Compression Methods

Frame size (from 720x576), then application of a video codec

low res DVD : 352x576

VCD, DVD, H.261 + H.263 (CIF) : 352x288

H.261 + H.263 (QCIF) : 176x144

NTSC : 320x240

Repetition and patterns

Averaging

Range Reduction

Frame difference

Key frame (every 100 frames or so) + differences

The Best introduction to Digital Video for the Web : http://stream.uen.org/medsol/digvid/html/mainmenu.html