Information Technology Reference
In-Depth Information
coding. Intra-VOPs are spatially coded, whereas P- and B-VOPs use motion-compensated prediction and a
spatially coded residual is transmitted. The vectors in MPEG-4 are subject to a prediction process not found in
MPEG-2. Note the use of texture padding before the coder. As the decoder will use MC padding in the course of
prediction, the encoder must also use it to prevent drift.
Figure 5.57:
An MPEG-4 object coder. See text for details.
The shape data are context coded in I-VOPs and the coding will be motion compensated in P- and B-VOPs.
However, the shape vectors can be predicted from other shape vectors or from the texture motion vectors where
these exist. These shape vectors are coded as prediction residuals. Figure 5.58 shows how the shape vector
prediction works. Around the bab to be coded are three designated shape vectors and three designated texture
vectors. The six designated vectors are scanned in the order shown and the first one which is defined will be used
as the predictor. Consequently if no shape vectors are defined, the texture vectors will be used automatically since
these are later in the scan.
Figure 5.58:
Shape vectors may be coded or taken from texture vectors. The decoder scans vectors is the
sequence shown and takes the first ones available.
Shape babs can exist in a variety of forms as Figure 5.47 showed. Opaque and transparent blocks are easily and
efficiently coded. Intra babs have no shape vector but need a lot of data. Inter babs may or may not contain a
shape vector according to object motion and only a residual is sent. On the other hand, under certain object
conditions, sufficient shape accuracy may result by sending only the shape vector.
5.25 Two-dimensional mesh coding
Mesh coding was developed in computer graphics to aid the rendering of synthetic images with perspective. As
Figure 5.59 shows, as a three- dimensional body turns, its two-dimensional appearance changes. If the body has a
flat side square-on to the optical axis whose texture is described by uniformly spaced pixels, after a rotation the
pixels will no longer be uniformly spaced. Nearly every pixel will have moved, and clearly an enormous amount of
data would be needed to describe the motion of each one. Fortunately this is not necessary; if the geometry of the
object is correctly sampled before and after the motion, then the shift of every pixel can be computed.
