The H.264/AVC standard improves coding efficiency significantly compared with existing standards. This is at the
expense of great computational complexity at the encoder. H.264/AVC adopts rate-distortion optimization (RDO)
technique to obtain the best result while maximizing visual quality and minimizing bitrates. To carry out RDO, the
encoder undergoes all possible combinations of coding modes exhaustively. H.264/AVC exploits directional spatial
correlation with adjacent blocks for intra prediction. There are 9 directional modes for 4x4 luma blocks, 4 directional
modes for 16x16 macroblocks and 8x8 chroma blocks, respectively. To determine the appropriate direction, based on
the RDO computation, the number of mode combinations in each MB is 592. For a fast intra mode decision, we propose
an algorithm to reduce the number of RDO computations using simple directional masks and adjacent mode
information. The proposed method carries out 4 RDO calculations at the most for 4x4 luma blocks, and 2 RDO
computations for 16x16 luma blocks and 8x8 chroma blocks, respectively. We verified the proposed algorithm by
implementing on JM reference software and comparing with the case of exhaustive search. Simulation results show the
proposed method reduces the ending time up to 70% with negligible loss of PSNR and bitrate increase.
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