Adaptive Macroblock Mode Selection for Reducing the Encoder Complexity in H.264

The H.264/AVC standard is a video compression standard that was jointly developed by the ITU-T Video Coding Experts Group and the ISO/IEC Motion Picture Experts Group. The H.264 video coding standard uses new coding tools, such as variable block size, quarter-pixel-accuracy motion estimation, intra prediction and a loop filter. Using these coding tools, H.264 achieves significant improvement in coding efficiency compared with existing standards. Encoder complexity, however, also increases tremendously. Among the tools, macroblock mode selection and motion estimation contribute most to total encoder complexity. This paper focuses on complexity reduction in macroblock mode selection. Of the macroblock modes which can be selected, inter8×8 and intra4×4 have the highest complexity. We propose two methods for complexity reduction of inter8×8 and intra4×4 by using the costs of the other macroblock modes. Simulation results show that the proposed methods save about 55% and 74% of total encoding time compared with the H.264 reference implementation when using a full search and a fast motion estimation scheme, respectively, while maintaining comparable PSNR.