I have some questions about lossless compression for webp.

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peiwei zhang

Dec 9, 2023, 8:27:14 AM12/9/23
to WebP Discussion
Hi there,
Recently I have just started learning lossless compression for webp. I don't understand the process of transform and entropy coding. I wonder if webp, in lossless compression, does the whole image have the same transform and entropy encoding? Or is it to block different areas of the image and adopt different transform and entropy coding forms? I was reading the EncoderAnalyze() function and didn't see a specific chunk, so I was skeptical. If it has a different transform and entropy encoding using different blocks, please let me know where I can see it and what the block size is. Thank you for your help.
Thanks again

peiwei zhang

Dec 9, 2023, 8:31:24 AM12/9/23
to WebP Discussion, peiwei zhang
And If the webp exists block in EncoderAnalyze(), when the image is encoding, does it use blocks?  

Vincent Rabaud

Dec 11, 2023, 10:08:09 AM12/11/23
to webp-d...@webmproject.org, peiwei zhang
The transforms, as defined in the specification at https://chromium.googlesource.com/webm/libwebp/+/refs/tags/v1.3.2/doc/webp-lossless-bitstream-spec.txt, are defined on the whole image. There is a notion of blocks though which applies to sub transforms. E.g., there can be a color transform, with parameters that are different depending on the block.

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