QuadPrior++: Multi-dimension Augmented Physical Prior for Zero-Reference Illumination Enhancement

Existing low-light enhancement methods typically rely on fitting data mappings (pixel-wise mappings through fully supervised methods or distribution-wise mappings through weakly supervised or self-supervised methods). However, their performance is heavily dependent on specific scenes and fails to adequately model the intrinsic prior of natural images, resulting in poor generalization. To tackle this challenge, we leverage the strengths of powerful generative diffusion models, conditioned on a thoughtfully designed prior, and propose a novel zero-reference low-light enhancement framework that gets rid of dependence on the distribution of low-light images. In detail, we address the most fundamental core by proposing an illumination-invariant prior derived from the theory of physical light transfer, bridging the gap between normal and low-light domains, and enabling zero-shot enhancement without the need for low-light-specific training. A prior-to-image restoration framework is built upon generative diffusion models, pre-trained on normal-light data. During inference, the framework extracts the illumination-invariant prior from low-light inputs and maps them back to high-quality images, naturally for low-light enhancement. Additionally, such intrinsic properties of illumination-invariant prior open up opportunities for distilling diffusion models into compact CNN-based networks. We propose a novel prior-injected distillation paradigm incorporating intensity, frequency, and gradient domain-augmented regularization comprehensively. This distillation framework not only reduces computational costs but also maintains high fidelity and perceptual quality in enhanced outputs, making it more efficient and practical for real-world applications. The approach further extends seamlessly to handle over-exposure scenarios, demonstrating its versatility in addressing complex lighting conditions. Extensive experiments demonstrate the superiority of our framework in various scenarios, as well as its strong interpretability, robustness, and efficiency.