A Macroblock Homogeneity Detection Method and its Application for Block Size Decision in H.264/AVC

Abstract

〈 Summary 〉 The variable block sizes for intra and inter coding in H.264/AVC achieves significant coding gain compared with coding a macroblock (MB) with fixed size. How-ever, extremely heavy computational burden is required when Rate Distortion Optimization (RDO) process runs in brutal force searching manner for selecting the optimal coding block. This paper proposes an MB homogeneity detection method to accelerate H.264/AVC intra and inter coding. All the luminance values of pixels in an MB are taken to calculate their entropy feature, which is defined as MB’s spatial homogeneity. Based on homogeneity judgment, 16 × 16 or 4 × 4 block size is appropriately selected for intra coding; Meanwhile, either the large blocks in { 16 × 16 , 16 × 8 , 8 × 16 } or sub-blocks in { 8 × 8 , 8 × 4 , 4 × 8 , 4 × 4 } are chosen for inter coding. Especially, a cost function is defined to select near optimal threshold for selecting optimal block size. Proposed methods are verified on a wide range of video sequences with different spatial-/motion characteristics. Sufficient simulations demonstrate that consistent encoding gain is achieved for all videos with different motion and spatial features. Encoding complexity for intra coding alone can be reduced by 31%– 34% and time savings for inter mode decision is 43.7%–58.7%, both with negligible loss in bitrate and PSNR.