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线积分卷积与双色调映射相结合的彩色素描模拟方法

潘龙1,2, 纪庆革1,2, 陈靖1,2(1.中山大学数据科学与计算机学院, 广州 510006;2.广东省大数据分析与处理重点实验室, 广州 510006)

摘 要
目的 目前学者已经设计了很多模拟油画、水彩、水墨等风格的非真实感绘制方法,而能够生成彩色素描的算法还不是很多。针对这一课题,在前人工作的基础上,结合线积分卷积与双色调映射技术,改进了一种彩色素描模拟方法。方法 首先基于K-means聚类对彩色图像进行分割,通过计算色彩差异性为每个区域指定两种基本色,并利用双色调映射技术计算每种颜色的密度。而后利用线积分卷积分别生成两个基本色层的素描纹理,并将两层纹理相融合来生成彩色纹理。与此同时,利用霓虹变换生成素描轮廓线。最后,将轮廓与彩色纹理相融合来得到彩色素描效果。结果 实验结果表明,本文方法能够实现由彩色图像到彩铅画的自动、实时转化。结论 本文方法从轮廓和纹理两个角度模拟了真实的彩铅绘画过程。基于K-means聚类的分割方法得到的结果能够更好地反映彩色图像的颜色分布特性。通过色彩差异性计算指定基本色的策略提高了该环节的效率,满足了实时性要求。由于粉笔、蜡笔等绘画风格的调色与彩铅画类似,本文不同颜色层上下叠加的方式可以扩展到对其他介质绘画的模拟当中。
关键词
Color sketch simulation combining line integral convolution with duotones color reproduction technique

Pan Long1,2, Ji Qingge1,2, Chen Jing1,2(1.School of Data and Computer Science, Sun Yat-sen University, Guangzhou 510006, China;2.Guangdong Province Key Laboratory of Big Data Analysis and Processing, Guangzhou 510006, China)

Abstract
Objective Non-photorealistic rendering is a technique which is committed to generate art style works. The goal of this technique is not only to represent the authenticity of graphics but also to simulate their art features, as well as their defects. This concept was first proposed in the 1980s. After years of development, non-photorealistic rendering techniques can simulate a numerous painting styles, including oil paintings, watercolor paintings, Chinese ink paintings, pencil sketches, and cartoons. The sketch is a kind of painting which presents the light and shade of an object by lines. The color sketch is a sketch painted by a small number of color pencils. Except for the light and shade, the color sketch represents color features of an object as well. When painting a color sketch, people follow the monochrome sketch and render different regions with disparate color pencils. People usually need to blend two or more colors to obtain the desired color because the types of pencils are limited. In addition, the color mixing of sketches differs from that of oil or watercolor paintings. Typically, people mix pigments on a palette. However, when doing a color sketch, they directly draw disparate color layers on the paper. The layers of stroke for each color are superimposed and interspersed to build the target color through optical blending. This feature creates a very special kind of texture and color style for the color sketch. Although many people are fond of sketches, mastering the skill of pencil drawing is not easy for everybody. Based on previous works and characteristics of color sketch, combining the duotone color reproduction with LIC techniques, we realized an improved way of generating color sketch. Method First, we fulfilled a monochrome sketch simulation process based on LIC(line integral convolution). On this basis, we realized a new approach for the color sketch generation based on color customization, which can automatically convert a digital image into a colored pencil drawing, by researching on the features of the color sketch and combining with the duotones color reproduction technique. The first step of this method is image segmentation. We use a primary color, as well as a secondary color, to paint each segmented region. Our method can automatically calculate the color set of each particular region. Such kind of color customization can successfully simulate the color mixing of sketch paintings, and it can also be expanded to the simulation of similar painting styles, such as chalk drawing and pastel drawing. In addition, we propose a way to superpose the real paper texture into digital images, which will make the final effect closer to real paintings. We achieve an ideal way to generate improved profile for the color sketch based on neon-transformation because the profile is one of the most important elements. We zoom the color of the original image before generating the white noise, which generates a clear hierarchy for the final effects, to improve the contrast ratio. We also fulfill a new way for image segmentation based on K-means cluster, which obtains suitable segmentation results for the color sketch generation. Result To validate the effectiveness of the proposed approach, we conducted extensive experiments. Experimental results demonstrate that our approach outperforms several state-of-the-art approaches on challenging automatic and real-time transformation from color image to color painting, as well as bridging the gap between them. In contrast to Yamamoto's method, the proposed technique incorporates the idea of hierarchical method and color scale transformation into linear integral convolution with two-tone color sketch mapping technology. As such, we obtain sketch texture closer to a hand-painted effect. Matsui's method can generate a soft texture. However, the entire stroke fusion process is really complicated and time-consuming. Particularly, this method does not consider the sketch contours and could be error-prone to lose structure characteristics of the object in drawing effect. Kim's method extracts basic image feature for sketch generation. This method also has high efficiency and can be well applied to mobile devices. However, this method only considers the contour perspective, thus ignoring the sketch texture effect which often suffers a decrease in accuracy compared to real color sketch. In the comparison of the two approaches, the proposed method considers contour and texture. Simulating color image from the contour and texture aspects helps in issuing object characteristics preservation and preventing potential unrealistic sketch transformation. Way's method can generate a soft and saturated but cluttering texture, thus missing the natural line sense of color sketch. We calculate texture direction by transforming image space to frequency domain for energy statistical characteristics to tackle this issue. The simulation results obtain obvious intensive line sense. In the color fusion aspect, Way's method is based on the color of the original image and only overlays strokes texture on the basic color. However, in the real drawing process, people can only use limited colors. We have established the library of basic colors to dispose this challenge. All colors used for sketch simulation are fused from the library and by two basic color layers. Simulation results show robustness to changing imaging conditions, as well as satisfactory human visual sense. Conclusion In general, our study reveals new insights into the drawing scheme, implementing method, algorithm complexity, and interactivity when simulating color sketch. First, the proposed method is accomplished using the image transformation approach. Therefore, the method can interactively transform color image to color sketch or any other basic color specified by the user. Second, our study encompasses contour and texture simulation, facilitating the simulation of a realistic painting process. Third, the proposed method finds inspiration from human drawing experience. We overlay and fuse different color layers to generate color texture. This way can also be employed to simulate a wide spectrum of painting style, such as chalk drawing and crayon drawing.
Keywords

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