Chroma coding for video at very low bit rates

Color pictures are usually compressed in a luminance-chrominance coordinate space. We consider the problem of encoding the chrominance information for very low bit rate video coding systems aimed at bit rates in the range 8 to 40 kbps. The challenge here is that the chrominance components typically get less than 10 to 20% of the total very low bit rate allocated for the video data. We found that it is sufficient to encode the chrominance information at 1/8 of the luminance resolution in both the horizontal and vertical directions. While, for many of the previous coding methods, the compression is performed independently for the luminance and chrominance coordinates, we propose a coding scheme which exploits the coded luminance data in coding and retrieving the chrominance components. The proposed video coder is an improved extension of an existing luminance-only coder so that color motion video can be coded at very low bit rates under fixed frame and bit rate constraints. It is based on a hybrid waveform coding technique with an implicit model-based component. Very good results were obtained for head-and-shoulders sequences even with chroma rates of less than 7% of the total very low bit rate. In addition, subjective tests indicate that the coded chrominance information improves the visual perception of noisy image features.