundaries for each image. If the images are transparent or do not cover the entire frame, as defined by the "max_frame_width" and "max_frame_height" fields of the "MHDR" chunk, 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 is not recognized by the decoder.
When "framing_mode=1", each image that becomes visible is a separate frame. "FRAM" chunks need not appear to separate them.
The following events trigger the display of a frame:
For example, the sequence
FRAM 1 SHOW 1 5will result in the display of five frames, each containing one of the images against the background according to its location and "CLIP" information.
The "frame_duration" value is the desired minimum time to elapse from the beginning of displaying one image until the beginning of displaying the next image.
Viewers are expected to display all of the images at once, if possible, or as fast as can be managed, without clearing the display and restoring the background between images. For example, the sequence
FRAM 2 SHOW 1 5 (shows images 1, 2, 3, 4, and 5) FRAMwill result in a single frame containing five images, each displayed according to its location and "CLIP" data.
When images in a frame overlap, viewers are expected to composite the later images against the partially completed frame that includes all earlier images.
When "framing_mode=3", a frame boundary occurs after each image is made visible, without clearing the display and restoring the background between frames until the next "FRAM" chunk is encountered.
When multiple images are made visible with a single "SHOW" chunk, a frame boundary occurs after each one. For example, the sequence
FRAM 3 SHOW 1 5 (shows images 1, 2, 3, 4, and 5) FRAMwould result in five frames being displayed, the first with only image 1, the second with images 1 and 2, etc., and the fifth frame with all five images visible. The resulting output is exactly equivalent to what would have been accomplished (but perhaps less efficiently) with
FRAM 2 SHOW 1 FRAM SHOW 1 2 (shows images 1 and 2) FRAM SHOW 1 3 (shows images 1, 2, and 3) FRAM SHOW 1 4 (shows images 1, 2, 3, and 4) FRAM SHOW 1 5 (shows images 1, 2, 3, 4, and 5) FRAMIf "MOVE" or "CLIP" chunks appear, they must appear before the images to which they pertain are made visible. Decoders can simply erase the display prior to displaying the first image, and then show each image as it is defined, without erasing the display between frames.
The "framing_mode" also affects the way decoders handle the "pHYs" chunk (see Paragraph 3.3.6, below).
The "frame_duration" field gives the duration of display, which is the minimum time that must elapse from the beginning of displaying one frame until the beginning of displaying the next (or between images, when "framing_mode=1"). It is measured in "ticks" using the tick length determined from "ticks_per_second" defined in the "MHDR" chunk.
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 "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 "frame_duration" and the "sync_timeout" fields elapses, processing can resume even though the processor has not received an indication that other processes have reached the synchronization point.
Note that the synchronization point does not occur immediately, but at the end of the frame that follows the "FRAM" chunk. 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 "frame_duration=0", "sync_timeout", and "sync_id", and the second establishing the synchronization point:
FRAM 0 0 0 1 1 0 1 0000 sync_timeout sync_id FRAM 0 "name"
The identifier "sync_id=0" is reserved to represent synchronization with a user input from a keyboard or pointing device. If multiple channels (not defined in this specification) are not present, viewers can ignore other values appearing in the "sync_id" list.
2 bytes: first_image (nonzero unsigned integer).
2 bytes: last_image (nonzero unsigned integer).
This field can be omitted if the visibility
byte is also omitted. If so, decoders must
assume the default value,
"last_image=first_image."
1 byte: show_mode (unsigned integer)
0: Make the images visible and display
them (set "visibility=0").
1: Make the images invisible (set
"visibility=1").
2: Don't change visibility; display those
that are visible.
3: Mark images "visible" but do not
display them.
4: Toggle visibility; display any that
are visible after toggling.
5: Toggle "visibility" but do not display
even if visible after toggling.
6: Make the next image in the cycle visible
and display it. Make all other images
in the cycle invisible.
7. Make the "next" image in the cycle
visible but do not display it. Make
all other images in the cycle invisible.
This field can be omitted. If so, decoders
must assume the default, "show_mode=0".
The decoder processes the images named in the "SHOW" chunk
in the order "first_image" through "last_image", and
resets the "visibility" flag for each of the images, and,
if directed, also displays the image if it is visible.
When the "SHOW" chunk is empty, the decoder displays all existing visible images, without changing their "visibility" status.
If "first_image > last_image" then the images are processed in reverse order. When "framing_mode=1" or when the "FRAM" chunk is not present, each image is displayed as a separate frame. When "show_mode=1, 3, 5, or 7", when "show_mode=2" and no image is visible, or when "show_mode=4" and an image becomes invisible, no frame is generated. Also, when the "SHOW" chunk is empty and there are no existing visible frames, no frame is generated.
When "show_mode=1, 4, 5, 6, or 7", images can be made invisible. This is not permitted when "framing_mode=2" or "framing_mode=3" in the "FRAM" chunk and the images have already been made visible 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.
When "show_mode=6 or 7", the decoder must examine the "visibility" of each image in the range "first_image" through "last_image", and make the next one after the first visible one it finds visible and the rest invisible. If the first visible one was "last_id", or none were visible, it must make "first_image" visible. When "first_image > last_image", the cycle is reversed. These modes can be useful in manipulating moving animated objects, where a group of sequential images represent different views of the object.
When "show_mode=0, 2, 4, or 6", an instance of each visible image will be displayed at the location specified by the "DEFI", "CLON", or "MOVE" chunk and clipped according to the boundaries specified by the "CLIP" chunk. 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.
Either of the following sequences would cause the image identified by "image_id=4" in a composite frame to blink:
LOOP 0 0 10 FRAM 2 SHOW 1 3 # show images 1 thru 5 SHOW 6 6 4 # toggle visibility of image 6 and show it SHOW 7 10 # show images 7 thru 10 FRAM ENDL LOOP 0 0 10 FRAM 2 SHOW 6 6 5 # toggle visibility of image 6 SHOW 1 10 2 # show visible images in 1 thru 10 FRAM ENDLIt is not necessary to follow an "IHDR-IEND", "BASI-IEND", or "DHDR-DEND" sequence or "PAST" chunk with a "SHOW" chunk to display the resulting image, if it was already made visible by the "DEFI" chunk that introduced the image. Similarly, the "CLON" chunk need not be followed by a "SHOW" chunk, if "visibility=0" in the "CLON" chunk.
It is not an error for the "SHOW" chunk to name an "image" that has not previously been defined. In such cases, nothing is done to the nonexistent image.
2 bytes: image (unsigned integer) image_id
to be given to the images that follow the
DEFI chunk.
1 byte: visibility (unsigned integer)
0: Make the images visible.
1: Make the images invisible.
This field can be omitted if the abstract_flag,
location, and clipping boundary fields
are also omitted. When it is omitted, the image
is made visible (visibility=0).
1 byte: abstract_flag (unsigned integer)
0: Make the images "abstract" (image can not
be the source for a PND)
1: Make the images "concrete" (image can be
modified by a PND).
This field can be omitted if location, and
clipping boundary fields are also omitted.
When it is omitted, the image is made "abstract"
(abstract_flag=0).
4 bytes: x_location (signed integer).
The x_location and y_location fields can be
omitted if the clipping boundaries are also
omitted. If so, decoders must assume default
values {x_location,y_location}={0,0}.
4 bytes: y_location (signed integer).
4 bytes: left_cb (signed integer). Left clipping
boundary. The left_cb, right_cb, top_cb, and
bottom_cb fields can be omitted as a group.
If so, decoders must assume default values
{0, max_frame_width, 0, max_frame_height}.
4 bytes: right_cb (signed integer).
4 bytes: top_cb (signed integer).
4 bytes: bottom_cb (signed integer).
If the image number for an image is nonzero, subsequent "DHDR",
"SHOW", "CLON", "MOVE", "CLIP", and
"DISC" chunks can use this number to identify it.
When the image number for an image is zero, it can be discarded immediately after it has been processed, and it can be treated as an "abstract" image, regardless of the contents of the "abstract_flag" field.
Negative values are permitted for the X and Y image location and clipping boundaries. The positive directions are downward and rightward from the upper left corner of the display.
The "DEFI" chunk values remain in effect until another "DEFI" chunk or a "SEEK" chunk appears. If no "DEFI" chunk is in effect (either because there is none in the datastream, or because a "SEEK" chunk has caused it to be discarded), the decoder must use the following default values:
image_id := 0
visibility := 0
abstract_flag := 0
x_location := 0
y_location := 0
left_cb := 0
right_cb := max_frame_width
top_cb := 0
bottom_cb := max_frame_height
If "image_id" is an identifier that already exists when an "DEFI", "IHDR" or "BASI" chunk is encountered, the parent image previously associated with the identifier is discarded (or disassociated with it, if the identifier was a partial clone of another image), along with any "MOVE" or "CLIP" data associated with it.
See the PNG specification [PNG] and the PNG Special Purpose Chunks document [PNG-EXT] for the format of the PNG chunks.
Any chunks between "IHDR" and "IEND" are written and decoded according to the PNG specification. The image width and height must not exceed "max_image_width" and "max_image_height" from the "MHDR" chunk.
If the PNG "sRGB", "gAMA", or "cHRM" chunks appear in the top-level MNG datastream but not in the PNG datastream, then the values are inherited from the top level as though the chunks had actually appeared in the PNG datastream. Data from such chunks appearing in the PNG datastream take preference over the inherited values. MNG applications that recreate PNG files must write these chunks in the output PNG files.
If the PNG "sPLT" chunk appears in the top-level MNG datastream, it takes preference over any "sPLT" chunk appearing in the PNG datastream. MNG applications that recreate PNG files should not copy top-level "sPLT" chunks to the output PNG files.
When "framing_mode != 1" from the MNG "FRAM" chunk, The PNG "oFFs" and "pHYs" chunks and any future chunks that attempt to set the pixel dimensions or the drawing location must be ignored by MNG viewers and simply copied (according to the copying rules) by MNG editors. When "framing_mode=1" (i.e. when each PNG image is an individual frame), these chunks can be treated as described in the PNG specification.
If "visibility=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.
If "image_id=0", there is no need to store the pixel data after displaying it.
If "abstract_flag=1" and "image_id != 0", the decoder must store the original pixel data losslessly, because it is possible that a subsequent PND stream might want to modify it. If "abstract_flag=0", the decoder can store the pixel data in any form that it chooses.
The first 13 bytes of the "BASI" chunk are identical to those of the "IHDR" chunk, and an optional 8 additional bytes provide 16-bit {red, green, blue, alpha} values, that are used to fill the entire basis image when the "IDAT" chunk is not present.
4 bytes: width (unsigned integer). 4 bytes: height (unsigned integer). 1 byte: bit_depth (unsigned integer). 1 byte: color_type (unsigned integer). 1 byte: compression_method (unsigned integer). 1 byte: filter_type (unsigned integer). 1 byte: interlace_type (unsigned integer). 2 bytes: red sample (unsigned integer). 2 bytes: green sample (unsigned integer).. 2 bytes: blue sample (unsigned integer). 2 bytes: alpha sample (unsigned integer).The alpha sample can be 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. The decoder is responsible for converting the color and alpha samples to the appropriate format and sample depth for the specified "color_type". When "color_type=3", the decoder must generate a palette of length 2^bit_depth, whose first entry contains the given {red, green, blue} triple, and whose remaining entries are filled with zeroes.
The "BASI" a datastream contains PNG chunks, but is not necessarily a PNG datastream. It can be incomplete or it can deviate in certain ways from the PNG specification. It can serve as a parent image for a PND 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 are:
A "BASI" chunk appearing in a MNG datastream must be preceded by a "DEFI" chunk that gives a nonzero "image_id" and the location and visibility for the basis image. When "visibility=0" the resulting image, after the pixel samples are filled in, must be a legal PNG datastream. The "abstract_flag" can be either 0 (abstract) or 1 (concrete), depending on whether the basis image is intended for subsequent use by a PND datastream or not.
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.
See Chapter 4, The PND Format, below, for the format of the PND datastream. Any chunks between "DHDR" and "DEND" are written and decoded according to the PND format, and the resulting image is displayed if its "visibility=0". The image width and height must not exceed "max_image_width" and "max_image_height" from the "MHDR" chunk. The image must have "abstract_flag=1"
2 bytes: source_id (nonzero unsigned integer) identifier
of the parent image to be cloned.
2 bytes: clone_id (nonzero unsigned integer) identifier
to be given to the clone (new copy) to serve as
the image_id of the new image.
1 byte: clone_type (unsigned integer)
0: full clone of image data.
1: partial clone; only the location, clipping
boundaries, and visibility are copied and
a link is made to the image data.
2: renumber image (this is equivalent to
"CLON old_id new_id 1 DISC old_id").
This field can be omitted if the "visibility"
field is also omitted. If it is omitted, the
clone_type defaults to zero (full clone).
1 byte: visibility (unsigned integer)
0: Make the clone visible.
1: Make the clone invisible.
This field can be omitted if the abstract_flag and
location fields are also omitted. When it is
omitted, the image is made visible (visibility=0).
1 byte: abstract_flag (unsigned integer)
0: abstract_flag is the same as that of the parent
image.
1: Make the clone "abstract" (abstract_flag=0).
This field can be omitted if the location fields
are also omitted. When it is omitted, the image
retains the abstract_flag of the parent image.
1 byte: loca_delta_type (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 image.
This field, together with the x_location and
y_location fields, can be omitted. When they
are omitted, decoders must assume default
values {x_location,y_location}={0,0}.
4 bytes: x_location or delta_x_location
(signed integer).
4 bytes: y_location or delta_y_location
(signed integer).
Negative values are permitted for the X and Y image position. The
positive directions are downward and rightward from the upper left
corner of the display.
The clone is initially identical to the parent image except for the location and visibility. It has the same clipping boundaries as the parent image. Subsequent "DHDR", "SHOW", "CLON", "CLIP", "MOVE", "PAST", and "DISC" chunks can use the "clone_id" to identify it.
Subsequent chunks can modify, show, or discard a full clone or modify its associated visibility, location and clipping boundaries without affecting the parent image, or they can modify, show, or discard the parent image or modify its associated data without affecting the clone.
The "abstract_flag" byte must be zero when the "clone_type" byte is nonzero.
If an image has partial clones, and the image data in a parent image or any of its partial clones is modified, the parent image and all of its partial clones are changed. Decoders must take care that when the parent image or any partial clone is discarded, the image data is not discarded until the last remaining one of them is discarded. Only the location, visibility, and clipping boundaries can be changed independently for each partial clone.
The "PAST" chunk contains a 2-byte "destination_id" and 9 bytes giving a "target location", plus one or more 30-byte sequences.
2 bytes: destination_id (unsigned integer)
1 byte: target_delta_type (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
4 bytes: target_x (signed integer)
4 bytes: target_y (signed integer)
2 bytes: source_id (unsigned nonzero integer)
an image to be pasted in.
1 byte: composition_mode (unsigned integer)
0: composite_over
1: replace
2: composite_under
1 byte: orientation (unsigned integer)
0: same as source image
2: flipped left-right, then up-down
4: flipped left-right
6: flipped up-down
1 byte: offset_delta_type (unsigned integer)
0: offsets are measured from {0,0}
1: offsets are measured from
{target_x, target_y}
4 bytes: xoffset or delta xoffset (signed integer)
4 bytes: yoffset or delta yoffset (signed integer)
1 byte: boundary_delta_type (unsigned integer)
0: boundaries are measured from {0,0}
1: boundaries are measured from
{target_x, target_y}
4 bytes: left_pb or delta left_pb (signed integer)
4 bytes: right_pb or delta right_pb (signed integer)
4 bytes: top_pb or delta top_pb (signed integer)
4 bytes: bottom_pb or delta bottom_pb (signed integer)
etc.
The destination image must have the "abstract" property
(abstract_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". The number of source images is ((chunk_length-11)/30).
The "xoffset" and "yoffset" distances and the clipping boundaries are measured, in pixels, positive rightward and downward from either the {0,0} position 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. If the source image has been flipped or rotated, "xoffset" and "yoffset" give the location of its new upper lefthand corner.
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_id's" appear in the list (but a decoder can do the composition in any order it pleases, provided that the resulting destination image is the same as if it had actually performed each composition in the specified order.
The "MOVE" or "CLIP" information associated with the "destination_id" and the "source_id's" is not used in the "PAST" operation (but if a decoder is simultaneously updating and displaying the "destination_id", the "MOVE" and "CLIP" for the "destination_id" is used in the display operation).
The chunk contains a sequence of zero or more two-byte image identifiers. The number of images to be discarded is the the chunk's data length, divided by two.
2 bytes: discard_id (nonzero unsigned integer) image
identifier that can be discarded. All
information pertaining to the corresponding
image can be discarded and the identifier can
be reused by a DEFI chunk.
etc.
If the "DISC" chunk is
empty, all images except those preceding the "SAVE" chunk
can be discarded. If a "SAVE" chunk has not been encountered,
all images can be discarded. Note that each appearance of a "SEEK"
chunk in the datastream implies an empty "DISC" chunk.
When an image is discarded, any location, visibility, and clipping boundary data associated with it is also discarded.
The appearance of an "image_id" in the "discard_id" list, when no such image has been stored, or when the image has already been discarded, should not be treated as a fatal error.
When the image is a partial clone or is the source of a partial clone that has not been discarded, only the "MOVE" or "CLIP" data can be discarded. The image data must be retained until the last remaining partial clone is discarded.
The "MOVE" chunk gives the position, measured downward and to the right of the upper left corner of the display, in pixels, where the named image or group of images is to be located.
The chunk's contents are:
2 bytes: first_image (nonzero unsigned integer).
2 bytes: last_image (nonzero unsigned integer).
1 byte: loca_delta_type (unsigned integer).
0: MOVE data gives x_location and
y_location directly.
1: New positions are determined by adding
the MOVE data to the position of the
parent image.
4 bytes: x_location or delta_x_location
(signed integer).
4 bytes: y_location or delta_y_location
(signed integer).
The new location applies to a single image,
if "first_image = last_image",
or to a group of consectutive image_ids, if they are different.
Negative values are permitted for the X and Y image position. The
positive directions are downward and rightward.
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 Paragraph 3.2.9, below.
It is not an error for the "MOVE" chunk to name an "image" that has not previously been defined. In such cases, nothing is done to the nonexistent image.
2 bytes: first_image (nonzero unsigned integer).
2 bytes: last_image (nonzero unsigned integer).
1 byte: clip_delta_type (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 image_image.
4 bytes: left_cb or delta_left_cb (signed integer).
4 bytes: right_cb or delta_right_cb (signed integer).
4 bytes: top_cb or delta_top_cb (signed integer).
4 bytes: bottom_cb or delta_bottom_cb (signed
integer).
The new clipping boundaries apply to a single image,
if "first_image = last_image",
or to a group of consecutive images, if they are different.
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 all of the following are true:
0 <= x < max_frame_width (from the MHDR chunk) 0 <= y < max_frame_height left_fb <= x < right_fb (from the FRAM chunk) top_fb <= y < bottom_fb left_cb <= x < right_cb (from the CLIP chunk) top_cb <= y < bottom_cbIt is not an error for the "MOVE" chunk to name an "image" that has not previously been defined. In such cases, nothing is done to the nonexistent image.
When an image_id is discarded, its image property set, which includes the "CLIP" data, is also discarded.
2 bytes: red_background (unsigned integer).
2 bytes: green_background (unsigned integer).
2 bytes: blue_background (unsigned integer).
1 byte: mandatory_background (unsigned integer)
0: background color is advisory; applications
can use it if they choose to.
1: background color is mandatory; applications
must use it.
This byte can be omitted; if so, the background
color is advisory.
Viewers are expected to composite
every frame in the MNG datastream, whether it be a PNG or PND datastream or
a group of PNG or PND datastreams delimited by "FRAM" chunks,
against a fresh copy of the background, clipped to the frame boundaries
given in the "FRAM" chunk (they might actually follow some
other procedure, but the final appearance of each frame must be the same
as if they had).
The "BACK" components are always interpreted in the current color space as defined by any top-level "gAMA", "cHRM", "sRGB" chunks that have appeared prior to the "BACK" chunk in the MNG datastream. If no such chunks appear, the color space is unknown.
Multiple instances of the "BACK" chunk are permitted in a MNG datastream.
The "BACK" chunk can be omitted. If a background is required and the "BACK" chunk is omitted, then the viewer must supply its own background.
The chunk contains an image identifier and a name:
2 bytes: snapshot_image (unsigned nonzero integer) n bytes: snapshot_name (Latin-1 text)Note that the "snapshot_name" is associated with the snapshot, not with the "snapshot_image" nor its future contents; discarding the image identified by "snapshot_image" will not affect the snapshot. The "snapshot_name" means nothing inside the scope of the MNG specification. If two "eXPI" chunks use the same name, it's the outside world's problem (and the outside world's prerogative to regard it as an error). 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.
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 PND images, but they would normally have smaller dimensions and fewer colors than the MNG frames. They can be defined with the visibility field set to "invisible" if they are not intended to be shown as a part of the regular display.
Multiple instances of the "eXPI" chunk are permitted in a MNG datastream.
1 byte: fPRI_delta_type (unsigned integer).
0: Priority is given directly.
1: Priority is determined by adding
the fPRI data to the previous
value, modulo 256.
1 byte: priority or delta_priority (signed
integer). Value to be assigned to
subsequent frames until another fPRI
chunk is reached.
1 byte: max_priority (unsigned integer). Maximum
priority that will appear in any subsequent
fPRI chunk (max_priority is always given
directly without regard to fPRI_delta_type).
While 256 distinct values of "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 PND datastreams, the "initial"
frames could be preceded by a "fPRI" with "priority=128"
and followed by one with "priority=0", and the best representative
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 could display everything.
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 Examples 5 and 8.
Viewers that care about the priority can assume "priority=255" for any portion of the MNG datastream that is processed prior to the first "fPRI" chunk.
Multiple instances of the "fPRI" chunk are permitted.
The "nEED" chunk contains a list of keywords that the decoder must recognize. Keywords are typically private critical chunk names.
n bytes: keyword 1 byte: null separator etc.The "nEED" chunk should be placed early in the MNG datastream, preferably immediately 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. 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.
[During the draft phase of the development of this specification, you can specify that the datastream is written in accordance with a certain draft version of MNG, you can use "nEEDdraft nn"].
1 byte: termination_action (unsigned integer)
0: Show the last frame indefinitely.
1: Cease displaying anything.
2: Discard all "nonfrozen" images and repeat
the animation starting immediately after
the SAVE chunk (or immediately after the
MHDR chunk, if a SAVE chunk is not present).
3: Repeat the animation starting immediately
after the last SEEK chunk in the datastream.
If no SEEK chunk is present, repeat the
animation starting immediately after the
SAVE chunk (or immediately after the MHDR
chunk, if a SAVE chunk is not present).
The MNG top-level "pHYs" chunk can be nullified by a subsequent empty "pHYs" chunk appearing in the MNG top level.
[Would this be less confusing if we used a different name for this top-level chunk, e.g. pSIz? Is there a way of eliminating the ugly interdependence on framing_mode while maintaining the functionality?]
The following 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 the appearance of a subsequent empty chunk with the same chunk name. Such empty chunks are not legal PNG chunks and must only appear in the MNG top level.
The following PNG chunks are also defined at the MNG top level. They provides values that take precedence over those that might be provided in subsequent PNG datastreams and provide values to be used when they are not provided in subsequent PNG datastreams:
When a decoder needs to chose 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 preference to the palette from the MNG level, to avoid spurious frame-to-frame color changes.
MNG editors that write PNG datastreams should ignore the "sPLT" and "pHYs" data from the MNG level and simply copy any "sPLT" and "pHYs" chunks appearing within the PNG datastreams.
No provision is made in this specification for storing a PND datastream as a standalone file. A PND datastream will normally be found as a component of a MNG datastream. Applications that need to store a PND 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 PND datastream, and the "MEND" chunk.
The decoder must have available a parent (decoded) image from which the original chunk data is known. The parent image can be the result of decoding a PNG, another PND datastream, or it could have been generated by a PNG-like datastream introduced by a "BASI" chunk.
The new image is always of the same basic type (at present only PNG is defined) as the parent image.
The decoder must not have modified the pixel data in the parent image 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 PND decoder the unmodified pixel data along with the values for the "gAMA", "cHRM", and any other recognized chunks from the parent image datastream.
A PND datastream consists of a "DHDR" and "DEND" enclosing other optional chunks (if there are no other chunks, the decoder simply copies the parent image, and displays it if its "visibility=0").
Chunk structure (length, name, CRC) and the chunk-naming system are identical to those defined in the PNG specification. Definitions of "compression_type", "filter_type", and "interlace_type" are also the same as defined in the PNG specification.
The "DHDR" chunk can contain 4, 12, or 20 bytes:
2 bytes: image (nonzero unsigned integer).
Identifies the parent image from which changes
will be made. This is also the image number
of the basis image, which can be used as the
parent image for a subsequent PND datastream.
1 byte: image_type.
0: Image type is unspecified. An IHDR or IPNG
chunk must be present.
1: Image type is PNG. IHDR and IPNG can be
omitted under certain conditions.
1 byte: delta_type.
0: Entire image replacement.
1: Block pixel addition, by samples, modulo
2^bit_depth.
2: Block alpha addition, by samples, modulo
2^bit_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)
color type 4 or color type 6.
3: Block pixel replacement.
4: Block alpha replacement.
5: No change to pixel data.
4 bytes: block_width (unsigned integer). Omit when
delta_type=5.
4 bytes: block_height (unsigned integer). Omit when
delta_type=5.
4 bytes: block_x_location (unsigned integer),
measured in pixels from the left edge of
the parent image. Omit when delta_type=0
or when delta_type=5.
4 bytes: block_y_location (unsigned integer),
measured in pixels from the top edge of
the parent image. Omit when delta_type=0
or when delta_type=5.
The "image" must identify an existing image, and the image
must be a "concrete" image, i.e. it must have the
property "abstract_flag=1".
The image type, whether given explicitly as 1 or implied by the presence of an "IHDR" or "IPNG" chunk, must be the same as that of the parent image.
When "delta_type=0", the width and height of the child image are given by the block_width and block_height fields.
For all other values of "delta_type", the width and height of the child image are inherited from the parent image.
When "delta_type=1, 2, 3, or 4", 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 image. The block must fall entirely within the parent image.
The parent image must have been derived from a PNG datastream or from a sequence of PND datastreams that depend upon a PNG datastream.
The compression method, filter method, and interlace method need not be the same as those of the parent image.
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 "interlace method", filtered according to the "filter method" and compressed according to the "compression method" given in the "IHDR" chunk. The actual pixel values are calculated using the method defined in the "delta_type" field of the "DHDR" chunk. 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.
An encoder calculates the new sample values from the samples in the parent image and those in the new image by subtracting the parent image samples from the new image samples, modulo 2^sample_bit_depth. When decoding the "IDAT" chunk, the new image bytes are obtained by adding the delta bytes to the parent image bytes, modulo 2^sample_bit_depth. This is similar in operation to the PNG SUB filter, except that it works by samples instead of by bytes.
When "color_type=3", the deltas are differences between index values, not between color samples.
The "bit_depth" of the data must match that of the parent image, and "color type" is 0 (grayscale), regardless of the "color_type" of the parent image. The parent image 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.
The "bit_depth" and "color_type" of the data must match that of the parent image. The compression method, filter method, and interlace method need not be the same.
The "DEND" chunk is empty.
1 byte: new color_type.
1 byte: new bit_depth.
1 byte: fill_method
0: left-bit-replication
1: zero fill
When a decoder encounters the "PROM" chunk, it must promote
the pixel data. The cases are:
The "PROM" chunk is not permitted to "demote" a parent image to an image 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.
If the "IHDR" chunk is present, its "width", "height", "bit_depth", and "color_type" fields are ignored. The values for these parameters are inherited from the parent image or from the "PROM" chunk.
The "compression_method", "interlace_type", and "filter_type" fields, if different from those of the parent image, are used in decoding any subsequent "IDAT" chunks, and the new values will be inherited by any subsequent image that uses this image as its parent.
See the PNG specification for the format of the PNG chunks. 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 image. Except for IDAT, any chunks appearing between "IHDR" and "IEND" are always treated as replacements or additions and not as deltas.
The "IPNG" chunk can be used instead of the "IHDR" chunk if the "IHDR" chunk is not needed for resetting the value of "compression_method", "filter_type", or "interlace_type". The purpose of this chunk is to identify the beginning of the PNG datastream, so decoders can start interpreting PNG chunks instead of PND chunks. The decoder must treat this datastream as though the "IHDR" chunk were present in the location occupied by the "IPNG" chunk.
The "IHDR" chunk can also be omitted when "image_type=1" and the PNG stream begins with either a "PLTE" 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 nor an "IDAT" chunk, then either the "IPNG" or "IHDR" must be present to introduce the PNG datastream.
A "gAMA", "cHRM", or similar chunk existing in the parent image would not affect the pixel data inherited by this PND 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 image are the raw pixel data that existed prior to any transformations that were applied while displaying the previous frame.
If the "PLTE" chunk is present, it need not have the same length as that inherited from the parent image, but it must contain the complete palette needed in the child image. If it is shorter than the palette of the parent image, decoders can discard the remaining entries. They can also truncate any "tRNS" data inherited from an indexed-color parent image. If it is longer, and a new "tRNS" chunk is not present in an indexed-color image, the "tRNS" data should be extended with opaque entries.
[What about defining an IPLT chunk that can give an incomplete PLTE, that would only overwrite palette entries without truncating the resulting PLTE? This might be useful for palette-animation applications.]
When processing the "tRNS" chunk, if "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 image.
MNG viewers must ignore "oFFs" and "pHYs" chunks that appear inside a PNG stream, except when "framing_mode=1" from the MNG "FRAM" chunk. datastream begins with a PNG signature instead of a MNG signature. MNG editors are expected to treat them as unknown chunks that will be handled as described in Paragraph 4.1.7, below.
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 Paragraph 4.1.7, below.
The "IEND" chunk can be omitted, if it would be the last chunk in the PND datastream before the "DEND", or when no PNG chunks are present.
Chunk name etc.If multiple name appear in the "DROP" chunk, it is shorthand for multiple "DROP" chunks.
4 bytes: Chunk name (ASCII text)
1 byte: Polarity (unsigned integer)
0: only
1: all-but (only/all-but)
Keywords (null-separated Latin-1 text strings)
The chunk name must be the name of a chunk that begins with a
null-terminated text string. Some parent image 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 image chunk whose keyword does not appear in the keywords list is
inhibited.
Use caution when printing or displaying keywords (Refer to Security considerations, Chapter 11, below).
Chunk name
Order type
0: anywhere
1: after IDAT
2: before IDAT
3: before IDAT, but not before PLTE
4: before IDAT, but not after PLTE
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 image which is not inhibited by "DROP/DBYK" will not be broken by this PND, 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 image, 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 PND, and knows where it may 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 image or in the PND, it is a fatal error unless the applier knows how to handle it. The specification of the critical chunk may include provisions for this scenario.
The copy-safe status of an unknown chunk is determined from the chunk name, just as in PNG [LINK]. 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 the "SAVE" chunk, this affects the copy-safe status of all top-level unknown chunks in the entire MNG datastream.
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 stream can have ordering rules only with respect to the "SAVE", "SEEK" chunks, "IHDR-IEND", "DHDR-DEND", and "BASI-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", and "DHDR-DEND" sequences is governed by the PNG specification, and any changes inside such sequences have no effect on top-level chunks.
[This one is ugly.] The copy-safe status of chunks inside a "DHDR-DEND" sequence depends on the copy-safe status of its parent image.
Pixels in PNG 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:
Ac = 1 - (1 - At)(1 - Ab) s = At / Ac t = (1 - At) Ab / Ac Rc = s Rt + t Rb Gc = s Gt + t Gb Bc = s Bt + t BbWhen the bottom pixel is fully opaque (Ab = 1.0), the function reduces to:
Ac = 1 Rc = At Rt + (1 - At) Rb Gc = At Gt + (1 - At) Gb Bc = At Bt + (1 - At) BbWhen 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:
Ac = 1 - (1 - At)(1 - Ab) Rc = Rt + (1 - At) Rb Gc = Gt + (1 - At) Gb Bc = Bt + (1 - At) BbAs mentioned in the PNG specification, the equations become much simpler when all the alpha values are integers, and the RGB samples need not be linear in that case.
The following information must be retained, for each nonzero "image" that is defined and not subsequently discarded:
When the encoder knows that image data will not be needed by subsequent frames, it can make life easier for decoders by declaring that the image is "abstract", by using "image_id=0", and by using the "DISC" or the "SEEK" chunk.
When an error occurs within a image datastream, such as an unknown critical PNG chunk or a missing parent image where one was required, only that image should be abandoned and the associated "image" should be discarded.
MNG editors, on the other hand, should be more strict and reject any datastream with errors unless the user intervenes.
If "PLTE" is present in a PND datastream, the new palette is used in displaying the image defined by the PND; 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 PND without a "PLTE" chunk, that has been declared by the "DHDR" "image" field to depend on the other.
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 reduced global palette, to help decoders build an appropriate palette when necessary.
See [RFC-1738], Uniform Resource Locators (URL).
<URL:ftp://ds.internic.net/rfc/rfc2083.txt>
also available at
<URL:ftp://swrinde.nde.swri.edu/pub/png/documents/>.
<URL:ftp://ds.internic.net/rfc/rfc2045.txt>
<URL:ftp://ds.internic.net/rfc/rfc2048.txt>
<URL:ftp://ds.internic.net/rfc/rfc1738.txt>
An infinite or just overly long loop could give the appearance of having locked up the machine, as could an unreasonably long inter-frame delay or a misplaced "sync_id" with a long "sync_timeout" value. Therefore a decoder should always provide a simple method for users to escape out of a loop or delay, either by canceling the MNG entirely or just proceeding on to the next "SEEK" chunk (The "SEEK" chunk makes it safe for a viewer to resume processing after it encounters a corrupted portion of a MNG datastream).
No known additional security concerns are raised by this format.
copy file.png file.mngThe resulting MNG file looks like:
\211 P N G \r \n ^z \n # PNG signature IHDR 720 468 8 0 0 0 0 # Width and height, etc. IDAT ... IEND
\212 M N G \r \n ^z \n # MNG signature
MHDR 256 300 # Width and height
30 # 30 ticks per second
tERm 2 # When done, repeat from SAVE indefinitely
FRAM 1 "begin" 0 2 0 0 0 30 # set frame_duration to 1 sec
DEFI 1 IHDR ... IDAT ... IEND # Eight PNG datastreams
DEFI 2 IHDR ... IDAT ... IEND # are read and stored as
DEFI 3 IHDR ... IDAT ... IEND # abstract images, and
DEFI 4 IHDR ... IDAT ... IEND # are displayed as they
DEFI 5 IHDR ... IDAT ... IEND # are read.
DEFI 6 IHDR ... IDAT ... IEND
DEFI 7 IHDR ... IDAT ... IEND
DEFI 8 IHDR ... IDAT ... IEND
SAVE
SHOW 1 8
MEND
\212 M N G \r \n ^z \n # MNG signature
MHDR 720 468 # Width and height
1 # 1 tick per second
FRAM 1 "" 0 2 2 0 2 0 600 0 # set frame_duration to 0
# sync_timeout to 600 sec and sync_id list to {0}
SAVE
SEEK "Briefing to the Workforce"
IHDR ... IDAT ... IEND # DEFI 0 is implied
SEEK "Outline"
IHDR ... IDAT ... IEND
SEEK "Our Vision"
IHDR ... IDAT ... IEND
SEEK "Our Mission"
IHDR ... IDAT ... IEND
SEEK "Downsizing Plans"
IHDR ... IDAT ... IEND
MEND
\212 M N G \r \n ^z \n # MNG signature
MHDR 720 468 # Width and height
1 # 1 tick per second
DEFI 1 1 # define image 1, invisible, abstract
IHDR ... IDAT ... IEND
FRAM 2 "" 0 2 2 0 2 0 600 0 # set frame_duration to 0
# sync_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 FRAM
SEEK "Outline"
CLON 1 2 DHDR 2 ... IDAT ... IEND SHOW 2 FRAM
SEEK "Our Vision"
CLON 1 2 DHDR 2 ... IDAT ... IEND SHOW 2 FRAM
SEEK "Our Mission"
CLON 1 2 DHDR 2 ... IDAT ... IEND SHOW 2 FRAM
SEEK "Downsizing Plans"
CLON 1 2 DHDR 2 ... IDAT ... IEND SHOW 2 FRAM
MEND
\212 M N G \r \n ^z \n # MNG signature
MHDR 720 468 # Width and height
30 # 30 ticks per second
tEXtTitle\0Sample Movie
fPRI 128 255 # default frame priority is "medium"
FRAM 1 "" 0 2 0 0 0 3 # set frame_duration 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
DEND # 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
DEND # block at a slightly different location
SEEK "frame 3" # Ok to restart here because a
# complete PNG frame follows
fPRI 255 255 # This is the representative frame that
IHDR 720 468 ...# will be displayed by single-frame
IDAT ... # viewers.
IEND
fPRI 128 128 # Return to medium frame priority
DHDR 1 1 1 720 468 0 0 # Another PNG-delta frame
IDAT ... # The entire 720x468 rectangle changes
DEND # this time.
SEEK "end"
MEND # End of MNG datastream
\212 M N G \r \n ^z \n # MNG signature
MHDR 1024 512 # Width, height
1 # ticklength
bACk 1 16448 16448 52800 # Must use sky blue background
DEFI 1 1 # Define invisible abstract thumbnail image.
IHDR 64 64 4 3 0 0 0
IDAT
IEND
eXPI 1 "thumbnail 1"
DEFI 1 1 # Also define a larger thumbnail.
IHDR 96 96 4 3 0 0 0
IDAT
IEND
eXPI 1 "thumbnail 2"
DISC # Discard the thumbnail image.
FRAM 2 "Two views of the data"
DEFI 1 0 0 1 6 6 # Define first (bottom) image
IHDR 500 500 16 0 .. # A 16-bit graylevel image
gAMA 50000
IDAT ...
IEND # End of image
CLON 1 2 0 0 1 0 518 6 # Make a full "concrete" clone
DHDR 2 1 5 # Modify it (no change to pixels).
ORDR faLT 2 # establish chunk placement
gAMA 100000 # gamma value is 100000 (gamma=1.0)
tEXtComment\0The faLT chunk is described in ftp://swrinde....
faLT ... # Apply pseudocolor to parent image
DEND # End of image
DEFI 3 0 0 1 900 400 # Overlay near lower right-hand corner
IHDR 101 101 2 3 ...
gAMA 50000 # We need a new gAMA because
PLTE ... # this is not a PND datastream
tRNS ... # It's 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
\212 M N G \r \n ^z \n # MNG signature
MHDR 512 512 # Start of MNG datastream
30 # ticklength
FRAM 2 "frame 1" 0 2 0 0 0 3 # First frame
# sets frame_duration=3 ticks
DEFI 1 # Define image 1 (abstract, LOCA 0 0)
IHDR 512 512 ... # It's a full-display PNG image
etc # Chunks according to PNG spec
IEND # SHOW 1 is implied by DEFI 1
DEFI 2 0 1 0 300 200 # Define image 2, concrete
IHDR 32 32 ... # It's 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 # make a full clone of it as image 3
0 400 500 # Location for image 3
DHDR 3 1 3 0 0 0 0 # Modify image 3
tRNS ... # Make it semitransparent
DEND # 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 100" # Another frame
etc.
FRAM
etc. # More frames
MEND # End of MNG datastream
\212 M N G \r \n ^z \n # MNG signature
MHDR 512 512 # Start of MNG datastream
30 # ticklength
FRAM 2 "frame 1" 0 2 0 0 0 3 # First frame
# sets frame_duration=3 ticks
DEFI 1 # Define image 1 (abstract, LOCA 0 0)
IHDR 512 512 ... # It's a full-display PNG image
etc # Chunks according to PNG spec
IEND # SHOW 1 is implied by DEFI 1
DEFI 1 # background
IHDR ... IDAT ... IEND
DEFI 10 1 0 x0 y0 # static part of sprite
IHDR ... IDAT ... IEND
DEFI 11 1 0 x0 y0 # view 1 of animated part
IHDR ... IDAT ... IEND
DEFI 12 1 0 x0 y0 # view 2 of animated part
IHDR ... IDAT ... IEND
DEFI 13 1 0 x0 y0 # view 3 of animated part
IHDR ... IDAT ... IEND
DEFI 14 1 0 x0 y0 # view 4 of animated part
IHDR ... IDAT ... IEND
LOOP 0 0 1
FRAM
MOVE 10 14 1 dx dy # Move animated icon {dx, dy}
SHOW 1 # Show background
SHOW 10 # Show static part
SHOW 11 14 # Select the next view of the
ENDL 0 # animated part and show it
FRAM
MEND
\212 M N G \r \n ^z \n # MNG signature
MHDR 64 64 # Width, height
30 # Tick length
bACk 0 52800 52800 52800 # "Browser gray" default background
FRAM 1 "" 0 2 0 0 0 3 # Set frame_duration=3 ticks
DEFI 1 0 1 # visible 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 0 255 # Give the fade-in sequence a low priority
CLON 1 2 # Make a working abstract copy of the image
# that will be modified during the low-priority
# part of the datastream. It's a full clone.
DHDR 2 1 2 64 64 0 0
tRNS 0 1 1 1 .. # change all nontransparent pixels to alpha=1
DEND
DHDR 2 1 3 64 64 0 0
tRNS 0 20 20 20 ...# Change alpha to 20 for all
DEND # originally nontransparent pixels
DHDR 2 .... # Repeat with increasing values of alpha.
tRNS 0 40 40 40 ...
DEND
etc.
DHDR 2 ...
trns 0 240 240 240 ...
DEND
DISC 2 # Discard the working copy
fPRI 0 255 255 # 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
\212 M N G \r \n ^z \n # MNG signature
MHDR 150 150 # Width, height
1 # Tick length (unused in this example)
tEXtTitle\0Weather modeling results
tEXtComment\0The pcAL, xsCL, ysCL, zsCA, and tsCL chunks
in this file are written according to the PNG Sci-vis
chunks specification version 19960921 and pCAL
specification version 19961230 available at
ftp://swrinde.nde.swri.edu/pub/png-group/documents/
xsCL x\0 [sig] kilometers\0 0\0 150
ysCL x\0 [sig] kilometers\0 0\0 150
zsCA x\0 [sig] Height (kilometers)\0 0\0 15
tsCL x\0 [sig] Time (hours)\0 0\0 24
pcAL x\0 [sig] 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's image 0
DHDR 1 1 0 # Source=0, PNG, pixel subtraction,
150 150 0 0 # Block is entire image
IDAT ... # IHDR is omitted; everything matches top
DEND # IEND is also omitted
etc. # Repeat DHDR through DEND 148 more times
SEEK
FRAM # End of first block
etc. # Repeat FRAM through SEEK 19 more times
SEEK
MEND # End of MNG
\212 M N G \r \n ^z \n # MNG signature MHDR 1024 768 1 # Start of MNG datastream FRAM 2 DEFI 1 1 0 0 -64 # Set up an offscreen "abstract" copy IHDR 128 64 ... # of the tile PLTE ... IDAT ... # Nothing will be displayed because it's IEND ... # outside the 1024 by 768 composite frame 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 # 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 MENDHere's a better approach, which creates a reusable tiled image by pasting the small image into the corner of a large blank image, and then repeatedly pasting the growing image back into itself.
\212 M N G \r \n ^z \n # MNG signature
MHDR 1024 768 1 # Start of MNG datastream
DEFI 1 1 0 0 -64 # Set up an offscreen "abstract" copy
IHDR 128 64 ... # of the tile
PLTE ...
IDAT ... # Nothing will be displayed because it's
IEND ... # outside the 1024 by 768 composite frame
DEFI 2 # The abstract image to be tiled
BASI 1024 768 8 2 0 0 0 # initially all pixels are zero
IEND
PAST 2 0 0 0 # destination and target location
# src mod orient offset clipping
1 0 0 0 0 0 0 0 128 0 64
2 0 0 0 128 0 0 0 256 0 64
2 0 0 0 256 0 0 0 512 0 64
2 0 0 0 512 0 0 0 1024 0 64
2 0 0 0 0 64 0 0 1024 0 128
2 0 0 0 0 128 0 0 1024 0 258
2 0 0 0 0 256 0 0 1024 0 512
2 0 0 0 0 512 0 0 1024 0 768
# end of PAST chunk data
MEND
\212 M N G \r \n ^z \n # MNG signature
MHDR 512 256 # Width and height on screen
30 # Tick length
FRAM 2 "" 0 2 0 0 0 1 # frame_duration = 1 tick
DEFI 1 1 0 0 256 # Define image 1 but don't display now
# Initially it's 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
FRAM
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
SHOW 1 2 # Show the two images
ENDL 0
MEND
begin
write "MHDR" chunk
write "BACK" chunk
saved_images := 0
frame_duration := 0
first_frame := TRUE
if(loops>1) "write tERm 2" chunk
for subimage in gif89a file do
if(frame_duration != gif_duration) then
frame_duration := gif_duration
write "FRAM 2 x 0 2 2 0 2 0 frame_duration 0" chunk
first_frame := FALSE
else if(first_frame == TRUE)then
write "FRAM 2" chunk
first_frame := FALSE
else
write "FRAM" chunk
endif
if(x_loc == 0 AND y_loc == 0) then
write "DEFI saved_images 1 1 1" chunk
else
write "DEFI saved_images 1 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" chunk
write "MEND" chunk
end
Where "<image>" represents a PNG or PND datastream containing the
GIF frame converted to PNG format.
Caution: if you write such a program you might have to pay royalties in order to use it or to convey it to anyone else.
Phone: (410) 278-6554
EMail: glennrp@arl.mil or randeg@alumni.rpi.edu
End of MNG Specification. Expires 21 Jul 1997