Efficient Cost Measures for Motion Compensation at Low Bit Rates

Abstract

We present and compare methods for choosing motion vectors for block-based motion-compensated video coding. The primary focus is on videophone and video- conferencing applications, where low bit rates are neces- sary, where motion is usually limited, and where the amount of computation is also limited. In a typical block-based motion-compensated video coding system, motion vectors are transmitted along with a lossy encoding of the resid- uals. As the bit rate decreases, the proportion required to transmit the motion vectors increases. We provide ex- perimental evidence that choosing motion vectors explic- itly to minimize rate (including motion vector coding), sub- ject to implicit constraints on distortion, yields better rate- distortion tradeo s than minimizing some measure of pre- diction error. Minimizing a combination of rate and distor- tion yields further improvements. Although these explicit- minimization schemes are computationally intensive, they provide invaluable insight which we use to develop practi- cal algorithms. We show that minimizing a simple heuristic function of the prediction error and the motion vector code- length results in rate-distortion performance comparable to explicit-minimization schemes while being computationally feasible. Experimental results are provided for coders that operate within the H.261 standard.