[FFmpeg-devel] Scaling PAL8 images with alpha

[Soft Works]

> -----Original Message-----
> From: ffmpeg-devel <ffmpeg-devel-bounces at ffmpeg.org> On Behalf Of
> Soft Works
> Sent: Friday, 24 September 2021 19:03
> To: FFmpeg development discussions and patches <ffmpeg-
> devel at ffmpeg.org>
> Subject: Re: [FFmpeg-devel] Scaling PAL8 images with alpha
> 
> 
> 
> > -----Original Message-----
> > From: ffmpeg-devel <ffmpeg-devel-bounces at ffmpeg.org> On Behalf Of
> Michael
> > Niedermayer
> > Sent: Friday, 24 September 2021 17:40
> > To: FFmpeg development discussions and patches <ffmpeg-
> devel at 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