Diagnosing and Improving Vector-Quantization-Based Blind Image Restoration

Vector-Quantization (VQ) based discrete generative models are widely used to learn powerful high-quality (HQ) priors for blind image restoration (BIR). In this paper, we diagnose the side-effects of discrete VQ process essential to VQ-based BIR methods: 1) confining the representation capacity of HQ codebook, 2) being error-prone for code index prediction on low-quality (LQ) images, and 3) under-valuing the importance of input LQ image. These motivate us to learn continuous feature representation of HQ codebook for better restoration performance than using discrete VQ process. To further improve the restoration fidelity, we propose a new Self-in-Cross-Attention (SinCA) module to augment the HQ codebook with the feature of input LQ image, and perform cross-attention between LQ feature and input-augmented codebook. By this way, our SinCA leverages the input LQ image to enhance the representation of codebook for restoration fidelity. Experiments on four typical VQ-based BIR methods demonstrate that, by replacing the VQ process with a transformer using our SinCA, they achieve better quantitative and qualitative performance on blind image super-resolution and blind face restoration. The code and pre-trained models are publicly released at https://github.com/lhy-85/SinCA.