NTIRE 2021 Challenge on Quality Enhancement of Compressed Video: Methods and Results. - Ren Yang; Radu Timofte; Jing Liu 031; Yi Xu 003; Xinjian Zhang; Minyi Zhao; Shuigeng Zhou; Kelvin C. K. Chan; Shangchen Zhou; Xiangyu Xu 002; Chen Change Loy; Xin Li 106; et al. (CoRR, 2021)