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.

1. INTRODUCTION

The increasing demand of quality of experience triggers the evolution of video services in a direction of higher resolution and higher dimension. With the high quality video, the viewing quality can be improved. As a consequence, the corresponding data amount increases and brings challenges to the network bandwidth and storage capacity. How to compress video content in an efficient manner becomes an essential research topic for both industry and academia. In this regard, many video coding projects have been launched in the past decades by different standardization organizations and research institutes [1][2].