Efficient and Fast Coefficient Sign Inference for Video Coding

In the modern video coding standards, transform coefficient coding takes large portion of output bit-stream. The absolute value of coefficients are usually represented with high-efficiency variable-length code, while the signs are binarized with 1-bit fixed-length code, which ignores the apriority constraints on sign combinations beyond signs. In this paper, we present a fast and efficient sign inference algorithm for improving the coding efficiency of coefficients signs. By considering the correlation of pixels across the transform block boundary, we define a criterion of continuity to constrain the combination space of signs in the block, and shorten the bits will be used. A given number of signs are hidden during bypass coding, and inferred with the high-precision criterion. Then the infer-error is coded with arithmetic coding. Meanwhile a sign bit redefining strategy is introduced, which is based on the invariants of the linear orthogonal separable transform. The strategy ensures that the codec is able to choose up to 4 bits for inferring each block, and avoids the complex decision mechanism introduced by rate distortion optimization. Experiment results show that about 0.6% in average and up to 1.1% BD-rate saving is archived and only less than 1% encoding time complexity increased compared with HEVC baseline.