JVET-Q0425 AHG15: QP offsets for adaptive colour transform [R. Sjöberg, M. Pettersson, M. Damghanian, D. Saffar (Ericsson)]
This contribution proposes to add four new QP offset syntax elements to the PPS in VVC that are applied for coding units that use the adaptive colour transform (ACT). The proposed new syntax element names are pps_act_y_qp_offset_plus5, pps_act_cb_qp_offset_plus5, pps_act_cr_qp_offset_plus3 and pps_act_cbcr_qp_offset_plus5. A gating flag, pps_act_qp_offsets_present_flag, is also proposed.
This contribution also proposes to modify the clipping that affect the range of the variable qP.
The contribution claims that additional flexibility and improved visual quality in some use cases can be achieved through the introduction of four new QP offset syntax elements for ACT in the PPS similar to corresponding support in HEVC. HEVC additionally supports ACT offsets in the slice header but that is not included in this contribution.
The contribution further claims that the current VVC draft applies normalization compensation offsets for the derivation of the quantization value qP such that it can exceed the range of 0 to 63+QpBdOffsetY. This contribution proposes to add clipping to ensure that qP stays within this range also when ACT is enabled.
Proposed specification text on top of JVET-Q0041-v2_DraftText.docx is attached with the contribution.
In a version 2 of the contribution, clipping is done during derivation of the quantization parameter qP for all of Y, Cb, Cr and CbCr since the proponents understanding of the current ACT design is that the quantization values used in the deblocking process shall not include normalization terms. The authors of this contribution propose that the new proposed PPS syntax element values are handled as offset values to the normalization values in the quantization derivation process.
This was almost identical with JVET-Q0510/JVET-Q0511. No need for presentation was identified.