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HL syntax in SHVC and 3D extensions (36)

1. Generic HLS issues (2)

14.1.97.1.1.1.1.1.297JCTVC-P0043 Version 1/MV-HEVC/SHVC HLS: Access unit boundary detection [M. M. Hannuksela (Nokia)]

Discussed 01-10 a.m. (GJS).

The contribution discusses problems related to access unit boundary detection and contains the following three proposals (one with two alternatives):

1. 
   It is proposed to clarify that the decoders shall use access unit delimiter NAL units with any value of nuh_layer_id in the determination of the start of a new access unit.
   
2. 
   Regarding the presence of the access unit delimiter NAL unit when there is no base layer picture present, either of the following alternatives is proposed:
   

1. 
   It is proposed to require the presence of the access unit delimiter NAL unit when there is no base layer picture present in the access unit.
   
2. 
   It is proposed to allow indication of access unit boundaries by external means. When external means are not in use, it is proposed to require the presence of the access unit delimiter NAL unit when there is no base layer picture present in the access unit.
   

3. 
   It is proposed to require the presence of first_slice_segment_in_pic_flag as the first syntax element in all VCL NAL units with nuh_layer_id equal to 0.
   

It is asserted that the access unit (AU) boundary detection has the following problems currently:

1. 
   The current AU boundary specification specifies one coded picture to be an access unit.
   

It is specified that the first VCL NAL unit of a coded picture after the last VCL NAL unit of the previous coded picture starts a new access unit. The intent in SHVC/MV-HEVC is to allow several coded pictures, each having different values of nuh_layer_id, in the same access unit.

2. 
   The contribution asserted that version 1 decoders must be able to detect boundaries of AUs that do not contain an HEVC base layer picture.
   

It is allowed to have access units where the base layer picture is not present for example to enable a base layer @ 30 Hz and a spatial or quality enhancement layer @ 60 Hz.

If there is no NAL unit present that starts a new access unit (e.g. an access unit delimiter) and also if there is no base layer picture present in the access unit (AU), it is asserted that HEVC v1 decoders may consider the following coded enhancement layer pictures as a part of the previous access unit, while SHVC/MV-HEVC decoders are intended to consider them as part of a new access unit. Consequently, it is asserted that the HRD parameters for AU-based CPB operation may become ambiguous and may be interpreted differently by HEVC v1 decoders and SHVC/MV-HEVC decoders.

A similar issue occurs in hybrid codec scalability, when the AVC base layer pictures would either not be present in the HEVC bitstream or would be encapsulated in NAL units that are not interpreted to start a new access unit.

3. 
   It should be clarified whether version 1 decoders shall consider NAL units with nuh_layer_id greater than 0 in the AU boundary determination.
   

However, in the discussion, it was remarked that non-nested HRD parameters and AU boundary detection for version 1 decoders must consider EL-only AUs to not be separate AUs.

It was remarked that the version 1 text may not be fully clear in that regard, and that this should be clarified.

Decision (BF/Corrigendum): Clarify the text such that decoders conforming to profiles specified in Annex A do not use NAL units with nuh_layer_id > 0 for AU boundary detection and that non-nested HRD parameters describe Annex C operation for this type of AU boundary detection.

14.1.97.1.1.1.1.1.298JCTVC-P0139 MV-HEVC/SHVC HLS: Header parameter set (HPS) [M. M. Hannuksela, H. Roodaki (Nokia)]

Discussed 01-10 a.m. (GJS).

It is asserted that in JCT-3V common test conditions (without multiple slices per picture), the overhead of enhancement-layer (EL) slice headers is on average about 3.4% when compared to the EL bit rate only for both MV-HEVC and 3D-HEVC and about 1.0 and 1.2% (for MV-HEVC and 3D-HEVC, respectively) when compared to the total bit rate. The motivation of the contribution is to reduce the EL slice header overhead by a header parameter set (HPS) design, which enables the inheritance of slice header syntax elements from the HPS.

HPS was proposed earlier in JCTVC-J0109 for HEVC version 1. The HPS design in this contribution is asserted to be similar to that of JCTVC-J0109 with the addition that repetitive slice header patterns e.g. for an entire IRAP picture period could be included in the HPS and addressed either by slice_pic_order_cnt_lsb values or an indicated index hps_entry_idx in the slice header.

In version 2 of the contribution, illustrative figures were added on the use cases for how the proposed HPS could be used.

The HPS, of course, would only be used by the ELs.

The HPSs could be shared across multiple pictures as well as across multiple slices per picture.

An encoder would be able to choose whether to use an HPS or send an ordinary SH.

The proposed HPS scheme would send not just one set of SH data but a list of them, and the applicable index into the list would be derived either by sending an index in the SH or by using POC LSBs.

No cross-verification was provided.

It seemed too late in the design process for the current projects for considering a change of this magnitude.