[FFmpeg-devel] Scaling PAL8 images with alpha

[Soft Works]

> -----Original Message-----
> From: ffmpeg-devel <ffmpeg-devel-bounces at ffmpeg.org> On Behalf Of
> Michael Niedermayer
> Sent: Saturday, 25 September 2021 22:47
> 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 10:34:32PM +0200, Michael Niedermayer wrote:
> > 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

I already did that and it improved the output a lot. Not as good 
as PS, but it's OK for that simple font example.
(I have updated the Gist and added the yellow-font result from 
the changed filter) 

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

Not anymore in the year 2021 as the algorithms are known and established,
but there are still many image editing programs that can't do it well.

Neither does the palettegen implementation btw.

I have added another conversion example to the Gist, comparing 
the results from PS, PngQuant, paletteuse/gen (normal +
patched for alpha support).

Kind regards,
softworkz