VBSMC, Variable block-size motion compensation, is used in most video encoding formats designed after MPEG-2, including MPEG-4 ASP and AVC as well as VC-1. In older video encoding technologies motion from one frame to the next must be evaluated in 16x16 blocks, meaning that if a few pixels out of the 256 in a block change the entire block must be updated. VBSMC allows the block size to be reduced so that changes in small areas don't necessarily require larger areas to be updated in later pictures. Additionally, block size for motion compensation can be different between different areas of the picture, as well as between frames.
