Page 173 - Video Coding for Mobile Communications Efficiency, Complexity, and Resilience

150 Chapter 6. Multiple-Reference motion Estimation Techniques

displacement of ±15 pels, SAD as the distortion measure, restricted mo-
tion vectors, and reconstructed reference frames. Motion vectors are coded
using the median predictor and the VLC table of the H.263 standard. The
frame signal (in case of INTRA) and the DFD signal (in case of INTER)
are transform encoded according to the H.263 standard. The encoder does
not employ rate-constrained motion estimation and mode decision. Thus,
motion estimation simply chooses the motion vector that minimizes the
SAD measure without any bit-rate considerations. The INTRA=INTERde-
cision is based on heuristic thresholds and is given by the following [142]:
INTRA mode is chosen if A¡(SAD(d) − 500); (6.1)

where �
P
A = |f c (x; y) − B| (6.2)
(x;y)∈B
and
f c (x; y)
�
P
B = (x;y)∈B ; (6.3)
256
where d =(d x ;d y ) is the motion vector of macroblock B in the current
frame f c and SAD(d) is the SAD between the macroblock in the current
frame and a corresponding macroblock in the reference frame shifted by d.
SR-RC This is a single-reference rate-constrained encoder. It is the same as
SR, but it uses rate-constrained motion estimation and mode decision as
de)ned in the high-complexity mode of the H.263 test model, near-term,
version 10 (TMN10) [142]. In this mode, motion estimation chooses the
motion vector that minimizes the following Langrangian cost function:
J MOTION = D MOTION + MOTION R MOTION ; (6.4)

where D MOTION is the SAD between the macroblock in the current frame
and the corresponding macroblock in the reference frame shifted by d,
R MOTION is the number of bits used to encode the motion vector d, and
MOTION is a Lagrange multiplier related to the quantization parameter
QP using
MOTION =0:92 × QP: (6.5)
To decide the mode, two Langrangian cost functions, one for each mode,
are calculated as follows:

J INTRA = D INTRA + MODE R INTRA ; (6.6)
J INTER = D INTER + MODE R INTER ; (6.7)