> -----Original Message----- > From: ffmpeg-devel <ffmpeg-devel-boun...@ffmpeg.org> On Behalf Of > Soft Works > Sent: Friday, 24 September 2021 19:03 > To: FFmpeg development discussions and patches <ffmpeg- > de...@ffmpeg.org> > Subject: Re: [FFmpeg-devel] Scaling PAL8 images with alpha > > > > > -----Original Message----- > > From: ffmpeg-devel <ffmpeg-devel-boun...@ffmpeg.org> On Behalf Of > Michael > > Niedermayer > > Sent: Friday, 24 September 2021 17:40 > > To: FFmpeg development discussions and patches <ffmpeg- > de...@ffmpeg.org> > > Subject: Re: [FFmpeg-devel] Scaling PAL8 images with alpha > > > > On Fri, Sep 24, 2021 at 10:30:31AM +0000, Soft Works wrote: > > > Hi, > > > > > > for a new filter, I want to rescale PAL8 subtitle bitmaps where > the palette > > includes > > > colors with alpha. > > > > > > From what I’ve seen, swscale doesn’t support PAL8-to-PAL8, only > PAL8-to- > > BGR8 > > > which doesn’t support alpha and the palette is fixed with 256 > entries > > defined by > > > convention, while I would ideally like to be able to allow the > following: > > > > > > - constrain the output to use the same palette as the input > > > - adaptively quantize it to a palette with a configurable number > of colors > > > > > > Thus it's about the palette quantization (with or without > dithering) after > > > scaling in RGBA. (or some cool algorithmic trick I'm not aware > of) > > > > > > Is there any existing code that I could reuse? The closest I > could find > > > is pngenc, but I'm wondering whether there's something > else/better > > > somewhere in the ffmpeg libs that I haven't seen? > > > > There are some non linear scaling filters which may make sense when > you do > > not have the full linear space available see: > > hqx, epx, xbr > > you could also try some neural net stuff > > Photoshop can perfectly do what is needed, but their algorithms are > not > public.
I've put a few thing together to illustrate what I'm talking about: https://gist.github.com/softworkz/deef5c2a43d3d629c3e17f9e21544a8f Meanwhile I found what I need: https://github.com/ImageOptim/libimagequant Interestingly, they are comparing their lib specifically to the Photoshop feature that I mentioned: https://pngquant.org/vsphotoshop.html In the 2000's, the PS implementation for image quantization, optimization and compression (profanely named "Save for Web") had remained unparalleled for years, that's why it was one of my first thoughts. I hadn't heard about libimagequant before, but it seems to do exactly what is needed for rescaling PAL8 to PAL8 at a high quality. Before spending any time on it, I wanted ask whether that library would be acceptable for adding it as a (GPLv3) reference to the project and as a dependency to my filter? Kind regards, 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".
