Transparency gets lost when using palettegen / paletteuse

[Carl Eugen Hoyos]

When using palettegen / paletteuse to encode to pal8, transparency gets lost.

$ ffmpeg -i alpha.png -vf palettegen pal.png
ffmpeg version N-71320-gc4b2017 Copyright (c) 2000-2015 the FFmpeg developers
  built with gcc 4.7 (SUSE Linux)
  configuration: --enable-gpl
  libavutil      54. 22.100 / 54. 22.100
  libavcodec     56. 34.100 / 56. 34.100
  libavformat    56. 29.100 / 56. 29.100
  libavdevice    56.  4.100 / 56.  4.100
  libavfilter     5. 13.101 /  5. 13.101
  libswscale      3.  1.101 /  3.  1.101
  libswresample   1.  1.100 /  1.  1.100
  libpostproc    53.  3.100 / 53.  3.100
Input #0, png_pipe, from 'alpha.png':
  Duration: N/A, bitrate: N/A
    Stream #0:0: Video: png, pal8, 21x21 [SAR 5:6 DAR 5:6], 25 tbr, 25 tbn, 25 tbc
Output #0, apng, to 'pal.png':
  Metadata:
    encoder         : Lavf56.29.100
    Stream #0:0: Video: apng, rgba, 16x16 [SAR 1:1 DAR 1:1], q=2-31, 200 kb/s, 25 fps, 25 tbn, 25 tbc
    Metadata:
      encoder         : Lavc56.34.100 apng
Stream mapping:
  Stream #0:0 -> #0:0 (png (native) -> apng (native))
Press [q] to stop, [?] for help
[Parsed_palettegen_0 @ 0x35ec900] 4(+1) colors generated out of 4 colors; ratio=1.000000
[apng @ 0x35f9040] Only a single frame so saving as a normal PNG.
frame=    1 fps=0.0 q=0.0 Lsize=       0kB time=00:00:00.04 bitrate=  24.2kbits/s
video:0kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 30.107527%

$ ffmpeg -i alpha.png -i pal.png -lavfi paletteuse out.gif
ffmpeg version N-71320-gc4b2017 Copyright (c) 2000-2015 the FFmpeg developers
  built with gcc 4.7 (SUSE Linux)
  configuration: --enable-gpl
  libavutil      54. 22.100 / 54. 22.100
  libavcodec     56. 34.100 / 56. 34.100
  libavformat    56. 29.100 / 56. 29.100
  libavdevice    56.  4.100 / 56.  4.100
  libavfilter     5. 13.101 /  5. 13.101
  libswscale      3.  1.101 /  3.  1.101
  libswresample   1.  1.100 /  1.  1.100
  libpostproc    53.  3.100 / 53.  3.100
Input #0, png_pipe, from 'alpha.png':
  Duration: N/A, bitrate: N/A
    Stream #0:0: Video: png, pal8, 21x21 [SAR 5:6 DAR 5:6], 25 tbr, 25 tbn, 25 tbc
Input #1, png_pipe, from 'pal.png':
  Duration: N/A, bitrate: N/A
    Stream #1:0: Video: png, rgba, 16x16, 25 tbr, 25 tbn, 25 tbc
Output #0, gif, to 'out.gif':
  Metadata:
    encoder         : Lavf56.29.100
    Stream #0:0: Video: gif, pal8, 21x21 [SAR 5:6 DAR 5:6], q=2-31, 200 kb/s, 25 fps, 100 tbn, 25 tbc (default)
    Metadata:
      encoder         : Lavc56.34.100 gif
Stream mapping:
  Stream #0:0 (png) -> paletteuse:default
  Stream #1:0 (png) -> paletteuse:palette
  paletteuse -> Stream #0:0 (gif)
Press [q] to stop, [?] for help
frame=    1 fps=0.0 q=0.0 Lsize=       1kB time=00:00:00.04 bitrate= 185.0kbits/s
video:1kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: unknown

[LordHDL]

Would like to see this tackled. Other command line tools for image processing have been problematic for me.

[llogan]

Consider offering a bounty. Developers are just volunteers and have limited time (and sometimes motivation, interest, etc). A bounty *may* help provide incentive if you are impatient.

And of course patches are always welcome from anyone.

[skavish]

anybody is willing to fix it? $500 for the fix.
thanks!

[bjorn]

I'm going to look into this and see if I can submit a patch. I'm new to FFMPEG, so if anyone has any hints about what files to look at or anything that would be helpful.

[Carl Eugen Hoyos]

You only have to look at paletteuse, afair, palettegen supports transparency.

[bjorn]

I've got a fix for this that handles stills, but there's a bug in the gif codec that doesn't handle the resulting transparencies correctly when animating, so animated gifs coming out of this are not correct. I'm going to try to work on that this weekend.

@skavish: what are you looking for for the bounty?

[Carl Eugen Hoyos]

This ticket was fixed by Bjorn Roche in aba926e7d654b6dfab680130c92e571ee9a956f0 (and subsequent commits by Clément), I opened ticket #6813 for a gif encoding issue.