Global motion compensation
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Global motion compensation (GMC) is a
.Operation
Global motion compensation describes the motion in a scene based on a single
Each macroblock in such a frame can be compensated using global motion (no further motion information is then signalled) or, alternatively, local motion (as if GMC were off). This choice, while costing an additional bit per macroblock, can improve prediction quality and therefore reduce residual.
Because the transforms used in global motion compensation are only added to the encoding stream when used, they do not have a constant bitrate overhead. A predicted frame which uses GMC is called an S-frame (sprite frame) while a predicted frame encoded without GMC is called either a
Implementations
DivX offers 1 warp-point GMC encoding: This enables easier hardware support in DivX certified and non-certified devices. But as 1 warp-point GMC limits the global transform to panning operation only (since panning can be described using blocks), this implementation rarely improves video quality.
Xvid offers 3 warp-point GMC encoding: As a result, it currently has no hardware support.
Criticism
GMC failed to meet expectations of dramatic improvements in motion compensation, and as a result it was omitted from the
Due to the extra decoding CPU cost of global motion compensation, most hardware players do not support global motion compensation.
See also
- DivX
- MPEG-4 ASP
- Motion compensation
- Xvid