Page 100 - Video Coding for Mobile Communications Efficiency, Complexity, and Resilience

Section 3.5. The MPEG-4 Standard 77

possible values. If the corresponding pel belongs to the object, then the element
is set to 255; otherwise it is set to 0. This matrix is sometimes referred to as
a binary mask or as a bitmap. Figure 3.12 shows the binary alpha plane of
the Balloon VOP.
Before encoding, the binary alpha plane is partitioned into 16 ×16 blocks
called binary alpha blocks (BABs). A BAB with all elements equal to 0 is
called a transparent BAB, whereas a BAB with all elements equal to 255
is called an opaque BAB. Each BAB is encoded separately. The main tools
used for encoding BABs are context-based arithmetic encoding (CAE) and
motion compensation. There are two variants of the CAE algorithm. One is
used with motion compensation and is called InterCAE, whereas the other one
is used without motion compensation and is called IntraCAE. There are seven
possible modes for encoding a BAB:
1. The BAB is Jagged transparent. In this case, no shape coding is neces-
sary. In addition, texture information is not coded for this BAB.

Figure 3.12: The binary alpha plane of the Balloon VOP