Combined mode selection and macroblock quantization step adaptation for the H.263 video encoder
The low bitrate video coding standard H.263 allows four modes of coding, and 31 quantization parameters for each macroblock in a sequence. A new quantization parameter can be set at the beginning of each group of blocks (GOB), and then adapted from macroblock to macroblock within the GOB in a constrained manner. In this work, for every GOB in a frame, an m-best search scheme is employed through a trellis to find the best modes and the best quantizer step changes, in a rate-distortion sense, for both when the starting quantization parameter is predetermined, and when it is to be selected optimally as well. Thus the algorithm performs the most generic kind of optimization at the macroblock level for the P-frames of H.263. It is demonstrated that a relatively low search depth is sufficient to produce coding results superior to established standard-compatible coders.