JVET-K0556 CE1-related: Constraint for binary and ternary partitions [C.-W. Hsu, T.-D. Chuang, C.-Y. Chen, Y.-W. Huang, S.-M. Lei (MediaTek)] [late]
Discussed Tuesday 1000 (GJS)
Virtual pipeline data units (VPDUs) are defined as non-overlapping MxM-luma(L)/NxN-chroma(C) units in a picture. In hardware decoders, successive VPDUs are processed by multiple pipeline stages at the same time; different stages process different VPDUs simultaneously. The VPDU size is roughly proportional to the buffer size in most pipeline stages, so it is said to be very important to keep the VPDU size small. In HEVC hardware decoders, the VPDU size is set to the maximum transform block (TB) size. Enlarging the maximum TB size from 32x32-L/16x16-C (as in HEVC) to 64x64-L/32x32-C (as in the current VVC) can bring coding gains, which results in 4X of VPDU size (64x64-L/32x32-C) expectedly in comparison with HEVC. However, in addition to quadtree (QT) coding unit (CU) partitioning, ternary tree (TT) and binary tree (BT) are adopted in VVC for achieving additional coding gains, and TT and BT splits can be applied to 128x128-L/64x64-C coding tree blocks (CTUs) recursively, which is said to lead to 16X of VPDU size (128x128-L/64x64-C) in comparison with HEVC. To reduce the VPDU size in VVC, a constraint for TT and BT is proposed, and the VPDU size is defined as 64x64-L/32x32-C for the following.
- Cond. 1: For each VPDU containing one or more CUs, the CUs are completely contained in the VPDU.
- Cond. 2: For each CU containing one or more VPDUs, the VPDUs are completely contained in the CU.
The contribution proposed to impose the constraint that, for each CTU, the above two conditions shall not be violated, and the processing order of CUs shall not leave a VPDU and re-visit it later.
For the current scheme, these constraints would be satisfied if three constraints are applied.
- Prohibit ternary split of edges longer than 64 (32 for chroma)
- Prohibit vertical split when width is 64 and height is 128 (half these for chroma)
- Prohibit horizontal split when width is 128 and height is 64 (half these for chroma)
It was reported that imposing these three constraints would have a significant coding efficiency impact. Another way to meet the constraint would be to set MAX_TT_SIZE and MAX_BT_SIZE to 64, likely accompanied by increasing the BT/TT depth.
Further study in a CE is neeed to test some approaches and determine the coding efficiency impact.
CE2 related – Loop filters (15)
Contributions in this category were discussed Sunday 15 July in Track B 0900–1220 (chaired by JRO).