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) Image.convert("P", palette=ADAPTIVE, colors=N) built the palette via quantize(), which maps each pixel to the nearest palette entry without dithering, then returned early -- silently dropping the dither argument. Build the adaptive palette first, then re-convert with the requested dither so Floyd-Steinberg is applied. Add a regression test asserting NONE vs FLOYDSTEINBERG produce different histograms.
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Summary
Image.convert("P", palette=ADAPTIVE, colors=N) silently dropped the
dither argument: quantize() maps each pixel to the nearest palette entry
without dithering, then the function returned early.
Fix: build the adaptive palette via quantize(), then re-convert with the
requested dither so Floyd-Steinberg is applied.
Fixes #5836
Testing
Added Tests/test_image_convert.py::test_adaptive_convert_honors_dither
asserting NONE vs FLOYDSTEINBERG produce different histograms for an
ADAPTIVE conversion.
Checklist
convertandquantize#5836)