Research On Fast Mode Selection Algorithm Of H.264

The H.264 video coding standard adopts many new coding tools, such as variable block size, multiple reference frames, quarter-pixel-accuracy motion estimation, intra prediction, loop filter, etc. Using these coding tools, H.264 achieves significant performance. Especially for reference frame, block size, and prediction direction, it can adaptively do the selection. However, the encoding complexity increases tremendously. Among these tools, the macroblock modes selection and the motion estimation contributes most to total encoding complexity. This paper focuses on complexity reduction in macroblock modes selection. On the basis of analyzing some fast mode selection algorithms, a new algorithm is presented, in order to reduce the time of encoding, in the precondition of reducing the quality of encoding obviously.For fast mode selection of H.264, the following research was done:(1)On the basis of JM8.6 reference software, the complexity of standard mode selection algorithm was analyzed based on the RD cost model. Then the essentiality of fast mode selection algorithm was discussed.(2)By rewriting some algoritthm in JM8.6 reference software, some statistic was done for the appearance probability of all the modes in the whole sequence. Then it found that the modes SKIP and inter16×16, whose complexity of encoding is tinny, dominated among other modes (the appearance probability arrives 80% on average), while the appearance probability of the modes inter8 X 8 and intra4 X 4, whose complexity of encoding is tremendous, is very small. Then, the feasible function to select modes fast was proposed. That is to fast select the modes whose appearance probability is large and restrict the selectable modes for modes whose appearance probability is small.(3)For fast selecting for SKIP mode, two common algorithms and another one proposed by ours was experimented, which was to compare the RD costs of SKIP and inter16×16 mode and decide if the best mode was SKIP mode. The correction percentage, missing percentage, PSNR and encoding time was compared. The result revealed that our method is a better one.(4)The algorithm which was to restrict the selectable sub-macroblock modes for inter8×8 mode by comparing the RD costs of inter16×16, inter16×8, inter8×16 was experimented. The result data showed that it reduced the time by 35% on average on the precondition of not reducing encoding quality obviously.(5)If fast intra mode selection algorithm was essential was analysed, then a simple algorithm for intra mode was propsed. That is to ignore intra4×4 mode for P frame and reserve the standard algorithm for I frame. The result showed that it reduced the time also by 35% on average on the precondition of not reducing encoding quality obviously.(6)In the end, the three algorithms were conbined and the experiment was done. The data shows that the reduction of time increased nearly to 40% while the PSNR just reduced nearly 0.05dB compared to standard algorithm. And the total bit also has a little reduction.