[FFmpeg-devel] Scaling PAL8 images with alpha

[Michael Niedermayer]

On Sat, Sep 25, 2021 at 03:46:25PM +0000, Soft Works wrote:
> 
> 
> > -----Original Message-----
> > From: ffmpeg-devel <ffmpeg-devel-bounces at ffmpeg.org> On Behalf Of
> > Michael Niedermayer
> > Sent: Saturday, 25 September 2021 16:30
> > To: FFmpeg development discussions and patches <ffmpeg-
> > devel at ffmpeg.org>
> > Subject: Re: [FFmpeg-devel] Scaling PAL8 images with alpha
> > 
> > On Sat, Sep 25, 2021 at 10:23:56AM +0200, Hendrik Leppkes wrote:
> > > 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.
> > 
> > +1
> > 
> > i see no reason why this would need an external dependancy
> > we have filters implementing much more complex things than scaling a
> > pal8
> > image
> 
> I dislike GPL3 as well and needing a library for this might 
> seem to be a bit too much for solving the problem.
> I'm just not sure whether it can be solved easily. I mean it
> can easily be solved in a basic way, but I'm not sure whether
> it would be possible to achieve the same quality in results.
> 
> I've put up here some examples and also a comparison of results
> from palettegen+paletteuse and Photoshop(which I assume to 
> be at the same level of quality like libimagequant).
> 
> https://gist.github.com/softworkz/deef5c2a43d3d629c3e17f9e21544a8f

is this comparing RGB vs RGBA palettes ?
I would imagine that for smooth edges of subtitles you need partly
transparent pixels. I am not sure palettegen supports that

please fix palettegen if that is the issue. Limiting a palette to
0% transparent and having 1 100% transparent color hardcoded may be the
problem here. Try handling alpha the same way red / green / blue is handled

Anyway i do not buy that palette generation is a super hard problem.

thx

[...]
-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

The smallest minority on earth is the individual. Those who deny 
individual rights cannot claim to be defenders of minorities. - Ayn Rand
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