基于梯度方向算子的H.264帧内模式选择算法

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

提出梯度方向算子的概念,基于该算子提出了一种H.264帧内模式快速选择算法。本文首先应用梯度方向算子计算编码宏块中各4×4亮度子块的纹理特征和灰度起伏特征,根据这两种特征参量削减4×4帧内候选预测模式。通过统计宏块中各子块的4×4候选预测模式信息,结合梯度方向强度门限判别法削减宏块的16×16候选预测模式,通过率失真优化算法计算得到最优亮度预测模式。进一步根据亮度宏块和色度宏块的对应关系,在亮度候选预测模式的基础上对色度宏块候选预测模式进行削减,最后计算得到最优色度预测模式。该算法削减了50%以上的帧内预测模式,减少了帧内预测模式选择的运算量,实验表明,该算法能够在峰值信噪比和码流比特率变化轻微的前提下减少50%以上的运算量。

A directional gradients operator is proposed in this paper,and a novel H.264 intra prediction was introduced 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.