Drop ImageMagick in favor of PIL

[tfuxu]

Moving from Wand (wrapper for ImageMagick) to Pillow would allow me to utilize hitherdither (see the comment below) library, which contains additional dithering algorithms and custom image palettes. As an added bonus, the packaging will become probably more straightforward, and the performance will improve¹.

Footnotes

1. Pillow Performance site ↩

[tfuxu]

Currently the plan is to instead use dither-go library, which contains more dithering algorithms, is much faster and according to original dither description: "is uniquely correct from a math and quality perspective".