> ffmpeg -i video.mkv -c:v libvpx -b:v 450k -minrate 450k -maxrate 450k -c:a libvorbis -b:a 48k -ac 2 video.webm >> output.txt
ffmpeg version 2.1.1 Copyright (c) 2000-2013 the FFmpeg developers
  built on Dec 25 2013 17:58:33 with gcc 4.2.1 (GCC) 20070831 patched [FeeBSD]
  configuration: --disable-libaacplus --disable-indev=alsa --disable-outdev=alsa --disable-libopencore-amrnb --disable-libopencore-amrwb --disable-lib
ass --disable-libcdio --disable-libcelt --disable-libfaac --disable-libfdk-aac --enable-ffserver --enable-fontconfig --enable-libfreetype --enable-fre
i0r --disable-gnutls --disable-libgsm --enable-iconv --disable-indev=jack --enable-libmp3lame --disable-libbluray --disable-libv4l2 --disable-indev=v4
l2 --disable-outdev=v4l2 --disable-libmodplug --disable-openal --disable-indev=openal --enable-libopencv --disable-libopenjpeg --disable-libopus --dis
able-libpulse --disable-indev=pulse --disable-outdev=pulse --disable-librtmp --enable-libschroedinger --disable-libspeex --enable-libtheora --enable-v
aapi --enable-vdpau --enable-libvorbis --disable-libvo-aacenc --disable-libvo-amrwbenc --enable-libvpx --enable-libx264 --enable-libxvid --prefix=/usr
/local --mandir=/usr/local/man --datadir=/usr/local/share/ffmpeg --enable-shared --enable-gpl --enable-postproc --enable-avfilter --enable-avresample
--enable-pthreads --enable-memalign-hack --disable-libstagefright-h264 --disable-libutvideo --disable-libsoxr --cc=cc --extra-cflags='-msse -I/usr/loc
al/include/vorbis -I/usr/local/include' --extra-ldflags='-L/usr/local/lib ' --extra-libs=-pthread --disable-debug --disable-ffplay --disable-outdev=sd
l --enable-nonfree
  libavutil      52. 48.101 / 52. 48.101
  libavcodec     55. 39.101 / 55. 39.101
  libavformat    55. 19.104 / 55. 19.104
  libavdevice    55.  5.100 / 55.  5.100
  libavfilter     3. 90.100 /  3. 90.100
  libavresample   1.  1.  0 /  1.  1.  0
  libswscale      2.  5.101 /  2.  5.101
  libswresample   0. 17.104 /  0. 17.104
  libpostproc    52.  3.100 / 52.  3.100
Input #0, matroska,webm, from 'video.mkv':
  Metadata:
    creation_time   : 2012-08-31 08:48:35
  Duration: 00:43:32.80, start: 0.000000, bitrate: 4886 kb/s
    Stream #0:0(eng): Video: h264 (High), yuv420p(tv, bt709), 1280x714 [SAR 1:1 DAR 640:357], 59.94 fps, 59.94 tbr, 1k tbn, 119.88 tbc (default)
    Metadata:
      title           : English
    Stream #0:1(eng): Audio: ac3, 48000 Hz, stereo, fltp, 384 kb/s (default)
    Metadata:
      title           : AC3 5.1
File 'video.webm' already exists. Overwrite ? [y/N] y
[libvpx @ 0x80741d020] v1.3.0
Output #0, webm, to 'video.webm':
  Metadata:
    encoder         : Lavf55.19.104
    Stream #0:0(eng): Video: vp8 (libvpx), yuv420p, 1280x714 [SAR 1:1 DAR 640:357], q=-1--1, 450 kb/s, 1k tbn, 59.94 tbc (default)
    Metadata:
      title           : English
    Stream #0:1(eng): Audio: vorbis (libvorbis), 48000 Hz, stereo, fltp, 48 kb/s (default)
    Metadata:
      title           : AC3 5.1
Stream mapping:
  Stream #0:0 -> #0:0 (h264 -> libvpx)
  Stream #0:1 -> #0:1 (ac3 -> libvorbis)
Press [q] to stop, [?] for help
frame=  246 fps= 13 q=0.0 size=     812kB time=00:00:04.24 bitrate=1565.9kbits/s