Of itu-t sg16 wp3 and iso/iec jtc1/SC29/WG11

JCTVC-M0226 Non-SCE3: Cross-check of Non-SCE3.3 Simplified Generalized Residual Prediction (JCTVC-M0275) [P. Lai, S. Liu (MediaTek)] [late]
JCTVC-M0321 Non-SCE3: Crosscheck for Simplified Generalized Residual Prediction (JCTVC-M0275) [W. Zhang, Y. Chiu (Intel)] [late]
JCTVC-M0297 Non-SCE3: Bandwidth reduction for combined inter mode [V. Seregin, M. Karczewicz (Qualcomm)]

This contribution presents additional results for combined inter mode tested in SCE3. It was noticed that combined mode introduces bandwidth increase comparing to HEVC single layer. In this contribution, combined mode usage is restricted for 8x8 coding unit with non 2Nx2N partition mode, with additional restriction of bi-prediction for 16x16 coding unit with non 2Nx2N partition mode. In the second method, bi-prediction is excluded for all coding units coded with combined mode. Experiments results show BD rate reduction from 0.2% to 2.6% for luma component and in a range from 1.5% to 8.8% for chroma components among RA 2x, RA 1.5x, RA SNR, LDP 2x, LDP 1.5x and LDP SNR test configurations with about 109% encoder complexity on average.

Method 3: Restriction of 8x8 bi-pred BR reduction 0.9% compared to SHM

Method 1: Restriction 8x8, 8x16, 16x8 bi-pred BR reduction 0.9% compared to SHM

Method 2: Restriction for all PU sizes (same as JCTVC-M0220, but combined prediction is not applied to chroma) BR red. 0.8% compared to SHM

Method 2 most promising

Differences w.r.t. JCTVC-M0220 – proponents were asked to identify differences and commonalities and report back – see notes regarding JCTVC-M0445.

(generally, the approach of JCTVC-M0220 and JCTVC-M0297 method 2 would enable a mode in intraBL that refidx already has, and gives decent gain without increasing the worst case complexity. However, both are not CE contributions, and cross-checks were only done very late – maturity of code?)

JCTVC-M0445 Uni-prediction for combined inter mode [V. Seregin, M. Karczewicz (Qualcomm), D.-K. Kwon, M. Budagavi (TI), T.-D. Chuang, Y.-W. Huang, P. Lai, S. Liu, S. Lei (MediaTek)– and others] [late]

(Presentation chaired by G. Sullivan.)

This contribution relates to M0297 and M0220, responsive to information requested during the meeting.

The contribution provides an analysis of two simplified solutions for combined prediction mode tested in SCE3.

A scheme was proposed in the contribution, based on the analysis of the prior proposed methods, characterized as follows.

	8x8 CU 2NxN, Nx2N	Uni-prediction	Bi-prediction Identical motion	Bi-prediction Non identical motion	Chroma
JCTVC-M0220	Not used	Combined prediction	Combined prediction	Bi-prediction	Combined prediction
JCTVC-M0297	Not used	Combined prediction	For Merge mode, bi-directional MV is converted to uni-L0 MV. For AMVP mode, inter direction signaling is restricted to be uni-direction.		Not used
Proposed	Not used	Combined prediction	Bi-prediction		Not used

A participant questioned the use of different processing for luma and chroma, and said that to justify a difference in processing between luma and chroma would need to be well justified. The presenter said that it works better as proposed. It was remarked that this might be indicative of a bug of some sort, and that we should avoid ad hoc selection of what looks the best at some point in time – that differences should be justified. Another participant indicated that with the alternative RefIdx approach no similar need for different processing of luma and chroma was observed.

Another participant remarked that differences between luma and chroma processing had also been proposed in other related CE contributions.

Another participant said that there should be alignment on the processing applied for luma and chroma in both the RefIdx and TextureRL approaches, so that this difference should not be applied.

Results of applying the proposed additional combined mode:

	Luma BD rate	Chroma BD rate	Enc	Dec
SCE3.2	−0.92	−4.35	113%	100%
M0220	−0.70	0.45	114%	102%
M0297	−0.79	−3.45	106%	99%
Proposed	TBD	TBD	TBD	TBD

We currently don't have a combined mode in the TextureRL approach.

It was remarked that the majority of the reported average gain is from LDP, and that this implies that there is no bipred in the RefIdx approach for LDP – thus this scheme somewhat "breaks the spirit" of LDP since it performs a type of bipred.

A participant suggested that there may be some adverse rounding effects in the proposed scheme.

No action.