Estimating reflectance and shape of objects from a single cartoon-shaded image

Although many photorealistic relighting methods provide a way to change the illumination of objects in a digital photograph, it is currently difficult to relight hand-drawn shading style in a digital illustration. The main difference is its soft color quantization and nonlinear color variations that cause noticeable reconstruction errors under physical reflectance assumption like Lambertian. As a first step to handle such a non-photorealistic shading property, we focus on the shading analysis of the most fundamental cartoon shading technique. Based on its color map shading representation, we propose a simple method to decompose the input shading into a smooth shape and nonlinear reflectance property. We also conducted simple ground-truth evaluations to compare our results with other existing approaches.