  The 2021 Image Similarity Challenge introduced a dataset to serve as a new
benchmark to evaluate recent image copy detection methods. There were 200
participants to the competition. This paper presents a quantitative and
qualitative analysis of the top submissions. It appears that the most difficult
image transformations involve either severe image crops or hiding into
unrelated images, combined with local pixel perturbations. The key algorithmic
elements in the winning submissions are: training on strong augmentations,
self-supervised learning, score normalization, explicit overlay detection, and
global descriptor matching followed by pairwise image comparison.
