JVET-P0147 AHG18: BDPCM in lossless coding [Z.-Y. Lin, T.-D. Chuang, C.-Y. Chen, C.-W. Hsu, Y.-W. Huang, S.-M. Lei (MediaTek)]

This contribution is to improve the block-wise differential pulse code modulation (BDPCM) mode in lossless coding. In VTM6.0, the BDPCM processes are implemented in the quantization-related functions, which are skipped if a BDPCM block is coded with lossless mode. To fix this problem, it is proposed to move the forward BDPCM and backward BDPCM outside the quantization function and inverse quantization function, respectively, so the BDPCM processes will not be skipped in lossless mode coding. To further improve BDPCM in lossless coding, two methods are proposed. In Method 1, BDPCM is applied to both luma and chroma. In Method 2, unlike always applying transform skip (TS) coefficient coding to BDPCM blocks in VTM6.0, non-TS coefficient coding can be applied to BDPCM blocks. Three schemes are developed for Method 2. To indicate applying non-TS or TS coefficient coding to BDPCM blocks, Scheme 1 proposes a flag in the sequence parameter set (SPS), and Scheme 2 proposes a flag in the picture parameter set (PPS). Scheme 3 always applies non-TS coefficient coding to BDPCM blocks coded with lossless mode. The results of combining the bug fix, Method 1, and Method 2 with Scheme 3 reportedly show 2.23% and 0.57% bit savings under AI and RA, respectively, when compared to VTM6.0-lossless software.

About method 1:

• Already decided: BDPCM chroma for 4:4:4 from CE8, however no benefit was shown for low QP range (mostly motivated by design consistency for RGB)
• No results on BDPCM benefit for 4:2:0 case

Establish CE to investigate benefit of BDPCM for chroma in case of lossless coding