Global Motion Compensation

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Global Motion Compensation (GMC) is a technique sometimes used when encoding video in a digital format, such as MPEG-4 (e.g. DivX and XviD codecs).

1 Operation
2 Implementations
3 Hardware compatibility
4 See also

[edit] Operation

GMC is most useful in scenes where panning, zooming, or rotation occurs. It enables movement to be described as a relatively simple transform instruction, rather than a constant frame by frame whole scene update. This may not substantially improve the apparent visual quality of the scene, but will greatly improve the resultant compression obtained.

[edit] Implementations

DIVX has only one GMC warp point specified. This was to enable easier hardware implementation. XVID states 3 warp points, and as a result, has less hardware support. The DIVX player does support 3 warp point GMC, and thus will play GMC XVID encoded streams. Because GMC transforms are only added to the encoding stream when used, they do not have a constant bit rate overhead.

[edit] Hardware compatibility

Because GMC is a relatively new technique in the world of digital video at time of writing, some hardware players still do not support the specification. For this reason, it is omitted from the default DIVX encoding profiles.