On Cross Component Adaptive Loop Filter for Video Compression

This paper introduces a video coding tool called the Cross Component Adaptive Loop Filter (CC-ALF). The goal of the tool is to improve chroma fidelity, and this is achieved by exploiting correlations between the luma and chroma channels during the loop filtering process. The tool analyzes the luma channel to determine a chroma residual that is added to a previously reconstructed chroma signal, where the analysis consists of applying a linear filter to the luma information. Adaptation of the filter is achieved by signaling the filter coefficients in the bit-stream and, furthermore, selectively enabling or disabling the process spatially across a picture. Results show the efficacy of the approach, where the performance is evaluated using the Common Test Conditions (CTC) defined by Joint Video Experts Team (JVET). The evaluations show that CCALF provides average chroma Bjøntegaard Delta improvement of 7.6%, 13.8% and 17.8% for all intra, random access, and low delay configurations. Additionally, an approach to shift these chroma gains to luma gains is considered and results in an average luma BD bitrate improvement of 0.9%.