XCiT: Cross-Covariance Image Transformers

• DOI: 10.48550/arxiv.2106.09681
• Publication Date: 2021-01-01
• Authors: El-Nouby, Alaaeldin; Touvron, Hugo; Caron, Mathilde; Bojanowski, Piotr; Douze, Matthijs; Joulin, Armand; Laptev, Ivan; Neverova, Natalia; Synnaeve, Gabriel; Verbeek, Jakob; Jégou, Hervé
• Keywords: FOS: Computer and information sciences; Computer Science - Machine Learning; Computer Vision and Pattern Recognition (cs.CV); Computer Science - Computer Vision and Pattern Recognition; 0202 electrical engineering, electronic engineering, information engineering; [INFO.INFO-CV]Computer Science [cs]/Computer Vision and Pattern Recognition [cs.CV]; 02 engineering and technology; Machine Learning (cs.LG)

International audience<br/>Following tremendous success in natural language processing, transformers have recently shown much promise for computer vision. The self-attention operation underlying transformers yields global interactions between all tokens, i.e. words or image patches, and enables flexible modelling of image data beyond the local interactions of convolutions. This flexibility, however, comes with a quadratic complexity in time and memory, hindering application to long sequences and highresolution images. We propose a "transposed" version of self-attention that operates across feature channels rather than tokens, where the interactions are based on the cross-covariance matrix between keys and queries. The resulting cross-covariance attention (XCA) has linear complexity in the number of tokens, and allows efficient processing of high-resolution images. Our cross-covariance image transformer (XCiT)-built upon XCA-combines the accuracy of conventional transformers with the scalability of convolutional architectures. We validate the effectiveness and generality of XCiT by reporting excellent results on multiple vision benchmarks, including (self-supervised) image classification on ImageNet-1k, object detection and instance segmentation on COCO, and semantic segmentation on ADE20k. We replace the self-attention, as originally introduced by Vaswani et al. [68], with a "transposed" attention that we denote as "cross-covariance attention" (XCA). Cross-covariance attention substi<br/>