Global Motion Compensation, or GMC, is a technique used in MPEG-4 encoding that allows greater compression for scenes with camera pans and other motion that affects the entire video frame simultaneously. By contrast, older formats like MPEG-2 always use separate calculations for each 16x16 pixel block in the frame, requiring more bits.
Due to the increased complexity of GMC over standard block based motion estimation, hardware MPEG-4 encoders often don't support it, making it risky to use for encoding in many situations. There are also differences in how its implemented in different MPEG-4 codecs, with DivX using a much simpler version than XviD. GMC complexity can be determined by the number of warp points supported. DivX uses a single warp point, while XviD can use up to three. Most hardware that supports GMC is only capable of rendering video with 1 warp point.
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