Pyramid Learnable Bandpass Filters for Ultra-High-Definition Image Demoiréing

Moiré patterns usually depend on the style of display grids and the position of shooting camera, appearing in the form of stripes, meshes or ripples, with various and irregular colors. Compared with low-resolution moiré images, high-definition (HD) and ultra-high-definition (UHD) moiré images exhibit more complex moiré patterns, e.g., wider distribution of moiré frequencies and higher coupling degree of moirés of different scales, which poses a greater challenge to the modeling capabilities of the model. To address these challenges, we propose a novel Pyramid Learnable Bandpass Filtering Network (PBNet) for demoiréing UHD images. Specifically, we propose a pyramid learnable bandpass filter (P-LBF) to perform multi-scale filtering in the same semantic context to obtain richer frequency domain information. The P-LBF contains three stages: aligning, filtering and fusing. First, we introduce a pyramid alignment (DA) to align neighbor pixels for eliminating the deviations raised by different styles of display grids and relative position of the shooting camera. Then, a pyramid filtering (PF) is conducted to model the complex and variable moiré patterns with aligned neighbor pixels. Finally, the frequency domain responses of these different scales are fused with a multi-dimensional feature fusion (MFF). The PBNet is constructed based on the P-LBF, incorporating a cross-layer feature fusion (CLF) module to facilitate more effective information interaction between features at different depths. Extensive experiments on four public datasets show that our model achieves state-of-the-art performance for both high- and low-resolution moiré images. The code is publicly available at: https://github.com/liuzhongqi1/PBNet.