  Plug-and-play Image Restoration (IR) has been widely recognized as a flexible
and interpretable method for solving various inverse problems by utilizing any
off-the-shelf denoiser as the implicit image prior. However, most existing
methods focus on discriminative Gaussian denoisers. Although diffusion models
have shown impressive performance for high-quality image synthesis, their
potential to serve as a generative denoiser prior to the plug-and-play IR
methods remains to be further explored. While several other attempts have been
made to adopt diffusion models for image restoration, they either fail to
achieve satisfactory results or typically require an unacceptable number of
Neural Function Evaluations (NFEs) during inference. This paper proposes
DiffPIR, which integrates the traditional plug-and-play method into the
diffusion sampling framework. Compared to plug-and-play IR methods that rely on
discriminative Gaussian denoisers, DiffPIR is expected to inherit the
generative ability of diffusion models. Experimental results on three
representative IR tasks, including super-resolution, image deblurring, and
inpainting, demonstrate that DiffPIR achieves state-of-the-art performance on
both the FFHQ and ImageNet datasets in terms of reconstruction faithfulness and
perceptual quality with no more than 100 NFEs. The source code is available at
{\url{https://github.com/yuanzhi-zhu/DiffPIR}}
