Inverse transform in Decoder

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Ritwik Ghosal

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Nov 5, 2023, 6:42:14 AM11/5/23
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Suppose an image just uses Color indexing transfoirm.
While going through the decoder I found that the Inverse transform is being applied inside the 'ProcessRows" function but the processrows function is getting called after each 'copy'-type backreference is seen. 
As a result the array responsible for storing the inverse trasnformed pixel ,'argb_cache_' gets updated for each new copy type reference.

Q.1) where does exactly the inverse transformed pixels stored ..as the argb_cache_ gets updated for each copy type reference?
  
Q.2)  And related to the above question, what is the pixel data used to write a decoded webp image to png image ?

Vincent Rabaud

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Nov 6, 2023, 7:37:23 AM11/6/23
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Hi,

ProcessRows is called whenever a row is finished and it is a multiple of NUM_ARGB_CACHE_ROWS, that includes after a 'copy'-type backreference, cf https://chromium.googlesource.com/webm/libwebp/+/24d7f9cb6ef1ef90a04d7b6c15d3477813f75ee0/src/dec/vp8l_dec.c#1237


Vincent


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