fl0x

ffmpeg -f x11grab -s 1824x984 -i :0.0 -s 800x600 -an ZIEL.mp4
ffmpeg version N-89616-ge6a1dfc9ce Copyright (c) 2000-2017 the FFmpeg developers
  built with gcc 6.3.0 (Raspbian 6.3.0-18+rpi1) 20170516
  configuration:
  libavutil      56.  7.100 / 56.  7.100
  libavcodec     58.  9.100 / 58.  9.100
  libavformat    58.  3.100 / 58.  3.100
  libavdevice    58.  0.100 / 58.  0.100
  libavfilter     7.  7.100 /  7.  7.100
  libswscale      5.  0.101 /  5.  0.101
  libswresample   3.  0.101 /  3.  0.101
[x11grab @ 0x33ca220] Stream #0: not enough frames to estimate rate; consider increasing probesize
Input #0, x11grab, from ':0.0':
  Duration: N/A, start: 1514381596.586506, bitrate: N/A
    Stream #0:0: Video: rawvideo (BGR[0] / 0x524742), bgr0, 1824x984, 29.97 fps, 1000k tbr, 1000k tbn, 1000k tbc
File 'ZIEL2.mp4' already exists. Overwrite ? [y/N] y
Stream mapping:
  Stream #0:0 -> #0:0 (rawvideo (native) -> mpeg4 (native))
Press [q] to stop, [?] for help
Output #0, mp4, to 'ZIEL2.mp4':
  Metadata:
    encoder         : Lavf58.3.100
    Stream #0:0: Video: mpeg4 (mp4v / 0x7634706D), yuv420p, 800x600, q=2-31, 200 kb/s, 29.97 fps, 30k tbn, 29.97 tbc
    Metadata:
      encoder         : Lavc58.9.100 mpeg4
    Side data:
      cpb: bitrate max/min/avg: 0/0/200000 buffer size: 0 vbv_delay: -1
frame=  760 fps= 31 q=31.0 Lsize=    1320kB time=00:00:25.32 bitrate= 427.0kbits/s dup=741 drop=0 speed=1.05x
video:1316kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 0.309224%
Exiting normally, received signal 2.