MPNGPNG - Mutli-PNG PNG spec
- SoniEx2 <email@example.com>
- From the APNG spec: (from which I copied a lot of stuff)
MPNGPNG is like APNG, but more powerful.
Most stuff here was taken from the APNG spec.
An MPNGPNG image contains siMG (Sub IMaGe) chunks, and may contain a ctLA (ConTroL Animation) chunk, a ctLF (ConTroL Frame) chunk or a ssEC (Sub SECtion) chunk.
MPNGPNG is better than MNG because it lets you shove whole MPNGPNGs in MPNGPNGs, with theoretically no depth limit. The top image color correction applies to the sub images, so you have to chain the color correction stuff, but meh w/e, this was meant to reuse the decoding subroutines for each sub-image anyway.
The "default image" is the image described by the standard
IDAT chunks, and is the image that is displayed by decoders that do not support MPNGPNG.
The "canvas" is the area on the output device on which the frames are to be displayed. The contents of the canvas are not necessarily available to the decoder. As per the PNG Specification, if a
bKGD chunk exists it may be used to fill the canvas if there is no preferable background.
The "output buffer" is a pixel array with dimensions specified by the width and height parameters of the PNG
IHDR chunk. Conceptually, each frame is constructed in the output buffer before being composited onto the canvas. The contents of the output buffer are available to the decoder. The corners of the output buffer are mapped to the corners of the canvas.
"Fully transparent black" means red, green, blue and alpha components are all set to zero.
A "frame" is a sub-image that is part of an animation. A "sub-image" may or may not be part of an animation.
For purposes of chunk descriptions, an "unsigned int" shall be a 32-bit unsigned integer in network byte order limited to the range 0 to (2^31)-1; an "unsigned short" shall be a 16-bit unsigned integer in network byte order with the range 0 to (2^16)-1; a "byte" shall be an 8-bit unsigned integer with the range 0 to (2^8)-1.
MPNGPNG is designed to allow incremental display of frames before the entire image has been read. This implies that some errors may not be detected until partway through the animation. It is strongly recommended that when any error is encountered decoders should discard all subsequent frames, stop the animation, and revert to displaying the default image. A decoder which detects an error before the animation has started should display the default image. An error message may be displayed to the user if appropriate.
A MPNGPNG stream is a normal PNG stream as defined in the PNG Specification, with four additional chunk types describing the animation and providing additional frame data.
To be recognized as a MPNGPNG, at least one valid
siMG chunk must appear in the stream. A valid
siMG chunk must contain either a
ctLF or a
ctLA chunk must appear in the stream when the animation should start. The
ctLA structure is described below.
Conceptually, at the beginning of each play the output buffer must be completely initialized to a fully transparent black rectangle, with width and height dimensions from the
The default image may be included as the first frame of the animation by the presence of a single
ctLF chunk on the main PNG. Otherwise, the default image is not part of the animation.
Subsequent frames are encoded in
siMG chunks, which are entire PNG files. Information for each frame about placement and rendering is stored in
IHDR chunks. The full layout of
ctLF chunks is described below. The
IHDR chunk is described in the PNG Specification.
The boundaries of the entire animation are specified by the width and height parameters of the PNG
IHDR chunk, regardless of whether the default image is part of the animation. The default image should be appropriately padded with fully transparent pixels if extra space will be needed for later frames.
Each frame is identical for each play, therefore it is safe for applications to cache the frames.
Frame Sequence Numbers
ctLF chunks have a 4 byte sequence/frame number. [TODO add more relevant stuff here, e.g. frame number or something. the original APNG spec stuff isn't relevant so it was removed.]
ctLA: The Animation Control Chunk
ctLA chunk is an ancillary chunk as defined in the PNG Specification. It may appear only once, and it also signifies when to start the playback.
ctLA chunk contains:
| byte | type | name | desc | | 0 | unsigned int | icount | Number of images. | | 4 | unsigned int | lcount | Number of times to loop this MPNGPNG. |
icount indicates the total number of images in the animation. This must equal the number of
ctLF chunks. 0 indicates a streaming/live animation or an unspecified number of frames. 1 is a valid value for a single-frame APNG. If this value does not equal the actual number of frames it should be treated as an error.
lcount indicates the number of times that this animation should play; if it is 0, the animation should play indefinitely. If nonzero, the animation should come to rest on the final frame at the end of the last play.
ctLF: The Frame Control Chunk
ctLF chunk is an ancillary chunk as defined in the PNG Specification.
ctLF chunk is present, then the
ssEC chunk must not be present.
- For the default image, if a
ctLFchunk is present it must appear on the main PNG.
- For all other frames, the
ctLFchunk must appear inside the siMG.
ctLF chunk is required for each frame.
| byte | type | name | desc | | 0 | unsigned int | seqnum | Frame sequence number. | | 4 | unsigned int | x_offset | X position at which to render this frame | | 8 | unsigned int | y_offset | Y position at which to render this frame | | 12 | unsigned short | delay_num | Frame delay fraction numerator | | 14 | unsigned short | delay_den | Frame delay fraction denominator | | 16 | byte | dispose_op | Type of frame area disposal to be done after rendering this frame | | 17 | byte | blend_op | Type of frame area rendering for this frame |
The frame must be rendered within the region defined by
height, the latter 2 which are available from the
IHDR chunk. The offsets must be non-negative, the dimensions must be positive, and the region may not fall outside of the default image. This means that for the default image
y_offset must be 0 (see below).
Constraints on frame regions:
`x_offset` >= 0 `y_offset` >= 0 `width` > 0 `height` > 0 `x_offset` + `width` <= default image `IHDR` width `y_offset` + `height` <= default image `IHDR` height
delay_den parameters together specify a fraction indicating the time to display the current frame, in seconds. If the denominator is 0, it is to be treated as if it were 100 (that is,
delay_num then specifies 1/100ths of a second). If the the value of the numerator is 0 the decoder should render the next frame as quickly as possible, though viewers may impose a reasonable lower bound.
Frame timings should be independent of the time required for decoding and display of each frame, so that animations will run at the same rate regardless of the performance of the decoder implementation.
dispose_op specifies how the output buffer should be changed at the end of the delay (before rendering the next frame).
Valid values for
| value | name | | 0 | MPNGPNG_DISPOSE_OP_NONE | | 1 | MPNGPNG_DISPOSE_OP_BACKGROUND | | 2 | MPNGPNG_DISPOSE_OP_PREVIOUS |
MPNGPNG_DISPOSE_OP_NONE: no disposal is done on this frame before rendering the next; the contents of the output buffer are left as is.
MPNGPNG_DISPOSE_OP_BACKGROUND: the frame's region of the output buffer is to be cleared to fully transparent black before rendering the next frame.
MPNGPNG_DISPOSE_OP_PREVIOUS: the frame's region of the output buffer is to be reverted to the previous contents before rendering the next frame.
If the first
ctLF chunk uses a
MPNGPNG_DISPOSE_OP_PREVIOUS, it should be treated as
blend_op specifies whether the frame is to be alpha blended into the current output buffer content, or whether it should completely replace its region in the output buffer.
Valid values for
| value | name | | 0 | MPNGPNG_BLEND_OP_SOURCE | | 1 | MPNGPNG_BLEND_OP_OVER |
MPNGPNG_BLEND_OP_SOURCE all color components of the frame, including alpha, overwrite the current contents of the frame's output buffer region. If
MPNGPNG_BLEND_OP_OVER the frame should be composited onto the output buffer based on its alpha, using a simple OVER operation as described in the "Alpha Channel Processing" section of the PNG specification [PNG-1.2]. Note that the second variation of the sample code is applicable.
Note that for the first frame the two blend modes are functionally equivalent due to the clearing of the output buffer at the beginning of each play.
ctLF chunk corresponding to the default image, if it exists, has these restrictions:
y_offsetfields must be 0.
As noted earlier, the output buffer must be completely initialized to fully transparent black at the beginning of each play. This is to ensure that each play of the animation will be identical. Decoders are free to avoid an explicit clear step as long as the result is guaranteed to be identical. For example, if the default image is included in the animation, and uses a
blend_op of APNG_BLEND_OP_SOURCE, clearing is not necessary because the entire output buffer will be overwritten.
Note, at the end of the
ctLF chunk is a 32-bit CRC checksum. The checksum is calculated using the ctLF chunk and includes the 'ctLF' bytes.
siMG: The Sub-Image Chunk
siMG chunk contains a PNG file.
siMG chunk is required for each frame.
| byte | type | name | desc | | 0 | unsigned int | streamid | The stream ID. | | 4 | unsigned int | part | The part ID. Specifies order of concatenation. | | 4 | X bytes | pngdat | Whole PNG file. Can be split across chunks. |
Chunks with the same
streamid get concatenated together, ordered by
ssEC: The Sub-Section Chunk
ssEC chunk contains information about how to render this sub-image onto the main image. It must be ignored for the top-level image.
ssEC chunk is present, then the
ctLF chunk must not be present.
| byte | type | name | desc | | 0 | unsigned int | x_offset | First X position at which to render this sub-image | | 4 | unsigned int | y_offset | First Y position at which to render this sub-image | | . | . | . | . | | . | . | . | . | | . | . | . | . |
The sub-image must be rendered within the region defined by
height, the latter 2 which are available from the
IHDR chunk. The offsets must be non-negative, the dimensions must be positive, and the region may not fall outside of the default image.
You can't put PNGs inside PNGs, but you can put MPNGPNGs inside MPNGPNGs.
The chunk names were chosen so to not conflict with APNG's chunk names. This means you can put an APNG inside a MPNGPNG.
You can display multiple animated MPNGPNGs inside a single MPNGPNG.
MFPNG 0.10 - Initial revision
MPNGPNG 0.11 - Rename from MFPNG to MPNGPNG, add support for "compositing" images, correct some "the following" to "this".
MPNGPNG 0.20 - siMG chunks shouldn't have frame number, that should be on the ctLF chunks, so you don't have to come up with random frame numbers to be able to use ssEC chunks/compositing images.
MPNGPNG 0.21 - Allow siMG to be rendered multiple times onto super-image.
MPNGPNG 0.22 - Streaming.
This work is licensed under the Creative Commons Attribution-ShareAlike license (CC-BY-SA) version 3.0 or any later version.
APNG Spec: https://wiki.mozilla.org/APNG_Specification APNG Spec Contributors: https://wiki.mozilla.org/index.php?title=APNG_Specification&action=history