> -----Original Message----- > From: ffmpeg-devel <ffmpeg-devel-boun...@ffmpeg.org> On Behalf Of > Clément Bœsch > Sent: Monday, October 31, 2022 1:30 AM > To: FFmpeg development discussions and patches <ffmpeg- > de...@ffmpeg.org> > Subject: Re: [FFmpeg-devel] [PATCH] Revert > "avfilter/vf_palette(gen|use): support palettes with alpha" > > On Sun, Oct 30, 2022 at 10:55:31PM +0000, Soft Works wrote: > [...] > > > Do you think it might make sense to put more weight on the > > alpha value by tripling it? So it would be weighted equally to the > > RGB value? > > You cannot mix alpha with colors at all, they are separate domains > and you > need to treat them as such. > > From paletteuse perspective what you need to do is first choose the > colors > in the palette that match exactly the alpha (or at least the closest > if > and only there is no exact match). Then within that set, and only > within > that one, you'd pick the closest color. > > From palettegen perspective, you need to split the colors in > different > transparency domain (a first dimensional quantization), then quantize > the > colors in each quantized alpha dimension. And when you have all your > quantized palettes for each level of alpha, you find an algorithm to > reduce the number of transparency dimensions or the number of colors > per > dimension to make it fit inside a single palette. But you can't just > do > the alpha and the colors at the same time, it cannot work, whatever > weights you choose.
Interestingly, pngquant which is supposed to have the best open source quantization algorithms seems to be using weights (albeit in a more sophisticated way) and does not handle alpha separately for calculating color distance, variance and averaging: https://github.com/ImageOptim/libimagequant/blob/a16c9ca66a24158496da02d86925cc0167831205/pam.h#L163-L182 https://github.com/ImageOptim/libimagequant/blob/a16c9ca66a24158496da02d86925cc0167831205/mediancut.c#L29-L49 https://github.com/ImageOptim/libimagequant/blob/a16c9ca66a24158496da02d86925cc0167831205/mediancut.c#L449-L476 softworkz _______________________________________________ ffmpeg-devel mailing list ffmpeg-devel@ffmpeg.org https://ffmpeg.org/mailman/listinfo/ffmpeg-devel To unsubscribe, visit link above, or email ffmpeg-devel-requ...@ffmpeg.org with subject "unsubscribe".
