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The ZopfliPNG pass re-encodes the default image, possibly with a new color type or bit depth (e.g. RGBA -> palette), and then copies the remaining chunks back unchanged. For an APNG those include the fdAT frames, which are still encoded in the original color type, so every frame after the first decodes to garbage. With -strip the animation chunks are not copied back and the APNG becomes a still image. OptiPNG already avoids reductions for APNGs, so skip the ZopfliPNG pass for files with an acTL chunk before IDAT. Other PNGs are unchanged (byte-identical output). Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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ECT breaks animated PNGs. The ZopfliPNG pass may reduce the default image (e.g. RGBA to palette) and then copies the
fdATchunks back unchanged, so every frame after the first decodes to garbage. With-stripthe animation is dropped instead.OptiPNG already skips reductions for APNGs, so this makes
Zopflipng()skip files with anacTLchunk. APNGs now keep all frames in every mode; above-1they are otherwise left as they are. Regular PNGs give byte-identical output to master.Tested with a 3-frame APNG in
-1,-3,-5 --strict,-9,--allfiltersand with-strip, and with truncated files in a hardened libc++ build.🤖 Generated with Claude Code