Is anyone familiar with any Unix-based software that can do motion detection in recorded video? I want a surveillance camera to record continuously, but would like to run some motion detection over the continuous files to generate timestamps that I can then feed into mencoder or something to get smaller files during times where motion was detected.

I found Motion, but it appears to want to operate on video streams directly from cameras using video4linux, not video files that are already recorded.

I don't think I need anything fancy, though the ability to mask off busy sections to be excluded from motion detection would be helpful.