Efficient Mode Decision Schemes for HEVC Inter Prediction

The emerging HEVC standard reduces the bit rate by almost 40% over the preceding state-of-the-art standard AVC with the same objective quality but at about 40% encoding complexity overhead. The main reason for HEVC complexity is inter prediction that accounts for 60-70% of the whole encoding time. This paper analyzes the rate-distortion-complexity (RDC) characteristics of the HEVC inter prediction as a function of different block partition structures and puts the analysis results into practice by developing optimized mode decision schemes for the HEVC encoder. The HEVC inter prediction involves three different partition modes: square motion partition (Square), symmetric motion partition (SMP), and asymmetric motion partition (AMP) out of which the decision of SMPs and AMPs are optimized in this work. The key optimization techniques behind the proposed schemes are 1) a conditional evaluation of the SMP modes, 2) range limitations primarily in the SMP sizes and secondarily in the AMP sizes, and 3) a selection of the SMP and AMP ranges as a function of the quantization parameter. These three techniques can be seamlessly incorporated in the existing control structures of the HEVC reference encoder (HM) without limiting its potential parallelization, hardware acceleration, or speed-up with other existing encoder optimizations. Our experiments show that the proposed schemes are able to cut the average complexity of HM encoder by 31-51% at a cost of 0.2-1.3% bit rate increase under the random access (RA) coding configuration. The respective values under the low-delay B (LB) coding configuration are 32-50% and 0.3-1.3%.