fix: Prevent color corruption when mixing local and global GIF palettes #256
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remiolivier-discord
approved these changes
Jun 3, 2025
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Summary
Fixes a color corruption issue in animated GIFs where frames with local color palettes would display incorrect colors in Discord and other viewers.
Problem
The issue was introduced in commit 52ba7f4 where transparency optimization logic incorrectly compared color indices from different color maps:
gcb.TransparentColorrefers to an index in the current frame's color map (local or global)e->gif->SBackGroundColorrefers to an index in the global color mapWhen a frame used a local color map, these indices could point to completely different colors, but the code incorrectly assumed they referenced the same palette. This caused transparency to be incorrectly removed, leading to visible color corruption.
Solution
Added a check
!e->frame_color_mapto only perform the transparency optimization when using the global color map, preventing index mismatches between local and global palettes.Testing
Files Changed
giflib.cpp: Added palette type check in transparency optimization logicResolves the regression reported by Discord users where GIFs processed by Lilliput showed "colors completely warped, flashing wildly" during playback.