Page 62 - Video Coding for Mobile Communications Efficiency, Complexity, and Resilience

Section 2.7. Interframe Coding 39

K
Symbol
N 3-D DCT Quantizer encoder
M
y
x
t

Figure 2.14: A 3-D transform coding system

2.7.2 Motion-Compensated Coding

One of the earliest approaches to interframe coding was conditional replenish-
ment (CR) [53]. In this method, the input frame is divided into “changed” and
“unchanged” regions with respect to a previously decoded reference frame, and
the addresses of this segmentation are coded. Unchanged regions need not be
coded because they can simply be copied from the reference frame, whereas
changed regions need to be coded. One way of coding them is to use one
of the intraframe coding methods discussed in Section 2.6. However, a more
eGcient approach is to predictively code them with respect to the correspond-
ing regions in the reference frame. In this case, the coded prediction error
signal is called the frame di)erence (FD) and the process is known as frame
di)erencing.
An improved performance can be obtained by improving the prediction of
changed regions. This can be achieved using motion estimation and compen-
sation. Changes between frames are mainly due to the movement of objects.
Using a model of the motion of objects between frames, the encoder estimates
the motion that occurred between the reference frame and the current frame.
This process is called motion estimation (ME). The encoder then uses this
motion model and information to move the contents of the reference frame
to provide a better prediction of the current frame. This process is known
as motion compensation (MC), and the prediction so produced is called the
motion-compensated prediction (MCP) or the displaced-frame (DF). In this
case, the coded prediction error signal is called the displaced-frame di)erence
(DFD). A block diagram of a motion-compensated coding system is illustrated
in Figure 2.15. This is the most commonly used interframe coding method.
The reference frame employed for ME can occur temporally before or af-
ter the current frame. The two cases are known as forward prediction and
backward prediction, respectively. In bidirectional prediction, however, two
reference frames (one each for forward and backward prediction) are employed