Motion Compensation

--------------------------------------------------------------------------------

Motion Compensation


An important element of MPEG is motion compensation, which is used in inter coding only. In this mode, the pels of the current picture are predicted by those of a previous reference picture (and, possibly, by those of a future reference picture). Pels are subtracted, and the differences (which should be small numbers) are DCT transformed, quantized, and encoded. The differences between the current picture and the reference one are normally caused by motion (either camera motion or scene motion), so best prediction is obtained by matching a region in the current picture with a different region in the reference picture. MPEG does not require the use of any particular matching algorithm, and any implementation can use its own method for matching macroblocks (see Section 6.4 for examples of matching algorithms). The discussion here concentrates on the operations of the decoder.

Differences between consecutive pictures may also be caused by random noise in the video camera, or by variations of illumination, which may change brightness in a nonuniform way. In such cases, motion compensation is not used, and each region ends up being matched with the same spatial region in the reference picture. If the difference between consecutive pictures is caused by camera motion, one motion vector is enough for the entire picture. Normally, however, there is also scene motion and movement of shadows, so a number of motion vectors are needed, to describe the motion of different regions in the picture. The size of those regions is critical. A large number of small regions improves prediction accuracy, whereas the opposite situation simplifies the algorithms used to find matching regions and also leads to fewer motion vectors and sometimes to better compression. Since a macroblock is such an important unit in MPEG, it was also selected as the elementary region for motion compensation.

Another important consideration is the precision of the motion vectors. A motion vector such as (15,-4) for a macroblock M typically means that M has been moved from the reference picture to the current picture by displacing it 15 pels to the right and 4 pels up (a positive vertical displacement is down). The components of the vector are in units of pels. They may, however, be in units of half a pel, or even smaller. In MPEG-1, the precision of motion vectors may be either full-pel or half-pel, and the encoder signals this decision to the decoder by a parameter in the picture header (this parameter may be different from picture to picture). It often happens that large areas of a picture move at identical or at similar speeds, and this implies that the motion vectors of adjacent macroblocks are correlated. This is the reason why the MPEG encoder encodes a motion vector by subtracting it from the motion vector of the preceding macroblock and encoding the difference.

A P picture uses an earlier I picture or P picture as a reference picture. We say that P pictures use forward motion-compensated prediction. When a motion vector MD for a macroblock is determined (MD stands for motion displacement, since the vector consists of two components, the horizontal and the vertical displacements), MPEG denotes the motion vector of the preceding macroblock in the slice by PMD and calculates the difference dMD=MD–PMD. PMD is reset to zero at the start of a slice, after a macroblock is intra coded, when the macroblock is skipped, and when parameter block_motion_forward is zero. The 1-bit parameter full_pel_forward_vector in the picture header defines the precision of the motion vectors (1=full-pel, 0=half-pel). The 3-bit parameter forward_ f_code defines the range.