Although many photorealistic relighting methods provide a way to change the illumination of objects in a digital photograph, it is currently difficult to relight digital illustrations having a cartoon shading style. The main difference between photorealistic and cartoon shading styles is that cartoon shading is characterized by soft color quantization and nonlinear color variations that cause noticeable reconstruction errors under a physical reflectance assumption, such as Lambertian reflection. To handle this non-photorealistic shading property, we focus on shading analysis of the most fundamental cartoon shading technique. Based on the color map shading representation, we propose a simple method to determine the input shading as that of a smooth shape with a nonlinear reflectance property. We have conducted simple ground-truth evaluations to compare our results to those obtained by other approaches.