NTIRE 2021 Challenge on Perceptual Image Quality Assessment. - Jinjin Gu; Haoming Cai; Chao Dong 005; Jimmy S. Ren; Yu Qiao 001; Shuhang Gu; Radu Timofte; Manri Cheon; Sung-Jun Yoon; Byungyeon Kang; Junwoo Lee; Qing Zhang; et al. (CoRR, 2021)