  Transformer architectures have exhibited remarkable performance in image
super-resolution (SR). Since the quadratic computational complexity of the
self-attention (SA) in Transformer, existing methods tend to adopt SA in a
local region to reduce overheads. However, the local design restricts the
global context exploitation, which is crucial for accurate image
reconstruction. In this work, we propose the Recursive Generalization
Transformer (RGT) for image SR, which can capture global spatial information
and is suitable for high-resolution images. Specifically, we propose the
recursive-generalization self-attention (RG-SA). It recursively aggregates
input features into representative feature maps, and then utilizes
cross-attention to extract global information. Meanwhile, the channel
dimensions of attention matrices (query, key, and value) are further scaled to
mitigate the redundancy in the channel domain. Furthermore, we combine the
RG-SA with local self-attention to enhance the exploitation of the global
context, and propose the hybrid adaptive integration (HAI) for module
integration. The HAI allows the direct and effective fusion between features at
different levels (local or global). Extensive experiments demonstrate that our
RGT outperforms recent state-of-the-art methods quantitatively and
qualitatively. Code and pre-trained models are available at
https://github.com/zhengchen1999/RGT.
