In this work, we investigate the capability of generating images from
pre-trained diffusion models at much higher resolutions than the training image
sizes. In addition, the generated images should have arbitrary image aspect
ratios. When generating images directly at a higher resolution, 1024 x 1024,
with the pre-trained Stable Diffusion using training images of resolution 512 x
512, we observe persistent problems of object repetition and unreasonable
object structures. Existing works for higher-resolution generation, such as
attention-based and joint-diffusion approaches, cannot well address these
issues. As a new perspective, we examine the structural components of the U-Net
in diffusion models and identify the crucial cause as the limited perception
field of convolutional kernels. Based on this key observation, we propose a
simple yet effective re-dilation that can dynamically adjust the convolutional
perception field during inference. We further propose the dispersed convolution
and noise-damped classifier-free guidance, which can enable
ultra-high-resolution image generation (e.g., 4096 x 4096). Notably, our
approach does not require any training or optimization. Extensive experiments
demonstrate that our approach can address the repetition issue well and achieve
state-of-the-art performance on higher-resolution image synthesis, especially
in texture details. Our work also suggests that a pre-trained diffusion model
trained on low-resolution images can be directly used for high-resolution
visual generation without further tuning, which may provide insights for future
research on ultra-high-resolution image and video synthesis.

通过重新扩张限制性卷积感知域以及引入分散卷积和抑噪无监督引导的方法，无需任何训练或优化，实现了超高分辨率图像生成，并解决了物体重复和不合理结构的问题，表现出卓越的性能。此外，该方法还提示了低分辨率训练模型可以直接用于高分辨率图像生成的可能性，为未来的超高分辨率图像合成研究提供了新的思路。