File: png-apng-proposal-20070331 This proposal is derived from the version 0.10 proposal posted on March 23, 2007, at bugzilla.mozilla.org, bug #257263. It is proposed to register animated PNG (APNG) chunks: acTL, fcTL, and fdAT. In this proposal, the new chunk names are shown in private form (second letter lowercase). Until the proposal is approved, the private form (second letter lowercase) must be used in any test implementations. It is proposed to document Animated PNG (APNG) in the Extensions to the PNG 1.2 Specification, Version 1.3.0, as follows: A. Change document version to 1.4.0. B. Add to the chunk ordering table: acTL No Before IDAT fcTL Yes Before (0 or 1) and after IDAT (multiple) fdAT Yes After IDAT C. Add paragraph 4 and renumber subsequent paragraphs 4. APNG Animation Chunks 4.1 Overview APNG is an extension of the [PNG][pngspec] format, adding support for animated images. It is intended to be a replacement for simple animated images that have traditionally used the [GIF][gifspec] format, while adding support for 24-bit images and 8-bit transparency. APNG is a simpler alternative to MNG, providing a spec suitable for the most common usage of animated images on the Internet. APNG is backwards-compatible with PNG; any PNG decoder should be able to ignore the APNG-specific chunks and display a single image. 4.2 Terminology 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 APNG. 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 preferrable 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. 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; a "byte" shall be an 8-bit unsigned integer. 4.3 Error Handling APNG 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. 4.4 Structure An APNG stream is a normal PNG stream as defined in the [PNG Specification][pngspec], with three additional chunk types describing the animation and providing additional frame data. To be recognized as an APNG, an `acTL` chunk must appear in the stream before any `IDAT` chunks. The `acTL` structure is described below. Logically, 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 `IHDR` chunk. The default image may be included as the first frame of the animation by the presence of a single `fcTL` chunk before `IDAT`. Otherwise, the default image is not part of the animation. Subsequent frames are encoded in `fdAT` chunks, which have the same structure as `IDAT` chunks, except preceded by a sequence number. Information for each frame about placement and rendering is stored in `fcTL` chunks. The full layout of `fdAT` and `fcTL` chunks is described below. 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. 4.5 Chunk Sequence Numbers The `fcTL` and `fdAT` chunks have a 4 byte sequence number. Both chunk types share the sequence. The purpose of this number is to detect (and optionally correct) sequence errors in an Animated PNG, since the PNG specification does not impose ordering restrictions on ancillary chunks. The first `fcTL` chunk must contain sequence number 0, and the sequence numbers in the remaining `fcTL` and `fdAT` chunks must be in order, with no gaps or duplicates. The tables below illustrates the use of sequence numbers for images with more than one frame and more than one `fdAT` chunk. If the default image is the first frame: Sequence number Chunk 0 `fcTL` first frame (before `IDAT`) 1 `fcTL` second frame 2 first `fdAT` for second frame 3 second `fdAT` for second frame .... If the default image is not part of the animation: Sequence number Chunk 0 `fcTL` first frame 1 first `fdAT` for first frame 2 second `fdAT` for first frame .... Decoders must treat out-of-order APNG chunks as an error. APNG-aware PNG editors should restore them to correct order using the sequence numbers. 4.6 `acTL`: The Animation Control Chunk The `acTL` chunk is an ancillary chunk as defined in the PNG Specification. It must appear before the first `IDAT` chunk within a valid PNG stream. The `acTL` chunk contains: byte 0 num_frames (unsigned int) Number of frames 4 num_plays (unsigned int) Number of times to loop this APNG. 0 indicates infinite looping. `num_frames` indicates the total number of frames in the animation. This must equal the number of `fcTL` chunks. 0 is not a valid value. 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. `num_plays` 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. 4.7 `fcTL`: The Frame Control Chunk The `fcTL` chunk is an ancillary chunk as defined in the PNG Specification. It must appear before the `IDAT` or `fdAT` chunks of the frame to which it applies, specifically: * For the default image, if a `fcTL` chunk is present it must appear before the first `IDAT` chunk. Position relative to the `acTL` chunk is not specified. * For the first frame excluding the default image (which may be either the first or second frame), the `fcTL` chunk must appear after all `IDAT` chunks and before the `fdAT` chunks for the frame. * For all subsequent frames, the `fcTL` chunk for frame N must appear after the `fdAT` chunks from frame N-1 and before the `fdAT` chunks for frame N. * Other ancillary chunks are allowed to appear among the APNG chunks, including between `fdAT` chunks. Exactly one `fcTL` chunk is required for each frame. Format: byte 0 sequence_number (unsigned int) Sequence number of the animation chunk, starting from 0 4 width (unsigned int) Width of the following frame 8 height (unsigned int) Height of the following frame 12 x_offset (unsigned int) X position at which to render the following frame 16 y_offset (unsigned int) Y position at which to render the following frame 20 delay_num (unsigned short) Frame delay fraction numerator 22 delay_den (unsigned short) Frame delay fraction denominator 24 dispose_op (byte) Type of frame area disposal to be done after rendering this frame 25 blend_op (byte) Type of frame area rendering for this frame The frame must be rendered within the region defined by `x_offset`, `y_offset`, `width`, and `height`. The offsets must be non-negative, the dimensions must be positive, and the region may not fall outside of the default image. Constraints on frame regions: `x_offset` >= 0 `y_offset` >= 0 `width` > 0 `height` > 0 `x_offset` + `width` <= `IHDR` width `y_offset` + `height` <= `IHDR` height The `delay_num` and `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 `dispose_op` are: value 0 APNG_DISPOSE_OP_NONE 1 APNG_DISPOSE_OP_BACKGROUND 2 APNG_DISPOSE_OP_PREVIOUS * APNG_DISPOSE_OP_NONE: no disposal is done on this frame before rendering the next; the contents of the output buffer are left as is. * APNG_DISPOSE_OP_BACKGROUND: the frame's region of the output buffer is to be cleared to fully transparent black before rendering the next frame. * APNG_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 `fcTL` chunk uses a `dispose_op` of APNG_DISPOSE_OP_PREVIOUS it should be treated as APNG_DISPOSE_OP_BACKGROUND. `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 `blend_op` are: value 0 APNG_BLEND_OP_SOURCE 1 APNG_BLEND_OP_OVER If `blend_op` is APNG_BLEND_OP_SOURCE all color components of the frame, including alpha, overwrite the current contents of the frame's output buffer region. If `blend_op` is APNG_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 Variation 2 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. The `fcTL` chunk corresponding to the default image, if it exists, has these restrictions: * The `x_offset` and `y_offset` fields must be 0. * The `width` and `height` fields must equal the corresponding fields from the `IHDR` chunk. 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. 4.8 `fdAT`: The Frame Data Chunk The `fdAT` chunk has the same purpose as an `IDAT` chunk. It has the same structure as an `IDAT` chunk, except preceded by a sequence number. At least one `fdAT` chunk is required for each frame. The compressed datastream is then the concatenation of the contents of the data fields of all the `fdAT` chunks within a frame. When decompressed, the datastream is the complete pixel data of a PNG image, including the filter byte at the beginning of each scanline, similar to the uncompressed data of all the `IDAT` chunks. It utilizes the same bit depth, color type, compression method, filter method, interlace method, and palette (if any) as the default image. Format: byte 0 sequence_number (unsigned int) Sequence number of the animation chunk, starting from 0 4 frame_data X bytes Frame data for this frame Each frame inherits every property specified by any critical or ancillary chunks before the first `IDAT` in the file, except the width and height, which come from the `fcTL` chunk. 4.9 Interaction with other PNG chunks If the PNG `pHYs` chunk is present, the APNG images and their `x_offset` and `y_offset` values must be scaled in the same way as the main image. Conceptually, such scaling occurs while mapping the output buffer onto the canvas. 4.10 Test Encoder/Decoder and Sample Images A test implementation in Mozilla is available at https://bugzilla.mozilla.org/show_bug.cgi?id=257197 Sample images are available from the APNG implementation page at http://littlesvr.ca/apng/ D. Add to the Appendix: Revision History * 30 April 2007 (version 1.4.0): * Added the acTL, fcTL, and fdAT animation chunks. D. Add to the References [GIF-SPEC] "Graphics Interchange Format 89a" available at http://www.w3.org/Graphics/GIF/spec-gif89a.txt E. Add to the Credits * Stuart Parmenter, pavlov@pavlov.net * Andrew Smith, asmith15@learn.senecac.on.ca * Vladimir Vukicevic, vladimir@pobox.com