Hi,
some time ago I needed an autogain/autolevels filter and was unsatisfied with what ColorYUV had to offer, so I wrote my own. The main improvement over ColorYUV(autogain=true) is that the amount of gain is averaged over a number of frames which helps with scenes that have flashing lights etc.
This filter was useful for a VHS transfer I had to do recently. Maybe somebody else will find it useful, too.
The parameters are explained on the web page and in the readme file.
URL:
http://ta41.zendurl.com/autolevels_en.php
Comments are always welcome, of course.