Improved Chroma from Luma Prediction in AV1 Based On Virtual Chroma Block Generation

Chroma from Luma (CfL) prediction is an efficient coding tool in AV1 which builds chroma prediction by implementing a linear model on luma pixels. To avoid transmission, the off-set factor in the linear model is set to the average of neighboring chroma pixels. An improved CfL algorithm is proposed to derive the offset factor based on a virtual chroma block. Such a block is constructed by using the chroma of the matched pixel which is determined according to the luma difference of neighboring regions and the co-located luma. Compared with libaom, the proposed CfL algorithm provides 0.31% and 0.15% weighted PSNR BD-rate saving under AI and RA configuration, respectively. Experimental results show that over 1.00% and 0.80% BD-rate saving can be achieved for chroma components under AI and RA configuration. With the proposed algorithm, the percentage of pixels with CfL as the optimal coding mode is increased.