I use the following command to convert a digital camera created mjpeg
to mpeg4 avi format.
ffmpeg -i DARKMJPEG.avi -f avi -vcodec mpeg4 -an DARKMPEG4.avi

Some of my movies are too dark.  Using GNU/Linux tools only, how do I
alter the contrast, brightness of an existing mpeg/divx AVI to an avi
file?  A plus if filters such as sharpening, etc are available.

Hi, linuxhack.

An alternative program for encoding movie files is the mencoder
program, which comes with the mplayer movie player. mplayer and
mencoder have been released under the GNU GPL.

You can change the alpha, brightness, contrast, and saturation during
the encoding process using the -vf switch and the eq2 filter. (Other
filters include blurring, cropping, scaling, but sadly not
sharpening).

The command to encode your movie would be similar to:
mencoder /INPUT.avi -ovc lavc -lavcopts vcodec=mpeg4 -oac mp3lame
-lameopts vbr=3 -vf eq2=1.0:1.3:0.5:1.0 -o /OUTPUT.avi

Information about the filters is available at:
http://www.sim.inf.ethz.ch/internal/movies_howto.html
(See 'Filtering' under the 'The Linux-Only Way' heading).

The manual for mplayer and mencoder is available at:
http://mplayerhq.hu/homepage/design7/info.html#docs

The following searches may also be useful:
://www.google.com/search?num=50&q=mencoder+brightness
://www.google.com/search?num=50q=encoding+ffmpeg+brightness+contrast

I hope this was of help.

-- wildeeo

You rock.  Just what I needed.