[FFmpeg-trac] #4444(swscale:new): Transparency gets lost when scaling pal8

[FFmpeg]

#4444: Transparency gets lost when scaling pal8
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             Reporter:  Carl Eugen   |                    Owner:  (none)
  Hoyos                              |
                 Type:  defect       |                   Status:  new
             Priority:  normal       |                Component:  swscale
              Version:  git-master   |               Resolution:
             Keywords:  pal8 alpha   |               Blocked By:
             Blocking:               |  Reproduced by developer:  0
Analyzed by developer:  0            |
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Comment (by Niklas Haas):

 So, the fundamental problem here is that swscale does not support "true"
 PAL8 output. Instead a hack is performed by converting instead to BGR8 and
 then just tagging that as PAL8 (using avpriv_set_systematic_pal2).

 The proper handling of paletted output would require generating a suitable
 palette and then dithering the image using that set of colors. (See
 vf_palettegen/paletteuse)

 With the new graph dispatch infrastructure, it would be fairly
 straightforward to incorporate those filters as extra output passes,
 although it will require moving some code around. However, I propose
 simply dropping the "systematic" PAL8 output hack for now, at least in the
 case of inputs with an alpha channel.
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Ticket URL: <https://trac.ffmpeg.org/ticket/4444#comment:1>
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