  This paper introduces a novel dataset for video enhancement and studies the
state-of-the-art methods of the NTIRE 2021 challenge on quality enhancement of
compressed video. The challenge is the first NTIRE challenge in this direction,
with three competitions, hundreds of participants and tens of proposed
solutions. Our newly collected Large-scale Diverse Video (LDV) dataset is
employed in the challenge. In our study, we analyze the proposed methods of the
challenge and several methods in previous works on the proposed LDV dataset. We
find that the NTIRE 2021 challenge advances the state-of-the-art of quality
enhancement on compressed video. The proposed LDV dataset is publicly available
at the homepage of the challenge: https://github.com/RenYang-home/NTIRE21_VEnh
