rfc9649v2.txt   rfc9649.txt 
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For an animated image, this chunk contains the _global parameters_ of For an animated image, this chunk contains the _global parameters_ of
the animation. the animation.
Background Color: 32 bits (_uint32_) Background Color: 32 bits (_uint32_)
The default background color of the canvas in [Blue, Green, Red, The default background color of the canvas in [Blue, Green, Red,
Alpha] byte order. This color MAY be used to fill the unused Alpha] byte order. This color MAY be used to fill the unused
space on the canvas around the frames, as well as the transparent space on the canvas around the frames, as well as the transparent
pixels of the first frame. The background color is also used pixels of the first frame. The background color is also used
when the Disposal method is 1. when the Disposal method is 1.
Note: Notes:
* The background color MAY contain a nonopaque alpha value, even * The background color MAY contain a nonopaque alpha value, even
if the _Alpha_ flag in the 'VP8X' Chunk (Figure 7) is unset. if the _Alpha_ flag in the 'VP8X' Chunk (Figure 7) is unset.
* Viewer applications SHOULD treat the background color value as * Viewer applications SHOULD treat the background color value as
a hint and are not required to use it. a hint and are not required to use it.
* The canvas is cleared at the start of each loop. The * The canvas is cleared at the start of each loop. The
background color MAY be used to achieve this. background color MAY be used to achieve this.
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* Raw data: This consists of a byte sequence of length = width * * Raw data: This consists of a byte sequence of length = width *
height, containing all the 8-bit transparency values in scan height, containing all the 8-bit transparency values in scan
order. order.
* Lossless format compression: The byte sequence is a compressed * Lossless format compression: The byte sequence is a compressed
image-stream (as described in Section 3) of implicit dimensions image-stream (as described in Section 3) of implicit dimensions
width x height. That is, this image-stream does NOT contain any width x height. That is, this image-stream does NOT contain any
headers describing the image dimensions. headers describing the image dimensions.
Rationale: The dimensions are already known from other sources, so | Rationale: The dimensions are already known from other sources,
storing them again would be redundant and prone to errors. | so storing them again would be redundant and prone to errors.
Once the image-stream is decoded into Alpha, Red, Green, Blue Once the image-stream is decoded into Alpha, Red, Green, Blue
(ARGB) color values, following the process described in the (ARGB) color values, following the process described in the
lossless format specification, the transparency information must lossless format specification, the transparency information must
be extracted from the green channel of the ARGB quadruplet. be extracted from the green channel of the ARGB quadruplet.
Rationale: The green channel is allowed extra transformation steps | Rationale: The green channel is allowed extra transformation
in the specification -- unlike the other channels -- that can | steps in the specification -- unlike the other channels -- that
improve compression. | can improve compression.
2.7.1.3. Bitstream (VP8/VP8L) 2.7.1.3. Bitstream (VP8/VP8L)
This chunk contains compressed bitstream data for a single frame. This chunk contains compressed bitstream data for a single frame.
A bitstream chunk may be either (i) a 'VP8 ' Chunk, using 'VP8 ' A bitstream chunk may be either (i) a 'VP8 ' Chunk, using 'VP8 '
(note the significant fourth-character space) as its FourCC, _or_ (note the significant fourth-character space) as its FourCC, _or_
(ii) a 'VP8L' Chunk, using 'VP8L' as its FourCC. (ii) a 'VP8L' Chunk, using 'VP8L' as its FourCC.
The formats of' VP8 ' and 'VP8L' Chunks are as described in Sections The formats of' VP8 ' and 'VP8L' Chunks are as described in Sections
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