x11grab input returns fully transparent image

[llogan]

jpg output works as expected. out20110506.png attached.

$ ffmpeg -f x11grab -r 25 -s 800x600 -i :0.0 -vframes 1 out20110506.png
ffmpeg version git-N-29638-g95f163b, Copyright (c) 2000-2011 the FFmpeg developers
  built on May  6 2011 14:01:20 with gcc 4.6.0 20110415 (prerelease)
  configuration: --prefix=/usr --disable-optimizations --enable-x11grab --enable-gpl
  libavutil    51.  2. 1 / 51.  2. 1
  libavcodec   53.  3. 0 / 53.  3. 0
  libavformat  53.  0. 3 / 53.  0. 3
  libavdevice  53.  0. 0 / 53.  0. 0
  libavfilter   2.  4. 0 /  2.  4. 0
  libswscale    0. 14. 0 /  0. 14. 0
[x11grab @ 0x1aacd40] device: :0.0 -> display: :0.0 x: 0 y: 0 width: 800 height: 600
[x11grab @ 0x1aacd40] shared memory extension found
[x11grab @ 0x1aacd40] Estimating duration from bitrate, this may be inaccurate
Input #0, x11grab, from ':0.0':
  Duration: N/A, start: 1304719812.085890, bitrate: 384000 kb/s
    Stream #0.0: Video: rawvideo, bgra, 800x600, 384000 kb/s, 25 tbr, 1000k tbn, 25 tbc
[buffer @ 0x1abb160] w:800 h:600 pixfmt:bgra
Output #0, image2, to 'out20110506.png':
  Metadata:
    encoder         : Lavf53.0.3
    Stream #0.0: Video: png, bgra, 800x600, q=2-31, 200 kb/s, 90k tbn, 25 tbc
Stream mapping:
  Stream #0.0 -> #0.0
Press [q] to stop encoding
frame=    1 fps=  0 q=0.0 Lsize=      -0kB time=0.04 bitrate=  -4.4kbits/s    
video:72kB audio:0kB global headers:0kB muxing overhead -100.029685%

[llogan]

Tested on x86_64 Arch Linux.

[cehoyos]

Work-around is to use -pix_fmt rgb24

Not a regression, afaict.

[reimar]

FFmpeg captures with alpha channel, though RGB32 for X does not usually include alpha but instead has a constant 0.
That constant 0 ends up as "fully transparent" in the png.
If you remove the alpha channel (e.g. in GIMP decompose as RGB and then compose again) you will see the image is there.
-pix_fmt rgb24 is of course a better way to get the same effect.

[cehoyos]

But am I correct that x11grab should not assume a useful value for the alpha channel of bgra?

[cehoyos]

Should be fixed.

[llogan]

Thanks for the patch. It create a normal output on Arch Linux, but in an Ubuntu Oneiric 11.10 VM there are still some transparency artifacts, but I'm not sure why. Example attached as ticket163.png.