Complexity-Efficient Rate Estimation for Mode Decision of the HEVC Encoder

The rate-distortion (R-D) optimized mode decision is very important to obtaining full benefit from various powerful coding tools in the most recent video coding standard, namely the high efficiency video coding (HEVC). In deciding a certain coding mode, the R-D optimality criterion takes into account both distortion and bit rate. The bit-rate is generally measured from the number of bits in the bitstream that context-adaptive binary arithmetic coding (CABAC) generates through performing a series of binarization, context modeling, interval subdivision, renormalization, outstanding bit handling, and context updating. Its simplification is important to reduce the high complexity of actual bit rate estimation by CABAC. This paper proposes the efficient estimation of output bits by counting only the number of generated binarized output of the binarization process in CABAC. The proposed method only executes binarization, without performing context modeling, interval subdivision renormalization, outstanding bit handling, or context updating. We evaluate the performances of the proposed method using HEVC reference software, and our experimental results verify that the proposed scheme is very effective in reducing the complexity of rate calculation. Compared to the HEVC reference software that performs all the CABAC execution steps, the proposed method shows an average saving in rate calculation time for all intra, low delay, and random access conditions of 55.29%, 41.05%, and 42.96%, respectively.