[FFmpeg-devel] Scaling PAL8 images with alpha

[Hendrik Leppkes]

On Sat, Sep 25, 2021 at 5:00 AM Soft Works <softworkz at hotmail.com> wrote:
>
>
>
> > -----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?
>

External dependencies for what is supposed to be a rather base feature
are not ideal.
I know people have done some work in the past for palette generation
for GIF encoding, which looked like it was generating decent images.
Ideally that should be used and improved if necessary.

- Hendrik