Joint Collaborative Team on Video Coding (jct-vc)

5.10Motion compensation operation and interpolation filters

5.10.1Interpolation filters and MV precision

JCTVC-I0142 Multiplication free interpolation filter implementation [B. Li, H. Li (USTC), H. Yang (Huawei)]

This contribution proposes multiplication free interpolation filter implementation. It is only an optimization at code level without any RD performance difference. With this multiplication free implementation, about 10% encoding time saving and 8% decoding time saving was obtained.

This would be non-normative implementation optimization (no text change). Communicate with SW coordinator whether this is desirable. Comment by one expert: It may sometimes be better if the reference software is not deviating too much from the text description. (Revisit: Mark as Decis.(SW) if SW coordinator agrees).
JCTVC-I0550 Sum-128 luma/chroma interpolation filter [Hongbo Zhu (??)] [late] [miss]

Presenter not available and document not uploaded Sat 4/28

5.10.2Weighted prediction

JCTVC-I0109 New results of implicit weighted prediction [A. Tanizawa, T. Chujoh, T. Yamakage (Toshiba)]

This document reports results for the implicit weighted prediction on IBBBP (M=4) coding structure. AVC-based weighted prediction scheme was adopted in HM and WD. The explicit WP can achieve large coding gain for fading sequences with illumination variation. However, the gain of the implicit WP compared to the explicit WP on current coding structures (RA and LDB) is not so high. The main reason is that AVC based implicit WP focuses on IBBP (M=3) coding structure originally and the current coding structures of HM common test conditions are not effective. For example, in the case of RA coding structure, the some weighting factors deriving from the implicit WP are identical to the default weighting factor. In this contribution, it is reported that the implicit WP on IBBBP (M=4) coding structure is tested and reported. AVC-based implicit WP can get 3.3% BD-rate gain on average for black-fade/white-fade sequences.

Presentation not uploaded.

Implicit WP only used for B pictures

Explicit WP gives roughly 20% BR gain for the same fade sequences

Question: Is implicit WP useful at all? Even the one in AVC seems to be hardly used, and it cannot compete with the explicit signalling.

JCTVC-I0110 Improved implicit weighted prediction [A. Tanizawa, T. Chujoh, T. Yamakage (Toshiba)]

This document presents an improved implicit weighted prediction. Weighted prediction (WP) scheme consists of two modes, an explicit WP and an implicit WP. An implicit WP is a method of not explicitly encoding WP parameters, but implicitly deriving them from information of reference pictures. In AVC-based implicit WP, only weighting factors can be derived from the temporal distances among the target picture and two reference pictures used for bi-prediction in B-slices. However, the offset value is always set to 0. Therefore, there are issues in that the conventional implicit WP cannot compensate the pixel value additively and cannot be applicable to uni-prediction. In order to cope with these issues, the derivation process for implicit WP parameters based on image characteristics of the reference pictures stored in decoded picture buffer is proposed. The experimental results in HM software version 6.0 including AVC-based implicit WP are reported. The BD-rate gain of the proposed method for black/white-fade sequences is 16% to 34% compared to HM common condition anchor. It is reported that this scheme can reduce the decoding time.

Presentation not uploaded.

Reduction of decoding time likely due to lower bit rate (less CABAC processing)

Here, the identical processing for deriving the WP weights is simply shifted to the decoder. Even then, it performs slightly worse than the explicit case with identical algorithm. Several experts express opinion that this is not desirable (more dec complexity, error resilience, too specific for fades, explicit WP would be needed anyway).
JCTVC-I0128 Cross-check report of Toshiba proposal JCTVC-I0110 [P. Bordes (Technicolor)] [late]
JCTVC-I0115 Improvement of Implicit Weighted Prediction [P. Bordes, P. Andrivon (Technicolor)]

This document presents an improved implicit weighted prediction (IWP). Weighted prediction (WP) scheme consists in two modes, an explicit WP and an implicit WP. In Explicit WP, the weighting parameters are transmitted explicitly, while in Implicit WP they are derived from the temporal distances of the two reference pictures used for bi-prediction in B-slices.

In the current IWP, the offsets are unforced to zero and it is applicable to bi-prediction and B-slices only. In RA (hierarchical GOP) the weights of closest references are equal to defaults.

The proposed IWP allows to cope with these limitations by interpolating or extrapolating the Explicit WP parameters, in the case Explicit and Implicit WP are combined.

The experimental results in HM software version 6.0 are reported. The BD-rate gain depends on the proportion of frames using Explicit WP and Implicit WP (EP=Explicit Period). For EP=4, the BD-rate gain of the proposed method for black/white-fade sequences is 17% to 34%.

Implicit weights are interpolated from explicit weights (e.g. the two reference pictures in a B frame pyramid)

This could rather be interpreted as a method of coding for the explicit parameters, cannot replace the explicit signalling. Also here it does not give benefit in all cases compared to explicit, and the maximum benefit would be around 0.2-0.3%.
JCTVC-I0111 Cross-check of implicit weighted prediction (JCTVC-I0115) [A. Tanizawa, T. Chujoh (Toshiba)]

Conclusion on I0109, I0110 and I115: Explicit WP is still the most universal and best choice in terms of compression. Current implicit WP of CD is useless. Decision: Remove implicit WP from CD. (Tim Hellman provides text (new doc) and software)

JCTVC-I0260 On Weighted Prediction Parameter Signalling [Yong He, Jie Dong, Yan Ye, Eun S. Ryu (InterDigital)]
JCTVC-I0279 Revised text of explicit weighted prediction [A. Tanizawa, T. Chujoh, T. Yamakage (Toshiba)]
JCTVC-I0335 On weighted prediction signalling [Y. Chen, M. Coban, Y.-K. Wang, W.-J. Chien (Qualcomm)]
JCTVC-I0421 High-level Syntax: Improvements to weighted prediction for B slices [C. S. Lim, S. Mon Thet Naing (Panasonic)]

This was shortly discussed in the track A session: No evidence is given that the additional weighting parameters for uni-pred case give benefit.

JCTVC-I0439 On Explicit Weighted Prediction Signalling [Hendry, S. Park, Y. Jeon, B. Jeon (LG)]