Improved Intra Prediction of H.264/AVC

Abstract

H.264/AVC is the latest international video coding standard developed by ITU-T Video Coding Expert Group and the ISO/IEC Moving Picture Expert Group, which provides gains in compression efficiency of about 40% compared to previous standards (ISO/IEC 14496-10, 2004, Weigand et al., 2003). New and advanced techniques are introduced in this new standard, such as intra prediction for I-frame encoding, multi-frame inter prediction, small block-size transform coding, context-adaptive binary arithmetic coding (CABAC), deblocking filtering, etc. These advanced techniques offer approximately 40% bit rate saving for comparable perceptual quality relative to the performance of prior standards (Weigand ct al., 2003). H.264 intra prediction offers nine prediction modes for 4x4 luma blocks, nine prediction modes for 8x8 luma blocks and four prediction modes for 16 x 16 luma blocks. However, the rate-distortion (RD) performance of the intra frame coding is still lower than that of inter frame coding. Hence intra frame coding usually requires much larger bits than inter frame coding which results in buffer control difficulties and/or dropping of several frames after the intra frames in real-time video. Thus the development of an efficient intra coding technique is an important task for overall bit rate reduction and efficient streaming. H.264/AVC uses rate-distortion optimization (RDO) technique to get the best coding mode out of nine prediction modes in terms of maximizing coding quality and minimizing bit rates. This means that the encoder has to code the video by exhaustively trying all of the nine mode combinations. The best mode is the one having the minimum rate-distortion (RD) cost. In order to compute RD cost for each mode, the same operation of forward and inverse transform/quantization and entropy coding is repetitively performed. All of these processing explains the high complexity of RD cost calculation. Therefore, computational complexity of encoder is increased drastically. Using nine prediction modes in intra 4x4 and 8x8 block unit for a 16x16 macroblock (MB) can reduce spatial redundancies, but it may needs a lot of overhead bits to represent the prediction mode of each 4x4 and 8x8 block. Fast intra mode decision algorithms were proposed to reduce the number of modes that needed calculation according to some criteria (Sarwer et al.,2008, Tsai et al., 2008, Kim, 2008, Pan et al., 2005, Yang et al., 2004). An intra mode bits skip (IBS) method based on adaptive singlemultiple prediction is proposed in order to reduce not only the overhead mode bits but also computational cost of the encoder (Kim et al., 2010). If the neighbouring pixels of upper and left blocks are similar, only DC prediction is used and it does not need prediction mode bits or else nine prediction modes are computed. But the IBS method suffers with some drawbacks a) the reference pixels in up-right block are not considered for similarity