JVET-P0468 CE5-Related: Reducing multiplier count in CC-ALF [K. Misra, F. Bossen, A. Segall (Sharp Labs of America)]
This contribution proposes modifications to the Cross Component Adaptive Loop Filter (CC-ALF). It is asserted that the changes (i) reduce complexity and (ii) further align CC-ALF with the regular ALF processes. To maintain the coding efficiency of CC-ALF with these changes, this contribution further proposes to allow for four CC-ALF filters to be used per chroma component.
Here, complexity reduction is achieved by:
- Reducing the support of the filter to a 3x4 diamond shape with 8 unique coefficients. This reduces the number of multiplications by 43% compared to the 5x6 diamond shape that was tested in CE5-2.1 (and that had 14 unique coefficients).
- Limiting the bit depth of the CC-ALF output by clipping it to -2BitDepthC‑1 to 2BitDepthC‑1‑1, inclusive
In addition, to better align CC-ALF with regular ALF, the following changes are also proposed in the contribution:
- Limit filter selection to signalling at a CTU level
- Removal of the temporal layer CC-ALF coefficient buffers to harmonize with the APS concept
- Use of symmetric line selection at virtual boundary to harmonize with ALF
The performance of the proposed changes are evaluated under the Common Test Conditions (CTC) and are reported to provide an average chroma gain of 8.93% and 11.46% for AI and RA cases, respectively.
An additional configuration is also reported for supplemental information. The configuration uses 6 unique coefficients that are subject to a zero-gain constraint. It is reported that the configuration provides an average chroma gain of 8.58% and 11.11% for AI and RA cases, respectively.
The proponents suggested there was no need to review this contribution.