Page 99 - Video Coding for Mobile Communications Efficiency, Complexity, and Resilience

76 Chapter 3. Video Coding: Standards

Frame box shift

VOP
bounding box

internal MB

boundary MB

exterior MB

Figure 3.11: The bounding box of the Balloon VOP

there are three types of MBs: internal MBs, boundary MBs, and exterior MBs.
An internal MB lies completely inside the VOP, whereas a boundary MB lies
on the contour of the VOP; i.e., parts of it are inside the VOP and the other
parts are outside the VOP. An exterior MB, on the other hand, lies completely
outside the VOP. Note that the shape, size, and location of this bounding box
can change from one time instance to another. Thus, the absolute (frame)
coordinate system is used to de/ne such bounding boxes.
The following subsections brieJy describe the main building blocks of the
MPEG-4 VOP encoder.

3.5.2 Shape Coding
In the context of MPEG-4, shape information is referred to as alpha planes.
There are two types of alpha planes: binary and gray-scale. A binary alpha
plane de/nes which pels within the bounding box belong to the video object
at a given instant of time. A gray-scale alpha plane, on the other hand, is a
more general form of alpha planes, for it includes transparency information.

3.5.2.1 Binary Shape Coding

A binary alpha plane is represented by a matrix the same size as the bounding
box of the video object. Every element within this matrix can take one of two