Honor RGBA channel count in Pillow resize goldens - #30
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Golden loading mapped every non-L mode to 3 channels, so regenerated RGBA cases from eval/gen-pillow-goldens.py failed length checks and never exercised 4-channel pillowResize. The browser CF path always resizes getImageData pixels. Prefer the stored channels field, map RGBA to 4, and add a compact RGBA golden to the committed subset. Co-authored-by: felirami <felirami@users.noreply.github.com>
Co-authored-by: felirami <felirami@users.noreply.github.com>
Pillow.Image.resize on RGBA premultiplies alpha, so random-alpha goldens cannot match the independent-channel browser CF path. Resize each RGBA band with BICUBIC and store a compact 4-channel golden in the committed subset. Co-authored-by: felirami <felirami@users.noreply.github.com>
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August 15, 2026 22:22
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Summary
The Pillow golden loader treated every non-
Lmode as 3 channels. Regenerating witheval/gen-pillow-goldens.pywritesprod_down_rgbaasmode: RGBAandchannels: 4, so the length check failed (3145728vs2359296) and 4-channelpillowResizewas never byte-checked. The browser CF path always resizesgetImageDataRGBA pixels.A second issue showed up after the loader fix:
Image.resizeon RGBA converts to premultipliedRGBafirst, so random-alpha goldens cannot match independent-channelpillowResize. The generator now resizes each RGBA band with BICUBIC, which is the extension contract.Changes
goldenChannels()prefers the storedchannelsfield and mapsRGBAto 4. Older committed RGB/L entries withoutchannelsstill load frommode.noise()cases do not shift.compact_rgba(160x120 to 147x110, Pillow 12.3.0).Evidence
Reproduced against a regenerated
prod_down_rgbagolden before the fix: input length1024*768*4 = 3145728, loader expected1024*768*3 = 2359296.After the fix:
tests/pixel-resize.test.mjs19/19 pass, includingcompact_rgbaPIXEL_RESIZE_GOLDENSon the full 21-case regen (Pillow 12.3.0): 35/35 pass, includingprod_down_rgba(1024x768x4 to 587x440)npm test: 217/217 passPublic fixtures are directional only and are not a claim about Kenny's private held-out set.