International organisation for standardisation organisation internationale de normalisation

HL syntax in SHVC (6)

1. Generic HLS issues (1)

14.1.1.1.1.1.1.1.392JCTVC-O0251 SHVC HLS: On adaptive resolution change based on single_layer_for_non_irap_flag [V. Seregin, Y. Chen, Y.-K. Wang, A. K. Ramasubramonian (Qualcomm)]

Reviewed Tues. 29th (GJS).

See also notes for related contribution O0199.

In SHVC, a resolution change can be enabled by setting single_layer_for_non_irap_flag in the VPS equal to 1. This proposal discusses some issues related setting single_layer_for_non_irap_flag equal to 1, including some DPB related issues and other issues. It was proposed that, when a resolution switching occurs, all reference pictures in the DPB be as marked as "unused for reference" and other decoded pictures with PicOutputFlag equal to 0 be removed from the DPB. For other issues discussed, some bitstream constraints are proposed.

The first proposed action was in section 2 of the contribution.

It was noted that this interacts with the agreement to use a layer-specific DPB.

It was remarked that the indicated RPS behaviour could be achieved by, e.g., a VPS VUI indicator flag, or could be indicated on a picture basis, e.g., by an SEI message. The SEI message could indicate that no lower layer pictures will be used anymore, or only the last one in decoding order will be used, or only the one in the RPS with the highest POC value will be used, etc. Further study was encouraged, e.g., to draft such an SEI message.

It was remarked that the output order relationship between BL and EL for ARC should be studied and perhaps constrained.

Regarding section 3 in the contribution, it was remarked that the persistence might be difficult to specify, e.g., in relation to temporal sub-layering, in a way that would not be undesirably constraining, so no action was planned.

Regarding section 4 in the contribution, it was remarked that the alternative output layer flag would take care of this issue, so no action was planned.

The reaction regarding section 5 in the contribution was similar to that for section 3.

Regarding section 6, if there is a problem, it is not solved by the suggested text, and after discussion it seemed that no action was needed.

Regarding section 7, the extracted bitstream would still be decodable and potentially useful, so no action was determined to be needed.

2. Slice/picture skipping (4)

This category reviewed Tues. 29th (GJS).

It was remarked that skipping can only be done using low-level syntax in version 1 decoders, so skipping might not be considered to only be a high-level-syntax modification. It can hypothetically be expressed as high-level syntax, but is a low-level operation in a version 1 decoder.

14.1.1.1.1.1.1.1.393JCTVC-O0055 MV-HEVC/SHVC HLS: Skipped slice and use case [T. Yamamoto, T. Ikai, T. Tsukuba (Sharp)]

This contribution introduces an ROI-capable scalable video applications using skipped tiles. In the application, 1) a media box transfers modified a EL bitstream corresponding to the region requested by the users, 2) EL data is modified so that the outside-of-region part of the bitstream includes less data by replacing the outside-of-region with skipped tile, and 3) an SHVC decoder could receive whole BL bitstream and partial EL sub bitstream corresponding to the requested ROI.

This contribution proposes modified syntax for signalling skipped slice/tile:

• 
  SPS-level syntax to indicate whether all tiles included in CVS are motion constrained.
  
• 
  Slice-level non-significant slice/tile indication flag signalled when multiple tiles are used and when either all tiles are motion constrained or all tiles are inter-layer constrained.
  
• 
  Slice-level syntax for the number of skipped CTUs in skipped slice/tile based on maximum number of CTUs included in the slice/tile
  
• 
  Bitstream constraint on conformance cropping window to include only non-skipped tiles.
  

The proposal is focused on using tile-constrained decoding operations so that a "middle box" could replace an existing tile in the bitstream with an instruction to generate that tile by another process.

No action was taken due to the general concern expressed above (about contributions in the category of partial-picture operations currently expressed only in low-level syntax for version 1).

14.1.1.1.1.1.1.1.394JCTVC-O0095 AHG9: Skipped slice signalling [Y. He, Y. Ye, X. Xiu, Y. He (InterDigital)]

This contribution proposes syntax elements and decoding process to define enhancement layer skipped pictures or skipped slices. This contribution is a follow-up of JCTVC-N0209.

The contribution discusses skipped pictures as well as skipped slices. The discussion focused on whole-picture operation due to the general concern expressed above (about contributions in the category of partial-picture operations currently expressed only in low-level syntax for version 1).

The contribution proposes to enable a "skipped picture" in an enhancement layer when a direct reference layer is available.

A skipped picture is proposed to be generated by copying the (possibly upsampled) default inter-layer reference picture. It was suggested to use picture-level signalling of the dependent layer (proposed in O0265) rather than depending on default operation. Another suggestion was to use the first picture in the default reference picture list.

The contribution indicates the use case to be for encoder complexity mitigation for a "CPU-starved" condition, or for a "middle box" operation. A participant remarked that generating equivalent low-level syntax should not be difficult for the CPU-starved case. Such a case is further discussed in O0199.

It was remarked that in the middle box case, unless this is accomplishing something important that would not be accomplished by simply removing an EL picture from the bitstream, it may not be needed. The flag for indicating output of a lower layer when a target EL is not available may provide equivalent capability. Further study should be conducted to determine whether the proposed feature is needed.

It was also remarked that if the picture that is dropped is a reference picture, this envisioned dropping by a middle box could cause problems, in terms of that picture being later used as a collocated reference picture or as a reference for texture prediction.

14.1.1.1.1.1.1.1.395JCTVC-O0199 SHVC skip pictures [J. Samuelsson, J. Enhorn (Ericsson)]

This contribution expresses support for the skip_picture_flag from contribution JCTVC-N0209 with the addition of handling special cases. In the method proposed for skip pictures in JCTVC-N0209, motion compensation is performed by applying the re-sampled motion field from the closest reference layer to the corresponding temporal reference pictures in the current layer’s DPB. This contribution agrees with the asserted advantage in using motion information from another layer while keeping fidelity (details) from the current layer. In addition to what is proposed in JCTVC-N0209, this contribution proposed the following:

• 
  To use re-sampled sample values from the inter-layer reference picture when the picture used for inter prediction in the inter-layer reference picture has no corresponding picture in the enhancement layer or when motion vector prediction is not used from the inter-layer reference picture.
  
• 
  To use inter prediction with zero motion from the first picture in L0 (and L1 in case of a B slice) when no inter-layer reference picture is available.
  
• 
  A bitstream restriction to prohibit the use of skipped pictures when neither inter-layer reference pictures nor temporal reference pictures are available.
  
• 
  To allow the deblocking filter to be applied to skipped pictures.
  

The proponent noted that the proposal might not fulfil the HLS-only concept, but presented some of the rationale and use case discussion for information.

The contributor discussed adaptive resolution conversion (ARC) and its desire to have a "diagonal" referencing (a referencing across both time and resolution), whereas our draft requires only cross-layer or cross-temporal referencing.

The number of bits saved by having a special skip indicator instead of using CTU-level skipping was not estimated.

It was noted that we have a flag single_layer_for_non_irap_flag (in the VPS) for the ARC case.

(It was remarked that, historically, the skipping functionality had also been proposed for coding efficiency purposes, but this did not seem like a strong case for it.)

It was remarked that there might be a case for having an indicator of a mode of operation in which the bitstream is only allowed to have a non-skipped picture at one layer at a time.

It was remarked that enabling a diagonal prediction might be a cleaner approach for the ARC case. (A diagonal referencing was proposed in L0188.)

It was noted that the diagonal case could have better delay characteristics, assuming the encoder would not want to send many skips repeatedly in case it wants to switch up in the subsequent picture.

However, another participant remarked that enabling diagonal referencing would have a serious impact on various aspects of the design as it exists.

Regarding the indicator described above (a non-skipped picture at one layer at a time), the contributor suggested to change the semantics of single_layer_for_non_irap_flag. The indicator could also be in an SEI message. It was remarked that it might be desirable to put the indicator in the same place as single_layer_for_non_irap_flag, and that perhaps both could go into VPS VUI. It was suggested to put these as two flags in VPS VUI.

It was remarked that O0251 is related, as it discusses ARC.

It was suggested that the coding efficiency impact of requiring the encoder to provide the low-level bits for skipping an EL picture should be studied. There was some information provided previously in I0394 for frame doubling of 4k video – reporting 568 bits with skipped CTUs versus 96 bits, or with tiles 1464 bits versus 96 bits. For 1080p HD, 280 versus 96 and 1056 versus 96. It seemed like it might be better to not have the high-level alternative, if that is the rationale and it is within an encoder.

Further discussion Thu 31st (GJS). The text described above was provided in a -v3 version of the contribution and reviewed. An alternative expression in the -v4 version using more explicit constraining of the syntax by the decoder was also discussed. The group preferred the more explicit signalling. Decision: Adopt -v4.

14.1.1.1.1.1.1.1.396JCTVC-O0265 MV-HEVC/SHVC HLS: On signalling of enhancement layer skip picture [J. Chen, K. Rapaka, Y.-K. Wang, V. Seregin, X. Li, M. Karczewicz (Qualcomm)]

Detailed review was suggested not to be needed, as this raises the same issues discussed above in the context of O0199.

3. Signalling of cropped inter-layer reference (1)

14.1.1.1.1.1.1.1.397JCTVC-O0098 AHG9: On signalling of scaled reference layer offsets [K. Ugur, M. M. Hannuksela (Nokia)]

6.