Making screencast:

$ ffmpeg -f x11grab -s 1280x760 -r 25 -i :0.0+3840+0 -f alsa -ac 2 -i pulse -sameq video.mpg

Transcoding to webm:

$ffmpeg -i video.mpg -s 1280x760 -vpre libvpx-720p -b 3900k -pass 1 -an -f webm -y video.webm

Output of the /usr/share/ffmpeg/libvpx-720p.ffpreset:


vcodec=libvpx
g=120
rc_lookahead=16
level=216
profile=0
qmax=42
qmin=10
vb=2M
#ignored unless using -pass 2
maxrate=24M
minrate=100k