The contribution explores the relationship between the current CRA design and temporal layering.
This contribution presents a proposal to change the signaling of Clean Random Access (CRA) pictures and Temporal Layer Switching Points in what is referred to as Temporal Layer Access (TLA) pictures. It is proposed to replace the CRA Network Abstraction Layer (NAL) unit type with a TLA NAL unit type.
The proposed TLA NAL unit type imposes constraints on the bitstream and does not have an impact on the decoding process.
It is stated in the contribution that both random access information and temporal layer switching information is of high value to a network node and thus should be available in NAL unit header,
independent of data outside that NAL unit header, specifically in the Sequence Parameter Set (SPS) and Picture Parameter Set (PPS). It is further stated that a unified syntax and definition of TLA pictures makes the standard text more readable and comprehensive.
It was remarked that a CRA must be an intra picture.
The proposal suggests, for temporal_id values greater than 0, to replace the current CRA design with a temporal layer switching point indicator.
It was suggested that the current CRA design seems to not be useful for temporal layer 0. This seemed to be generally agreed. So it may be interesting to explore an alternative specification for the syntax flag in that case. Some misgivings were expressed about the specifics of the proposal, although the general idea appears interesting and potentially useful.
It was noted that this proposal changes the temporal layer switching behaviour from what is currently specified. This proposal disallows having a switching point picture that uses an earlier picture in the same temporal layer as a reference, and it was questioned whether this is desirable or not.
Further study was encouraged (delegate to an AHG).
JCTVC-G834 Cross-check result of Samsung’s proposal JCTVC-G533 by LG Electronics [Hendry, S. Park, B. Jeon (LG)] [late]
Not necessary to be presented – can be studied.
5.9.8 Decoded picture buffering and reference picture signaling
JCTVC-G157 Reference List Construction for Random Access Settings [Hendry, S. Park, B. Jeon (LG)]
(This had not been reviewed in the BoG relating to DPB topics.)
In the 6th JCT-VC meeting, based on contribution JCTVC-F433 and JCTVC-F701, reference picture list construction by using 3 higher quality and 1 nearest reference pictures has been adopted in the common conditions for low delay settings. This contribution proposes to construct default reference picture lists differently. When constructing RefPicListX, the proposed scheme suggests sorting reference pictures in Decoded Picture Buffer by POC first and then by the picture-level QP value relative to the QP value of current picture, instead of only by POC as it is done currently.
It was noted that the described behaviour is only correct for B pictures, and that the description is only a matter of the default list order; the default can be changed by reference picture list modification syntax. If this behaviour is desirable, it can be done explicitly by the encoder in this way.
It was remarked that this adds an extra sorting step and complication to the initialization.
It was noted that explicit mode reference picture marking can also be used to change the default list initialization values.
It was noted that larger aspects of reference picture list construction are being considered, and remarked that this may be an over-optimization relative to excessively emphasizing our common conditions configurations, which are not part of the standard – rather, they are just a matter of how we are using the standard in some example tests.
It was noted that the selection of QP values could end up being manipulated for purposes of reference picture list construction, which seems like an unusual repurposing, and might result in sending more PPS syntax structures so that this manipulation can be done.
The group found it interesting that gain was being reported due to using a different set and ordering of reference pictures than what is our current common conditions. So there could be an opportunity here for non-normative coding efficiency improvement of future common conditions (0.4% Y, 0.4% U, and 0.4% V for RA HE).
It was noted that G589 showed some gain (0.2%) relative to our current common conditions while using much less picture storage. See notes elsewhere. Revisit tThis aspect was discussed again on Nov 29 (FB chairing). Further study was encouraged.
JCTVC-G500 Cross-verification of LG's Reference List Construction for Random Access Settings (JCTVC-G157) by Panasonic [C. S. Lim, S. M. T. Naing, V.
Wahadaniah (Panasonic)]
JCTVC-G166 AHG21: Explicit Reference Pictures Signaling with Output Latency Count Scheme [Hendry, S. Park, B. Jeon (LG)]
Reviewed in BoG.
JCTVC-G832 Cross-check of "AHG21: Explicit Reference Pictures Signaling with Output Latency Count Scheme"(JCTVC-G166) [Y. Park, I.-K. Kim, C. Kim
(Samsung)] [late]
JCTVC-G198 AHG21: Inter reference picture set prediction syntax and semantics. [T.K.
Tan, C.S. Boon (NTT Docomo)]
Document JCTVC-F493 proposed the explicit signaling of reference pictures needed for the inter prediction of the current and future pictures, using buffer descriptions (reference picture sets). A
reference picture set is a set of ΔPOC values. ΔPOC values are picture order count (POC) of the reference pictures relative the current picture. Template reference picture sets are signaled in the picture parameter set (PPS) and referred to by each slice.
This contribution proposes to further reduce the amount of bits necessary for signaling the reference picture set by predicting the ΔPOC values using the ΔPOC values from a reference picture set already present in the PPS.
Based on the latest draft of the reference picture set syntax from the ad hoc group on Reference picture buffering and list construction (AHG21), the number of additional PPS signaling bits needed for the random access (RA) and low delay (LD) common conditions are reported to be 288 and 201 bits,
respectively. Using the proposed inter reference picture set prediction method, the numbers of bits needed are reported to be reduced to 144 and 106 bits, respectively. This represents a reduction of 50% and 47%, respectively.
It was noted that there are multiple inputs that remain under consideration regarding the APS and details of the RPS design.
It was suggested that this may be a degree of over-optimization within the context of a scheme that is not yet a really settled area of the design. There are also multiple ideas on the table that are available for compressing the number of bits needed for the RPSs. G643 was suggested as one example. However, it was also suggested that this proposal has been well studied and implemented, has good text, etc., and seems relatively mature. It was remarked that without this proposal, the current G1002 scheme would have an obvious redundancy in relation to cyclic picture structure encoding.
Decision: Adopted (Part 1 “full inter-RPS prediction”).
JCTVC-G314 AHG21: On DPB management [Y.-K. Wang, Y. Chen (Qualcomm)]
Reviewed in BoG.
JCTVC-G398 High-level Syntax: Marking process for non-TMVP pictures [B. Li (USTC), J. Xu (Microsoft), H. Li (USTC)]
(Chaired by J. Boyce.)
In the current HM design, MVP and merge candidates may be incorrect when packet loss exists. The error may influence motion vectors of all the following pictures. Disabling some pictures’ TMVP (non-TMVP pictures) may stop the error propagation for motion vectors. However, it cannot be always guaranteed that the error can be stopped with non-TMVP pictures. This contribution presents a mechanism to make sure that the error can be stopped after non-TMVP pictures
enable_temporal_mvp_flag had been previously adopted at Torino meeting to the PPS, but not included in the WD yet., Contribution proposes semantics changes for the use of that flag.
Suggestion to move enable_temporal_mvp_flag to SPS,which would have a cost of 3% if turned off, but may be worthwhile in error prone conditions.
Decision: Adopted solution 2.
Will work with editors to incorporate both this adoption and the previous BoG adoption of the enable_temporal_mvp_flag in the WD text and software.
JCTVC-G526 AHG21: Combined signaling for reference picture set [Y. Park, I.-K. Kim, C. Kim (Samsung)]
Reviewed in BoG.
JCTVC-G546 On high-level syntax for maximum DPB size and frame latency [Y. Park, K.
P. Choi, C. Kim (Samsung)]
Chaired by J. Boyce.
JCTVC-E339 proposed to move max_dec_frame_buffering and num_reorder_frames from the optional VUI to mandatory SPS. The JCTVC-F541 proposed to add max_latency_frames_plus1 or
max_latency_increase_plus1. We propose move max_dec_frame_buffering and add
max_latency_frames_plus1 in SPS. We propose the num_reorder_frames to be left in VUI without change.
If an encoder doesn’t send the VUI parameters, capabilities determination by decoder would be negatively impacted.
Without max latency, output is delayed.
Suggestion to also move num_reorder_frames to SPS.
Decision: Adopt put three syntax elements in the SPS, max_dec_frame_buffering, num_reorder_frames, and use max_latency_increase . (Also JCTVC-G779)
JCTVC-G779 Proposed constraint on reordering latency (for further consideration of JCTVC-F541) [G. J. Sullivan (Microsoft)]
Chaired by J. Boyce
This contribution repeats the content of JCTVC-F541 to propose to add an SPS-level parameter in HEVC that expresses a constraint on the maximum amount of reordering that can be applied to any frame in a coded video sequence. By comparing the latency status of each frame in the DPB to the value of the maximum latency constraint, a decoder can determine when the maximum latency limit has been reached, and can immediately output any frame that has reached this limit. It is asserted that this can enable the decoder to more rapidly identify frames that are ready for output than with the current syntax for a variety of video encoding structures that includes typical cases. It is also asserted that directly expressing such a limit on the amount of reordering latency allowed through the encoding-decoding process would be a useful characteristic to be established for system-level negotiation and characterization purposes.
This proposal essentially just repeated the content of JCTVC-F541 of the previous meeting, as the disposition of that contribution at the previous meeting was to essentially allow time for further study.
Suggestion to express value as POC difference rather than picture count difference.
Covered in JCTVC-G546.
JCTVC-G548 AHG21: Construction and modification of predefined reference picture sets and reference picture lists [V. Wahadaniah, C. S. Lim, S. M. T. Naing
(Panasonic)]
Reviewed in BoG.
JCTVC-G635 Coding with a unified reference picture list [M. Naccari (BBC), G. Van Wallendael (Ghent University), M. Mrak, D. Flynn (BBC)]
The unified reference picture list (LU - List Unified) was presented in contribution JCTVC-F549 with the aim of providing a simpler and more flexible structure to map the reference picture used during the inter coding process in the HM codec. The main idea behind the LU is to simplify mapping of reference pictures by using only a single reference list whereby reference frame pairs are stored. A reference pair consists of two reference frames (in the case of bi-directional prediction) or one reference frame and a null element (in the case of uni-directional prediction). It was asserted that the usage of the LU reduces the bitstream parsing and enables adding/removing some combinations of references in a more flexible fashion than the current HM design using two reference lists. In this context, this contribution addresses the reference list indexes usage in the current HM 4.0 codec and describes an implementation of the LU scheme based on the default HM reference settings. It is reported that the experimental results obtained for this implementation show that the LU scheme can handle usual HM conditions, while providing a space for more flexible selection of reference frames.
It was asked what is the impact of the scheme on coding efficiency? Some loss in compression was reported. It was suggested that this was due to the fact that the different method of coding of the reference picture indexes was not included in the R-D decision-making process. (At the previous meeting, some gain had been shown when using a similar but slightly different scheme, in a usage that included the scheme within the R-D optimization.)
It was remarked that this may require a difficult coupling of the encoder’s decision-making process of joint selection of the two reference pictures to use for references. For example, if there are 10 pictures in each list, then there would be 120 entries needed in the combined list. Initalizing, reordering, and
managing such a large list might get difficult. The overhead for reference picture list reordering might be large.
It was remarked that the only clear benefit would seem to be simplification of the parsing of the reference indexes at the PU level. Further study would be needed to identify and clarify whether a significant benefit can be shown for this concept.
JCTVC-G991 Cross-check of contribution JCTVC-G635: Coding with a unified reference picture list [J. Jung (Orange Labs)] [late]
JCTVC-G549 Syntax rearrangement for list combination [Y. Park, S. Jeong, C. Kim (Samsung)]
A list combination (LC) scheme was proposed for uni-prediction at B-slices to improve coding efficiency.
The current syntax bit ref_pic_list_combination_flag seems a bit redundant.
Proposes to put a default combined list length in the PPS and modify the slice-level syntax and remove that bit.
There were some differences in the proposed design aspects in the newer version of the proposal.
Another way to deal with that bit was suggested – further study was suggested.
JCTVC-G717 Improvements on reference picture buffering and list construction [Y. Yu, K. Panusopone, X. Fang, L. Wang (Motorola Mobility)]
This contribution proposes changes of reference picture construction of combined list and an explicit way for signalling collocated pictures according to the value of delta POC. The proposed scheme was reported to be more efficient to build the combined list and signal the collocated picture.
The proposal assumes the RPS (G021) style of buffer control.
It is proposed to allow any picture within list 0 or list 1 (or perhaps within the RPS list) to be specified to be the “collocated picture” (using an index syntax element in the RPS syntax or in the slice header).
Currently the collocated picture is always the first picture in list 0 or the first picture in list 1.
Some tests were done to see that the proposed technique works; however, there was no compression benefit shown overall. Further study would be needed to determine whether there may be a significant benefit for this concept.
Also proposed was a change of the default order of combined reference picture lists, based on pair-wise minimization of POC distance. Test results were not provided, so the work seemed somewhat
preliminary, and further study would be needed to determine whether it has value.