Disable/reduce gif quantization?

[sudofox]

Input image: https://www.hatena.ne.jp/images/what/what-riyo-img.gif

255 colors in image

Output image: https://images.weserv.nl/?url=https://www.hatena.ne.jp/images/what/what-riyo-img.gif

254 colors in image. Visible degradation in the frog. I like quantization, but I can't find any option to change how it behaves for GIFs. (also the fact that it didn't just chose one of the lesser-used colors while quantizing bugs me slightly)

[kleisauke]

It looks like ImageMagick applies by default a Riemersma dither during GIF encoding.
https://github.com/ImageMagick/ImageMagick6/blob/94026fbfb045d27576799d84ee63320ba7cbc7aa/magick/quantize.c#L2260-L2261

Dithers introduces by definition distributed distortion, but you're right, there's currently no way to change it (e.g. using Floyd-Steinberg dithering) or to turn if off (which could be useful for GIF to GIF processing). This requires adding an additional parameter in vips_magicksave that controls the dithering.

As for this particular image, I was able to reproduce this with the ImageMagick's convert tool, so maybe it's a bug there.

# Convert to PNG first to ensure *magick enables dithering
$ convert x.gif x.png
# Convert the PNG to GIF with a various set of dithers
$ convert x.png -dither None none.gif
$ convert x.png -dither Riemersma riemersma.gif
$ convert x.png -dither FloydSteinberg floyd-steinberg.gif

none.gif	riemersma.gif	floyd-steinberg.gif

[kleisauke]

We're now A/B testing libvips 8.12 in production, which includes a new gifsave operation that doesn't use ImageMagick for saving GIF images. This particular quantization bug seems to be fixed now, but please report if you see anything strange.

There's some discussion to add indexed read/write support to libvips. This would avoid unpacking to RGB(A) and re-quantization for GIF images that are processed without performing any operations on the image (like this example), but it's not there yet.