Fast intra mode decision algorithm based on directional gradients for H.264/AVC

A directional gradients operator is proposed in this paper, and a novel H.264 intra prediction was in-troduced based on this operator. First of all, an encoding 16×16 macro block was divided into sixteen 4×4 sub-block. Using this directional gradients operator, an encoding 4×4 luma sub-block’s texture feature and grey level fluctuation can be calculated. Based on these features, some dispensable 4×4 intra prediction modes were cut down. Furthermore, the 16×16 macro block’s texture feature and grey level fluctuation can be judged according to statistical information of each 4×4 sub-block’s candidate intra prediction modes. And the 16×16 candidate intra prediction modes can be decided by using the threshold of directional gradient strength value. The rate distortion optimization process should be done among the 4×4 and 16×16 candidate intra prediction modes and the best luma intra prediction mode should be decided. In addition, 8×8 chroma block intra prediction modes could also be reduced based on the corresponding luma macro block. In all, the number of intra prediction mode was reduced up to 50%, and the computational complexity was highly reduced. According to experiments, the proposed method could reduce the encoding time of overall sequence by about 50% without noticeable degradation of coding quality.