Encode transparency to flv1

[Carl Eugen Hoyos]

​http://thread.gmane.org/gmane.comp.video.ffmpeg.user/48140

$ ffmpeg -i tests/lena.pnm -pix_fmt yuva420p out.flv
ffmpeg version N-56801-g286beeb Copyright (c) 2000-2013 the FFmpeg developers
  built on Oct  2 2013 11:26:12 with gcc 4.3 (SUSE Linux)
  configuration: --enable-gpl
  libavutil      52. 46.100 / 52. 46.100
  libavcodec     55. 33.101 / 55. 33.101
  libavformat    55. 18.104 / 55. 18.104
  libavdevice    55.  3.100 / 55.  3.100
  libavfilter     3. 88.100 /  3. 88.100
  libswscale      2.  5.100 /  2.  5.100
  libswresample   0. 17.103 /  0. 17.103
  libpostproc    52.  3.100 / 52.  3.100
Input #0, image2, from 'tests/lena.pnm':
  Duration: 00:00:00.04, start: 0.000000, bitrate: N/A
    Stream #0:0: Video: ppm, rgb24, 256x256, 25 tbr, 25 tbn, 25 tbc
Incompatible pixel format 'yuva420p' for codec 'flv', auto-selecting format 'yuv420p'
Output #0, flv, to 'out.flv':
  Metadata:
    encoder         : Lavf55.18.104
    Stream #0:0: Video: flv1 (flv) ([2][0][0][0] / 0x0002), yuv420p, 256x256, q=2-31, 200 kb/s, 1k tbn, 25 tbc
Stream mapping:
  Stream #0:0 -> #0:0 (ppm -> flv)
Press [q] to stop, [?] for help
frame=    1 fps=0.0 q=3.8 Lsize=      13kB time=00:00:00.04 bitrate=2737.2kbits/s
video:13kB audio:0kB subtitle:0 global headers:0kB muxing overhead 1.694160%

[Elon Musk]

Is something like that even possible?
Even decoder supports yuv420p only.

[Carl Eugen Hoyos]

Not a valid ticket...