Learning Tone Curves for Local Image Enhancement

Image enhancement methods can be formulated as global transformations, local transformations, pixel-wise processing, or a mixture of these operations. Global transformations are limited in enhancing local image regions. Existing local and pixel-wise methods mitigate this issue, but give rise to the additional challenge of limited interpretability. Bridging the gap between global and local methods, we propose a local tone mapping network (LTMNet) that learns a grid of tone curves to locally enhance an image. Tone curves are commonly used by photo-editing software and offer an intuitive representation to photographers, facilitating subsequent customization of the image. Tone curves are also widely used in image signal processors (ISPs), making our method easy to deploy on cameras. Because existing datasets contain image enhancement and photofinishing beyond global and local tone mapping, we also propose a new dataset representative of local tone mapping—the LTM dataset. We evaluate our method on this new dataset as well as MIT-Adobe and HDR+ datasets. We show that the proposed LTMNet outperforms existing methods in local tone mapping while achieving competitive performance modeling additional photofinishing. Furthermore, we show that our method can be assistive in user-interactive photo-editing tools. Our code, model, and data will be released publicly at https://github.com/SamsungLabs/ltmnet .