  Blur was naturally analyzed in the frequency domain, by estimating the latent
sharp image and the blur kernel given a blurry image. Recent progress on image
deblurring always designs end-to-end architectures and aims at learning the
difference between blurry and sharp image pairs from pixel-level, which
inevitably overlooks the importance of blur kernels. This paper reveals an
intriguing phenomenon that simply applying ReLU operation on the frequency
domain of a blur image followed by inverse Fourier transform, i.e., frequency
selection, provides faithful information about the blur pattern (e.g., the blur
direction and blur level, implicitly shows the kernel pattern). Based on this
observation, we attempt to leverage kernel-level information for image
deblurring networks by inserting Fourier transform, ReLU operation, and inverse
Fourier transform to the standard ResBlock. 1x1 convolution is further added to
let the network modulate flexible thresholds for frequency selection. We term
our newly built block as Res FFT-ReLU Block, which takes advantages of both
kernel-level and pixel-level features via learning frequency-spatial
dual-domain representations. Extensive experiments are conducted to acquire a
thorough analysis on the insights of the method. Moreover, after plugging the
proposed block into NAFNet, we can achieve 33.85 dB in PSNR on GoPro dataset.
Our method noticeably improves backbone architectures without introducing many
parameters, while maintaining low computational complexity. Code is available
at https://github.com/DeepMed-Lab/DeepRFT-AAAI2023.
