Information Technology Reference
In-Depth Information
Figure 5.87 shows a spatial scaleability system in which the enhancement improves resolution. Spatial scaleability
may be applied to rectangular VOPs or to arbitrarily shaped objects using alpha data. In the case of irregular
objects the alpha data will also need to be refined. The mid-processor will upsample the base layer images to
provide base pixel data at the new, higher resolution, sampling grid. With respect to a sampling array upsampled
by a factor of two, motion vectors appear half as accurate and there are one quarter as many as are needed.
Consequently to improve prediction in the enhancement layer, the motion compensation must be made more
accurate.
Figure 5.87:
Spatial scaleability system requires the enhancement layer to improve the vector density in order to
minimize the residual bit rate.
The output of the base level decoder is considered to be a spatial prediction by the enhancement process. This
may be shifted by vectors from the enhancement bitstream. Figure 5.88 shows that a base level macroblock of 16 x
16 pixels predicted with one vector is upsampled to four macroblocks, each of which can be shifted with its own
enhancement vector. Upsampled base layer pixels which have been shifted by enhancement vectors form a new
high-resolution prediction. The encoder transmits the spatial residual from this prediction.
Figure 5.88:
Each base level macroblock corresponds to four enhancement macroblocks. Base vectors are used
to predict enhancement vectors.
Temporally scaleable decoding allows new VOPs to be created in between the VOPs of the base layer to increase
the frame rate. Figure 5.89 shows that enhancement I-VOPs are intra-coded and so are decoded without reference
to any other VOP just like a base I-VOP. P- and B-VOPs in the enhancement layer are motion predicted from other
VOPs. These may be in the base layer or the enhancement layer according to prediction reference bits in the
enhancement data. Base layer B-VOPs are never used as enhancement references, but enhancement B-VOPs
can be references for other enhancement VOPs.
