Alpha coding of arbitrary-shaped objects for low bit-rate MPEG-4

This paper presents a new scheme for compact shape-coding which can reduce the needed bandwidth for low bit rate MPEG-4 applications. Our scheme is based on a coarse representation of the alpha plane with a block size resolution of 8x8 pixels. This arrangement saves bandwidth and reduces the algorithm complexity (number of computations), as compared to the Content-based Arithmetic Encoding (CAE) algorithm. In our algorithm, we encode the alpha plane of a macroblock with only 4 bits, while then we can further reduce the number of encoding bits by using the Huffman code. The encoding blocks are only contour macroblocks, transparent macroblocks are considered as background macroblocks, while opaque macroblocks are considered as object macroblocks. We show that the amount of bandwidth saving with representing the alpha-plane can reach a factor of 9.5. Such a scheme is appropriate for mobile applications where there is a lack of both bandwidth and processing power. We also speculate that our scheme will be compatible to the MPEG-4 standard.