[FFmpeg-devel] Scaling PAL8 images with alpha

[Paul B Mahol]

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?
>
>
I'm against more GPLv3 dependency in lavfi.


> Kind regards,
> softworkz
>
>
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