Editing Motion compensation (section)

== Global motion compensation ==

In [[global motion compensation]], the motion model basically reflects camera motions such as:
* Dolly — moving the camera forward or backward
* Track — moving the camera left or right
* Boom — moving the camera up or down
* Pan — rotating the camera around its Y axis, moving the view left or right
* Tilt — rotating the camera around its X axis, moving the view up or down
* Roll — rotating the camera around the view axis

It works best for still scenes without moving objects.

There are several advantages of global motion compensation:
* It models the dominant motion usually found in video sequences with just a few parameters. The share in bit-rate of these parameters is negligible.
* It does not partition the frames. This avoids artifacts at partition borders.
* A straight line (in the time direction) of pixels with equal spatial positions in the frame corresponds to a continuously moving point in the real scene. Other MC schemes introduce discontinuities in the time direction.

MPEG-4 ASP supports global motion compensation with three reference points, although some implementations can only make use of one. A single reference point only allows for translational motion which for its relatively large performance cost provides little advantage over block based motion compensation.

Moving objects within a frame are not sufficiently represented by global motion compensation.
Thus, local [[motion estimation]] is also needed.