Image Processing Reference
Figure 12-4 Shape-coded outline.
The white blocks are ignored. They are outside the mask and need not be coded in
the mask layer or in the video. This conserves some bit rate. The light gray blocks strad-
dle the boundary of the alpha-coded shape and contain masking pixels that must be
coded. These blocks in the video must be fully coded even though they will be partially
masked on playback. The dark gray blocks are wholly inside the masked area. No mask-
ing pixels are coded because those blocks will be displayed without masking.