Page 63 - Video Coding for Mobile Communications Efficiency, Complexity, and Resilience

40 Chapter 2. Video Coding: Fundamentals

Encoded displaced-frame
Input frame difference (DFD)
Intraframe
+
_ encoder

Motion-compensated
prediction (MCP) Decoded DFD Intraframe
+
decoder
Decoded Decoded current frame
reference
Motion
frame Frame buffer
compensation
(delay)
(MC) Decoder
Motion information
Motion
estimation
(ME)

Figure 2.15: Motion-compensated coding system

and the two predictions are interpolated (the resulting predicted frame is called
B-frame). The most commonly used ME method is the block-matching mo-
tion estimation (BMME) algorithm [54]. In this algorithm, the current frame
is rst divided into blocks. The motion of each block is then estimated by
searching for the best-match block in the reference frame according to some
distortion measure. This search is usually restricted to a search window cen-
tered around the corresponding block in the reference frame. The motion of
the current block is then represented by a motion vector, which is the dis-
placement between the block and its best-match block in the reference frame.
The process of BMME is illustrated in Figure 2.16. Note that this algorithm
is based on a translational model of the motion of objects between frames.
It also assumes that all pels within a block undergo the same translational
movement. There are many other ME methods, but BMME is normally pre-
ferred due to its simplicity and good compromise between prediction quality
and motion overhead [55]. A more detailed discussion of BMME and other
ME methods is deferred to Chapter 4.
As illustrated in Figure 2.15, the DFD signal can be coded using any of
the intraframe coding methods discussed in Section 2.6. However, the most
commonly used method is transform coding, in particular block-based DCT
transform coding. This combination of block-matching motion-compensated
prediction and block-based DCT coding of the prediction error has proved
to be the most successful class of video coding methods. Today, most video