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Global Motion Compensation

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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Synonyms

Related glossary terms

Related software tools

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	DivX(Ad-supported) The latest version of the DivX. That is, you need to have this file installed if you want to watch DivX movies. And latest codec gives you the best quality.
	DivX (Win98/ME)(Shareware) The latest version of the DivX codec for Win98/ME platforms. You need to have this codec installed in case you want to watch or encode DivX movies.
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	DivX for Mac OS X(Shareware) DivX5 for Mac is _the_ official DivX codec for Mac -- it has been developed by DivX Networks, same guys who have developed DivX4 for Win. Excellent MPEG-4 based video codec.
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	XviD Bitrate Calculator(Freeware) Nice little bitrate and resolution calculator for XviD videos, which does everything you can ask from a bitrate calculator. Only nag is the installation of the tool, which can get painful.
	XviD Codec(Open source) XviD is an open source video codec that lets you play and create MPEG-4 compatible videos.