12.04 LTS

It appears that ffmpeg no longer works properly in this release for converting recordmydesktop's .ogv recordings to WebM. As it is no longer maintained, it's recommended to use its replacement avconv instead, provided in the libav-tools package (which seems to be automatically installed if ffmpeg was installed).

The command needed to use avconv for conversions is not too much different than from ffmpeg:

avconv -i input.ogv output.webm

Thus, if anybody's facing the same issue of trying to use only free packages to convert from .ogv to .webm in 12.04 (or presumably later), this is how I resolved it. This should be helpful if you're making recordings to show new features or document issues using video.

Actually, I think I can answer my own question after spending some time reading around and messing with the avconv command myself. I found that these settings seem to work perfect for what I need:

avconv -i input.mp4 \
       -c:v libvpx -qmin 20 -qmax 30 -ss 00:00:30 -t 00:01:00 -threads 2 \
       -c:a libvorbis \
       output.webm 

I realized that it was easier to just split the video using the command line also. The -ss hh:mm:ss splits the video starting at that point and -t indicates the time it records after.

The -an option will also drop the audio for uploading your WebM video to boards like 8chan, 7chan, 4chan, etc.

The -s option will "downsample" and use a new resolution for the video size. Such as -s 640x480, for example.

So in conclusion, if you were trying to convert part of a video (using all of the mentioned options) 1 minute in, for 30 seconds after that point, while scaling the resolution down and dropping audio, it would look like this:

avconv -i MyFavoriteMovie.mp4 \
       -c:v libvpx -qmin 20 -qmax 30 -ss 00:01:00 -t 00:00:30 -s 640x480 -an -threads 2 \
       -c:a libvorbis \
       output.webm

Just to give an example. This would create a 30 second clip with the given resolution.

Hope this helps other avconv beginners as well.