JVET-R0073 AHG9: Some cleanups on QP delta signalling [Z. Deng, L. Zhang, Y.-K. Wang, J. Xu, K. Zhang (Bytedance)]
This contribution proposes the following changes on QP delta signalling, asserted to make the design more consistent between luma and chroma for CU-level and picture/slice-level QP delta signalling:
- Regarding the CU-level luma QP delta control, it is proposed to:
- Add a slice-level on/off control flag (named cu_qp_delta_enabled_flag) for CU luma QP delta, and condition the presence based on the PPS-level on/off control flag. This would allow the encoder to send QP deltas at the CU level for some slices and not others.
It was noted that chroma QP ordinarily tracks luma QP and was remarked that the overhead for sending QP deltas when they are all zero-valued seems low (they are arithmetically coded and may be as infrequent as one per CTU or they can use a smaller region basis as selected by the encoder).
The proponent said that under some circumstances we have slice-level control of whether there are chroma QP offsets at the CG level without having slice-level control of whether there are luma QP offsets at the CG level. Another participant commented that this may be desirable for HDR, and another commented that the purpose of local luma QP control and local chroma QP control are somewhat different.
Another participant commented that the local chroma QP offsets are likely to be less frequently used than for luma. Another said that chroma QP offset usage might be only for content adaptive purposes and not used in some slices.
No action was agreed to be take on this.
- Rename the PPS-level on/off control flag to be pps_cu_qp_delta_enabled_flag for clearer wordings.
- Resolve an asserted error in the semantics of pps_cu_qp_delta_enabled_flag by using it to specify the presence of cu_qp_delta_abs and cu_qp_delta_sign_flag in both the transform unit syntax and the palette coding syntax.
Decision (expression of existing intent): The error identified in “c” should be corrected, and the editor should also consider the suggestion in “b”.
- Regarding the picture/slice-level chroma QP offsets signalling:
- Signal the picture-level chroma QP offsets (named ph_cb_qp_offset, ph_cb_qp_offset, and ph_joint_cbcr_qp_offset).
- Add a PPS switch flag (named chroma_qp_offset_info_in_ph_flag) to control whether to signal the chroma QP offsets in picture or slice-level (but never both).
- Rename the PPS present flag to be pps_chroma_qp_offsets_present_flag and use it to condition the presence of picture/slice level chroma QP offsets.
It was noted that the purpose of the luma and chroma controls are somewhat different since the luma changes are tracked by the chroma changes as a bit rate control functionality. That is different from the purpose of the chroma QP control, which is for just controlling an offset used when tracking luma or for very localized control.
Currently, chroma QP offsets are sent in the PPS or SH, but not the PH.
The luma QP delta is sent at the PH or SH.
JVET-R0272 and JVET-R0302 were said to be similar.
Since the purpose of the control functionality is different for the luma and chroma controls, it was not agreed that there is a need for changing the way these work to make them more similar, and no action was thus taken on this.
- Regarding the picture/slice-level luma QP delta signalling:
- Add an on/off control flag in the PPS (named pps_luma_qp_delta_present_flag) to condition the presence of picture/slice level luma QP delta.
See the notes above; no action was taken on this.