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webp-lossless-bitstream-spec: clarify image width changes

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After processing the Color Indexing Transform the image width is
subsampled by the transform's width_bits.

Bug: webp:611
Change-Id: If8d2d47dbe05e5c8085fb3fb4c1b63552be9028b
This commit is contained in:
James Zern 2023-07-14 15:03:09 -07:00
parent ee72299796
commit 84628e5611
1 changed files with 8 additions and 0 deletions
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8
doc/webp-lossless-bitstream-spec.txt
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@ -584,6 +584,14 @@ The values are packed into the green component as follows:
value at x / 8, and green values at x + 1 to x + 7 are positioned in order
to the more significant bits of the green value at x / 8.
After reading this transform, `image_width` is subsampled by `width_bits`. This
affects the size of subsequent transforms. The new size can be calculated using
`DIV_ROUND_UP` as defined [earlier](#predictor-transform).
```
image_width = DIV_ROUND_UP(image_width, 1 << width_bits);
```
5 Image Data
------------