-vcodec mpeg4
That could be your problem. MPEG-4 is a fairly good codec, but it's easily outperformed by h.264 – x264 being a popular example for an h.264 encoder. I'm not sure how big of a role the bitrate plays here, but it would help to just try x264. Therefore, use
-c:v libx264
to change your encoder.* In order to tune the quality, set the Constant Rate Factor to a level of your choice. Sane values are from 19 to 24, where lower values mean better quality.
avconv -f image2 -i Capture/%d.bmp -c:v libx264 -r 24 -crf 21 Capture.mkv
If you still see artifacts, you might want to give us more info, and update your post with the full, uncut console output from your avconv
command.
* -c:v
is the new syntax used instead of vcodec
, similar to -b:v
. Try to keep sticking to that.