Joint Collaborative Team on Video Coding (jct-vc)

5.12.9High-level syntax for weighted prediction

The following 5 documents are related to HL syntax and were discussed in Track B. Only I0421 had some aspects that are directly related to prediction and was shortly presented in track A.

JCTVC-I0260 On Weighted Prediction Parameter Signalling [Y. He, J. Dong, Y. Ye, E. S. Ryu (InterDigital)]

In the current HEVC CD, the explicit Weighted Prediction parameters in B slices are signalled either for the L0 and L1, or for the combined list LC. Switching of these two types of signalling is controlled by the flag ref_pic_list_combination_flag. In this contribution, a revised WP parameter signalling method is proposed with the goal to fix problems in the current scheme and to simplify the syntax design for WP parameter signalling.

In the context of the fact that I0125 was adopted, the remaining proposal is to add a flag for B slices when weighted prediction is enabled, and share the weights for both lists when the flag is set to 0.

It was remarked that it is undesirable for the flag to be there when unlikely to be used (e.g. when the lists contain different pictures).

The contribution relates to I0439.

It was suggested that we made the presence of the flag depend on something (such as decoder recognition of identical reference picture lists), the proposal seemed arguably useful. However, it seemed appropriate to conduct further study rather than trying to act at this time (e.g. considering the change already introduced by I0125).

Another sub-proposal was to be able to share weights for some individual pictures when the same picture appears in both lists but the lists are not identical.

A scheme for predicting the value of the WP parameters was also described.

For further study.

JCTVC-I0279 Revised text of explicit weighted prediction [A. Tanizawa, T. Chujoh, T. Yamakage (Toshiba)]

This topic was resolved by the action taken on I0125.

JCTVC-I0335 On weighted prediction signalling [Y. Chen, M. Coban, Y.-K. Wang, W.-J. Chien (Qualcomm)]

This topic was resolved by the action taken on I0125.

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 the uni-prediction case gives a benefit.

This topic was resolved by the action taken on I0125.

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

This topic was partly resolved by the action taken on I0125. It could still apply, but would prevent using multiple weights on the same reference picture. No action was taken on this.

5.12.10Multi-topic contributions

JCTVC-I0143 High level syntax modifications [B. Li, H. Li (USTC), H. Yang (Huawei)]

Reviewed in HLS BoG (chaired by J. Boyce).

This document presents several modifications of the high level syntax of the current HEVC design. The proposed modifications include: 1) a nal_ref_flag restriction for CRA pictures; 2) differentially signalling of per-temporal layer information; 3) the position of signalling per-temporal information in SPS; 4) the derivation of POC when num_reorder_pics is 0, 5) a proposed change to the motion merging.

Consideration of each item in the summary above is noted as follows:

1) CRA restriction on nal_ref_flag: nal_ref_flag shall be equal to 1 for CRA pictures.

The BoG recommended to adopt. Decision (Ed.): Adopt (this is just an editorial clarification of the existing intent).

2) To introduce a variable in order to save some bits in the SPS. There was a relevant SW NB comment to disallow smaller values. The BoG recommended to note the restriction recommended in the NB comment.

3) It was commented that the picture resolution should be syntactically earlier than these parameters. The BoG recommended no action.

4) The BoG recommended no action.

5) See the notes relating to JCTVC-I0414.

JCTVC-I0330 Comments on HEVC text spec draft 6 [Y.-K. Wang, M. Coban, Y. Chen, R. L. Joshi, J. Chon, A. K. Ramasubramonian (Qualcomm)]

Reviewed in HLS BoG (chaired by J. Boyce).

The contribution proposes to remove the constraint on maxPicOrderCnt – minPicOrderCnt.

It was asked whether, in the current specification, the picture corresponding to prefRefPic needs to be in the RPS. A participant indicated that there is currently no expressed requirement in this regard.

It was suggested that if we keep the constraint, the prefRefPic picture should be in included the set of pictures over which the difference constraint applies. Decision: Agreed (although the need for the constraint is planned for further study).

There were many parts in the contribution, identified as follows:

• 
  temporal_id semantics, distinguishing between VCL and non-VCL NAL units. Defining a temporal_id of an access unit. It was remarked that we may want to add to the definition section also. The relationship with TLA was questioned. It was requested to have some off-line work to ensure correctness and get the details sorted out – document I0600 was later provided as the result of that.
  
• 
  Tile syntax in SPS and PPS – see notes relating to JCTVC-I0113.
  
• 
  picture size signalling in multiples of 8. It was noted that other standards referring to the specification may benefit from referring directly to a syntax element. No action.
  
• 
  slice header byte alignment. In the text, the slice header may or may not be an integer number of bytes. The HM always has slice headers aligned. It is noted that ALF and SAO no longer use slice header signalling. Decision: At the end of the slice header, add a 1 bit followed by the number of 0 bits necessary to achieve byte alignment (regardless of whether tiles or wavefronts or neither are in use).
  
• 
  Editorial, new VLC code description. Editors can consider. No action.
  
• 
  Entropy slices. This has a sort of inverse spirit to that of I0229. After consideration, the proponent of I0330 withdrew this suggestion. Another aspect of what was proposed here (syntax cleanup for entropy slice) was addressed by other actions taken. It was proposed for cabac_init_flag and slice_qp_delta to either both be in the ES header or both not in it. Decision: Have only slice_type (0=B, 1=P, 2=I, 3=entropy/dependent) and the slice address in the ES header. No longer use entropy_slice_flag.
  
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  coding of slice address in the slice header. See notes relating to JCTVC-I0113.
  
• 
  Applicability of inter-RPS prediction. It was suggested to further change the inter-RPS to add a parameter to indicate whether it is being called by the SPS or the slice header. Further study was encouraged.
  
• 
  Regarding the relationship between TLA and temporal_id_nesting_flag, it was suggested to always mark as TLA (when not an IDR or CRA). Decision: When temporal_id_nesting_flag is equal to 1 and temporal_id > 0, nal_unit_type shall be equal to 3.
  
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  POC constraint. For further study.
  
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  Replace rbsp_trailing_bits( ) with byte_alignment( ) in slice_data. For further study. Decision (Ed.): As an editorial refinement, replace rbsp_trailing_bits( ) with the equivalent syntax of a 1 bit followed by the number of 0 bits necessary to achieve byte alignment in any such cases that do not end an RBSP.
  
• 
  Change num_reorder_pics[ i ] semantics. Decision (Ed.): Just an editorial correction – fix.
  
• 
  Semantics of list_modification_present_flag. Just an editorial issue – to be addressed by the editors (there is also a Sweden NB comment on this about integration problem on H0412). There is already a ticket #512 to track this in the bug tracking system.
  
• 
  Editorial comments. The editors can consider these suggestions. We should add ticket(s) to track this in the bug tracking system.
  

JCTVC-I0600 On semantics of temporal_id and related [Y.-K. Wang (Qualcomm), J. Boyce (Vidyo)] [late]

This document proposes the following, to follow up on the issues raised by I0330:

1) A change to the semantics of temporal_id

2) A change to the bitstream extraction process

3) Adding a restriction on the presence of PPS and APS

The first two proposal pieces were proposed in JCTVC-I0330, and the corresponding proposal pieces here provided refined text after discussion of JCTVC-I0330 at the same meeting.

This is a cleanup of the bitstream extraction process.

Decision: Adopt.