A Coordinate Displacement Compensation Based Quality Enhancement Method for Chroma Subsampling and Upsampling on VVC

Abstract

<div>Prior to encoding an input RGB full-color image I<sup>RGB</sup>, at the server side, performing chroma subsampling on the converted chroma image is a necessary step. After receiving the decompressed subsampled chroma image and luma</div><div>image at the client side, performing chroma upsampling is also a necessary step for reconstructing the RGB full-color image. In this paper, we consider seven commonly used chroma subsampling methods, denoted by C<sub>s</sub>, and four widely used chroma upsampling methods, denoted by C<sub>u</sub>. For each combination c<sub>s</sub>-c<sub>u</sub> in C<sub>s</sub>xC<sub>u</sub>, we first utilize the moment balance law to analyze the coordinate displacement (CD) bias problem occurring in c<sub>s</sub>. Next, for the combination c<sub>s</sub>-c<sub>u</sub>, we analyze the CD bias problem occurring in the transition from the server side to the client side. Then, we explain why the CD bias problem degrades the quality of the reconstructed RGB full-color images in the current coding system. To remedy this CD bias problem, a CD compensationbased (CDC-based) quality enhancement method is proposed to improve the quality of the reconstructed images. To the best of our knowledge, this is the first work in this research direction. Based on the IMAX, Kodak, SCI (screen content images), and Video datasets, the comprehensive experimental results have demonstrated that on the newly released versatile video coding (VVC) platform VTM-12.0, the proposed CDC-based quality enhancement method in our augmented coding system can achieve substantial quality improvement for 17 combinations in C<sub>s</sub>xC<sub>u</sub>.</div>