Wednesday, February 15, 2006

Screencasting for X11: an FFmpeg screen recorder

vnc2swf, vncrec, xvidcap. Either they can't record audio (vnc2swf,
vncrec) or they can't keep audio in sync (xvidcap). xvidcap is the most
advanced utility but it seem rather unmaintained and I also dislike
the internal code structure (amorphic functions that behaviours
strongly depend on semi-global state variables).

ffmpeg already comes with grabbing code, but only for V4L. The
requirements are not so different from X11 grabbing, so I decided to
hack a new X11 grabber into ffmpeg (after verifying that ffmpeg does
correct A/V syncing).

The result is a working mixture of videograb.c of ffmpeg
and capture.c/xtoffmpeg.c of xvidcap. I'm now able to do screencasts
from X11 where I'm able to comment the things I'm doing via

Here is the patch for ffmpeg-0.4.9-p20051216: ffmpeg-x11-screen-recorder.diff

Compile like:

./configure --enable-x11grab --enable-gpl

Run like:

./ffmpeg -vcodec mpeg4 -b 1000 -r 10 -g 300 -vd x11:0,0 -s 1280x1024 test.avi

The -vd option is required. The two values following the colon
describe the left upper coordinate of the grabbing region. The -s
option describes the grabbing size. I'm using a frame rate of 10fps
(-r 10) and a group size of 300 frames (so you have an intra frame
every 30 seconds).

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