[FFmpeg-devel] Lossy GIF encoding

Tomas Härdin tjoppen at acc.umu.se
Sun Feb 17 23:16:37 EET 2019

fre 2019-02-15 klockan 16:45 +0000 skrev Kornel:
> > > Alternatively, would it be OK to create a separate codec just for
> > > lossy compression? I would create a new GIF codec, define it under a
> > > new name, and declare it always takes AV_PIX_FMT_RGB24 or
> > > AV_PIX_FMT_RGBA. Is that a good approach?
> > 
> > This sounds more like something that belongs in libswscale (RGBA ->
> > PAL8 conversion) or could be done with a filter. An option like
> > ImageMagick's -fuzz could be added to the PAL8 quantizer in sws
> Unfortunately, the lossy technique depends on a layering violation. 
> The lossy compression alters each pixel in the image based on
> current, precise state of the LZW dictionary. Without doing both LZW
> and remapping at the same time I don't have information required to
> make the image optimally compressible.
> At best I could simulate LZW compression in swscale just for the
> purpose of remapping/dithering filter, and then expect the GIF codec
> to LZW-compress the remapped image again. However, that approach
> doesn't work in practice: the second LZW encoder pass ends up making
> different decisions than the first one, falls out of sync with the
> optimal lossy pattern, and ends up creating a large file.
> Given the constraint of true-color input and LZW integrated in one
> algorithm, where's the best place to put that code?

Aha. Yeah then a different encoder would be the appropriate solution.
It's neat that you can do this with LZW


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