14.5. Accelerating an MPEG-4 Decoder
One of the most difficult parts of encoding MPEG-4 video data is motion estimation, which searches adjacent video frames for similar pixel blocks to detect inter-frame movement in the picture. The motion-estimation search algorithm’s inner loop contains an SAD (sum of absolute differences) operation consisting of a subtraction, an absolute value, and the addition of the resulting value with the previously computed value.
For a QCIF (quarter common image format, 176 × 144 pixels) image frame, a 15-Hz frame rate, and an exhaustive-search motion-estimation scheme, SAD operations require about 641 million operations/sec. As shown in Figure 14.3, it’s possible to add SIMD SAD hardware capable of executing 16 pixel-wide ...
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