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 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.
Keywordsnon-photorealistic rendering cartoon shading relighting quantization
We would like to thank the anonymous reviewers for their constructive comments. We are also grateful to Tatsuya Yatagawa, Hiromu Ozaki, Tomohiro Tachi, and Takashi Kanai for their valuable discussions and suggestions. Additional thanks go to the AIM@SHAPE Shape Repository, Keenan’s 3D Model Repository for 3D models, and Makoto Nakajima, www.piapro.net for 2D illustrations used in this work. This work was supported in part by the Japan Science and Technology Agency CREST project and the Japan Society for the Promotion of Science KAKENHI Grant No. JP15H05924.
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