Results and findings of the 2021 Image Similarity Challenge.

The 2021 Image Similarity Challenge introduced a dataset to serve as a benchmark to evaluate 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 overlaying onto 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.