U. Glaeser

FIGURE 42.27 Motion estimation process in a block-based video coder.
FIGURE 42.28 Motion prediction in 2 ITU-T video coding standards.
SAD, is considered to most resemble the current MB and referred to as the “best match.” The displacement
vector between the currently coded MB and the matrix that spatially corresponds to its best match in
the previous frame is called the motion vector (MV) and the relative SAD is called the MB residual
matrix. If the smallest SAD is less than a certain threshold then the MB is INTER coded by sending the
MV and the DCT coefficients of the residual matrix, otherwise the MB is INTRA coded. The coordinates
of the MV are transmitted differentially using the coordinates of one or more MVs corresponding to
neighboring MBs (left MB in ITU-T H.261 or left, top, and top right MBs in ITU-T H.263 and ISO
MPEG-4) within the same video frame. Figures 42.27 and 42.28 illustrate the motion estimation and
prediction processes of contemporary video coding algorithms.
Subjective and Objective Evaluation of Perceptual Quality
The performance of a video coding algorithm can be simply subjectively evaluated by visually comparing
the reconstructed video sequence to the original one. Two major types of subjective methods are used
to assess the quality of perceptual video quality. In the first, an overall quality rating is assigned to the
image (usually last decoded frame of a sequence). In the second, quality impairment is induced on a
standard type image until it is completely similar to the reference image or vice versa.
© 2002 by CRC Press LLC