Low bit rate model-assisted H.261-compatible coding of video

We describe a method of object-selective quantizer control in a standard coding system based on MC DCT| CCITT Recommendation H.261. The approach is based on two novel algorithms, namely bu er rate modulation and bu er size modulation. By forcing the rate control algorithm to transfer a relatively small fraction|about 10{ 15% on average|of the total available bit rate from the coding of the non-facial to that of the facial area in headand-shoulders videoteleconferencing sequences, images with better-rendered facial features are obtained; i.e. blocky artifacts in the facial area are less pronounced and eye contact and lip-sync are preserved. The improvement was found to be perceptually signi cant on video sequences coded at the rate of 64 kbps, with 48 kbps allocated for the input (color) video signal in QCIF format.