A low-complexity rate-distortion model for motion estimation in H.263

The ITU-T low-bit-rate video coding standard H.263 supports efficient transmission of digital video over narrow-band telecommunication channels. The standard provides an excellent framework for low bit rate video coding, and leaves substantial design flexibility for tasks such as motion estimation to the implementer. We propose an alternative to the sum of absolute difference (SAD) metric that is used as the criterion for identifying motion vectors in many video coding implementations. The SAD usually fails to identify the rate-distortion optimal motion vector for a block because it does not take into account of the number of bits used to code the motion vector. One solution which has been proposed in the past is to use the Lagrangian cost J=D+λR as the cost function in motion estimation. This allows near optimal motion vector search in the rate-distortion sense but is too complex to be used in a practical video coding system. We present an alternative motion estimation algorithm that considers rate-distortion trade-offs in a low complexity framework, resulting in both a higher coding efficiency and a faster encoding speed