sergio@sergio-desktop:~/ffmpeg-git-20151106-64bit-static$ ./ffmpeg -video_size 700x420 -framerate 10 -f x11grab -i :0.0+50,0 -ab 96k -ar 44100 -b 345k -pix_fmt yuv420p test2.webm
ffmpeg version N-50966-gc878082-static http://johnvansickle.com/ffmpeg/  Copyright (c) 2000-2015 the FFmpeg developers
  built with gcc 5.2.1 (Debian 5.2.1-23) 20151028
  configuration: --enable-gpl --enable-version3 --disable-shared --disable-debug --enable-runtime-cpudetect --enable-libmp3lame --enable-libx264 --enable-libx265 --enable-libwebp --enable-libspeex --enable-libvorbis --enable-libvpx --enable-libfreetype --enable-fontconfig --enable-libxvid --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libtheora --enable-libvo-aacenc --enable-libvo-amrwbenc --enable-gray --enable-libopenjpeg --enable-libopus --enable-libass --enable-gnutls --enable-libvidstab --enable-libsoxr --enable-frei0r --enable-libfribidi --disable-indev=sndio --disable-outdev=sndio --cc=gcc
  libavutil      55.  5.100 / 55.  5.100
  libavcodec     57. 14.100 / 57. 14.100
  libavformat    57. 14.100 / 57. 14.100
  libavdevice    57.  0.100 / 57.  0.100
  libavfilter     6. 14.101 /  6. 14.101
  libswscale      4.  0.100 /  4.  0.100
  libswresample   2.  0.100 /  2.  0.100
  libpostproc    54.  0.100 / 54.  0.100
Input #0, x11grab, from ':0.0+50,0':
  Duration: N/A, start: 1446829248.314082, bitrate: N/A
    Stream #0:0: Video: rawvideo (BGR[0] / 0x524742), bgr0, 700x420, 10 fps, 10 tbr, 1000k tbn, 10 tbc
Please use -b:a or -b:v, -b is ambiguous
File 'test2.webm' already exists. Overwrite ? [y/N] y
[libvpx-vp9 @ 0x4d79840] v1.4.0-1490-g0134764
Output #0, webm, to 'test2.webm':
  Metadata:
    encoder         : Lavf57.14.100
    Stream #0:0: Video: vp9 (libvpx-vp9), yuv420p, 700x420, q=-1--1, 345 kb/s, 10 fps, 1k tbn, 10 tbc
    Metadata:
      encoder         : Lavc57.14.100 libvpx-vp9
Stream mapping:
  Stream #0:0 -> #0:0 (rawvideo (native) -> vp9 (libvpx-vp9))
Press [q] to stop, [?] for help
Past duration 0.999992 too large
frame=   10 fps=0.0 q=0.0 size=       0kB time=00:00:00.00 bitrate=N/A dup=0 dPast duration 0.999992 too large
    Last message repeated 1 times
frame=   15 fps= 14 q=0.0 size=       0kB time=00:00:00.00 bitrate=N/A dup=0 dframe=   21 fps= 13 q=0.0 size=       0kB time=00:00:00.00 bitrate=N/A dup=0 dPast duration 0.999977 too large
frame=   26 fps=9.5 q=0.0 size=     106kB time=00:00:01.40 bitrate= 618.7kbitsPast duration 0.958778 too large
frame=   32 fps=9.6 q=0.0 size=     115kB time=00:00:02.00 bitrate= 470.4kbitsframe=   35 fps=6.9 q=0.0 Lsize=     159kB time=00:00:05.10 bitrate= 254.9kbits/s dup=0 drop=15
video:158kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 0.438775%
Exiting normally, received signal 2.