Don't touch images already handled by pngquant about pngquant

[kornelski]

Ask Slicy to integrate pngquant library :)

If you run pngquant on image that is already paletted it won't degrade it (except when you use --quality option to deliberately lower the quality).

It will recompress it, so md5 might change.

[rolftimmermans]

It would actually be pretty useful to have an option like --quality that only applies when converting true colour images to palettes. Then when images with palettes are processed the number of colours is not reduced (i.e. maximum quality).

[kornelski]

I can't elegantly check whether file has been processed with pngquant already (I don't want to add any extra chunks to PNG files or keep database of them).

What I think would be useful is to ignore _.fs8.png and *.or8.png files when their *.png version is specified as argument. This way pngquant *.png run multiple times wouldn't create _.fs8.fs8.fs8.fs8.png etc.

@rolftimmermans that's an interesting suggestion. But what if somebody had 256-color images and wanted to reduce them further? e.g. down to 64 colors. For grayscale images or transparency masks that's reasonable optmization.

[rolftimmermans]

@pornel That's a reasonable scenario, but I guess then you would use the regular quality option. Anyway, ultimately I the best solution is probably to keep track of which images were compressed before calling pngquant and not relying on it to guess it for you.

[kornelski]

--skip-if-larger should handle that well enough now.