Cross-Color Channel Perceptually Adaptive Quantization for HEVC

Abstract

We propose a novel Coding Unit (CU) level luma-chroma perceptually adaptive quantization technique for HEVC, we name this technique C-BAQ. C-BAQ is designed to improve upon the luma-only adaptive perceptual quantization method adopted by JCT-VC. This luma-only perceptual quantization technique, named AdaptiveQP, has demonstrated coding improvements compared with Uniform Reconstruction Quantization, it increases or decreases the Quantization Parameter (QP) of an entire 2N × 2N CU based on the spatial activity of raw samples in the constituent luma Coding Block (CB). AdaptiveQP does not take into account the spatial activity of raw samples in the corresponding chroma Cb and chroma Cr CBs, however, which constitutes a significant shortcoming of the method. In contrast to AdaptiveQP, the proposed C-BAQ technique accounts for the spatial activity of the raw samples in both the luma CBs and also the chroma CBs when modifying the QP at the CU level. The primary objective with C-BAQ is as follows: to significantly decrease bitrates, in comparison with AdaptiveQP, without incurring a discernible loss of reconstruction quality. With respect to the evaluation of C-BAQ, we implemented the proposed method into HEVC HM 16.7 and subsequently performed comprehensive testing. In the simulations, we compared C-BAQ with AdaptiveQP and tested both methods on YCbCr 4:4:4, 4:2:2 and 4:2:0 JCT-VC test sequences using the base QPs 22, 27, 32 and 37 with the All Intra and Random Access encoding configurations. In the subjective visual quality evaluation, 81.25% of the participants either discerned no differences between C-BAQ coded sequences and AdaptiveQP coded sequences, or C-BAQ coded sequences were interpreted to be superior. In the objective visual quality evaluation, and in comparison with AdaptiveQP, C-BAQ achieves significant coding efficiency improvements, with a maximum BD-Rate reduction of 15.9% (Y), 13.1% (Cr) and 16.1% (Cb). Improved encoding times and decoding times are also attained, with maximum reductions of 6.0% and 11.0%, respectively.