Fast Coding-Unit Mode Decision for HEVC Transrating

Transrating is a bitrate transcoding technique that facilitates video applications in heterogeneous environments. This paper presents a fast coding-unit (CU) mode decision algorithm for transrating a once-encoded HEVC bitstreams to lower bitrate versions of diverse quality levels. The proposed method comprises three major parts. First, an early SKIP decision is based on the modes and motion vectors of the correlated CUs. Second, the CU depth search is restricted to a range estimated by a weighted average of the depth levels of correlated CUs and the weights are adapted to the quantization parameter. Third, a CU with a higher rate-distortion (RD) cost (judging from the RD costs of correlated CUs) will be directly divided into smaller CUs without evaluating RD costs of the remaining partitioning modes at the current depth level. Experimental results substantiate the superiority of the proposed method, which averagely reduces the encoding time by 55.01% with only 0.09 dB BD-PSNR degradation and 2.26% BD-rate increase.