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Do You Like Sclera? Sclera-region Detection and Colorization for Anime Character Line Drawings

Line drawing colorization is an important process in creating artwork such as animation, illustrations and color manga. Many artists color work manually, a process that requires considerable time and effort. In addition, colorizing requires special skills, experience, and knowledge, and this makes such work difficult for beginners. As a result, automated line drawing colorizing methods have significant market demand. However, it is difficult to paint artistic works. Many automated colorizing methods have been developed, but several problems arise in art colorized using these methods. For example, colors may be different in a region that should be painted the same color as another region, or a mismatch may occur between the input line drawing and the colorizing result due to difficulty in understanding the sketches, the inclusion of undesirable artifacts, and other issues. Anime character’s eyes are drawn in various styles, depending on the artists’ preferences. In some styles, eyes are overly abstract. In addition, in grayscale line drawings, the skin and sclera are both expressed in white in many cases. Therefore, the boundaries cannot always be determined using existing automated colorizing techniques. As a result, sclera are often painted the same color as the skin, and there is a mismatch between these regions in the line drawing and the colorizing results. Facial features are important in artworks that depict people, and excessive ambiguity at the boundary between the eyes and the skin may impair quality. Therefore, it is expected that sclera-region detection should improve the accuracy of automated colorizing of grayscale line drawings of people. This paper focuses on inconsistencies in the sclera region between line drawings and colorizing results; we aim to match the structure of line drawings and colorizing results by detecting the sclera regions in grayscale line drawings of people to improve the accuracy of automated colorizing (Figure 1). In our proposed framework, we perform machine learning using a pair of line drawings and a mask image. The sclera regions are labeled to create semantic segmentation models of the sclera regions. Then, to colorize the line drawing, the semantic segmentation models detect the sclera regions, and we apply these regions to the automated colorizing result. As a result, our framework maintains the correct sclera-region color. When using the semantic segmentation model, it is possible to detect sclera regions without requiring the user to add hints. In this paper, we propose two mask image creation methods: the manual type and the graph cut type. Compared with the manual type, the graph cut type can reduce the mask image creator’s burden.