JVET-O0562 AHG15: Flexible luma-to-chroma quantization parameter tables [F. Bossen, A. Segall (Sharp)]

This contribution was discussed 4 July at 2030 (chaired by GJS).

JVET-O0186, JVET-O0298, JVET-O0433, JVET-O0562, and JVET- O0650 are all related.

The current VVC draft defines a table that maps a luma quantization parameter value to a chroma quantization parameter value. The table is substantially the same as what was used in previous standards, and it is the authors understanding that the table was originally developed to manage the bit-allocation between luma and chroma channels at lower rates. However, compared to these previous standards, the VVC draft now contains additional tools that impact the balance of luma and chroma, such as dual tree support and cross-component linear model prediction (CCLM). Furthermore, it is anticipated that the VVC standard will be used to compress a larger amount of content types – including both standard dynamic range and high dynamic range content – that contain different relationships between the luma and chroma channels. As such, it is asserted that the current and fixed table may not be ideal, and it is proposed to have a programmable table to offer encoder flexibility.

The contribution notes that there is something different about QP for joint chroma coding.

The proposed syntax uses a piece-wise linear scheme (with no slope assumption).

The syntax is proposed to be at the PPS level.

Offsets both prior to and after the mapping table are proposed to be used.

Tests with not rounding the lambda value were reported, showing a more linear behaviour with increasing chroma QP.

It was commented that the chroma QP offset as currently used may not be needed if a general table is supported.