NPAR: Non-Photorealistic Animation and Rendering

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https://diglib7.eg.org/handle/10.2312/990

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Pixelated Image Abstraction
(The Eurographics Association, 2012) Gerstner, Timothy; DeCarlo, Doug; Alexa, Marc; Finkelstein, Adam; Gingold, Yotam; Nealen, Andrew; Paul Asente and Cindy Grimm
We present an automatic method that can be used to abstract high resolution images into very low resolution outputs with reduced color palettes in the style of pixel art. Our method simultaneously solves for a mapping of features and a reduced palette needed to construct the output image. The results are an approximation to the results generated by pixel artists. We compare our method against the results of a naive process common to image manipulation programs, as well as the hand-crafted work of pixel artists. Through a formal user study and interviews with expert pixel artists we show that our results offer an improvement over the naive methods.
Pixelating Vector Line Art
(The Eurographics Association, 2012) Inglis, Tiffany C.; Kaplan, Craig S.; Paul Asente and Cindy Grimm
Creating pixel art is a laborious process that requires artists to place individual pixels by hand. Although many image editors provide vector-to-raster conversions, the results produced do not meet the standards of pixel art: artifacts such as jaggies or broken lines frequently occur. We describe a novel Pixelation algorithm that rasterizes vector line art while adhering to established conventions used by pixel artists. We compare our results through a user study to those generated by Adobe Illustrator and Photoshop, as well as hand-drawn samples by both amateur and professional pixel artists.
Texture-Preserving Abstraction
(The Eurographics Association, 2012) Mould, David; Paul Asente and Cindy Grimm
Image abstraction traditionally eliminates texture, but doing so ignores the more elegant alternative of texture indication, e.g., suggesting the presence of texture through irregular silhouettes and locally chosen details. We propose a variant of geodesic image filtering which preserves the locally strongest edges, leading to preservation of both strong edges and weak edges depending on the surrounding context. Our contribution is to introduce cumulative range geodesic filtering, where the distance in the image plane is lengthened proportional to the color distance. We apply the new filtering scheme to abstraction applications and demonstrate that it has powerful structure-preserving capabilities, especially regarding preservation and indication of textures.
Shape-Free Hybrid Image
(The Eurographics Association, 2012) Sripian, Peeraya; Yamaguchi, Yasushi; Paul Asente and Cindy Grimm
Hybrid image is the image that changes interpretation according to viewing distance. By simply extracting the low and the high spatial frequency bands from two source images, the combination result image can be interpreted differently by viewing distance. This research finds the way to allow construction of hybrid image regardless of the source image's shape. Without the need to carefully pick the two images to be superimposed, hybrid image can be extended to use with any kind of image contents. There are two approaches for accomplishing shape-free hybrid image. Noise-inserted approach forces observers to perceive alternative low frequency image as meaningless noises in a close viewing distance, by manipulating contrast and details in the high frequency image. Color-inserted approach helps attract the visual attention for the high frequency image perception is also introduced in this research. Finally, hybrid image recognition experiment proves that our proposed method yield a better recognition rate over the original method while preserving hybrid image characteristic.
Combining Sketch and Tone for Pencil Drawing Production
(The Eurographics Association, 2012) Lu, Cewu; Xu, Li; Jia, Jiaya; Paul Asente and Cindy Grimm
We propose a new system to produce pencil drawing from natural images. The results contain various natural strokes and patterns, and are structurally representative. They are accomplished by novelly combining the tone and stroke structures, which complement each other in generating visually constrained results. Prior knowledge on pencil drawing is also incorporated, making the two basic functions robust against noise, strong texture, and significant illumination variation. In light of edge, shadow, and shading information conveyance, our pencil drawing system establishes a style that artists use to understand visual data and draw them. Meanwhile, it lets the results contain rich and well-ordered lines to vividly express the original scene.
Virtual Passepartouts
(The Eurographics Association, 2012) Ritschel, Tobias; Templin, Krzysztof; Myszkowski, Karol; Seidel, Hans-Peter; Paul Asente and Cindy Grimm
In traditional media, such as photography and painting, a cardboard sheet with a cutout (called passepartout) is frequently placed on top of an image. One of its functions is to increase the depth impression via the ''looking-through-a-window'' metaphor. This paper shows how an improved 3D effect can be achieved by using a virtual passepartout: a 2D framing that selectively masks the 3D shape and leads to additional occlusion events between the virtual world and the frame. We introduce a pipeline to design virtual passepartouts interactively as a simple post-process on RGB images augmented with depth information. Additionally, an automated approach finds the optimal virtual passepartout for a given scene. Virtual passepartouts can be used to enhance depth depiction in images and videos with depth information, renderings, stereo images and the fabrication of physical passepartouts
No Photos Harmed / Growing Paths from Seed - An Exhibition
(The Eurographics Association, 2012) Colton, Simon; Ferrer, Blanca Perez; Paul Asente and Cindy Grimm
We report on an exhibition centered around a dialogue between a Computational Creativity researcher presenting artwork generated by a computer program and a classically trained artist taking inspiration from the computational processes. The main purpose of the exhibition was to place software-generated art (where the program takes on some aesthetic and generative responsibilities, rather than acting as a mere tool) in both an art-production context and an art-historical context, by exploring the themes of creative responsibility and the loss of aura surrounding a work of art. A secondary purpose was to highlight the fact that computer generated art can be representational without relying on digital photographs as inputs. We describe certain technical hurdles we overcame in the production of the exhibition and the feedback we gained, in addition to elaborating on how the event and the project as a whole fits into an art-historical context. We conclude with brief details of another exhibition involving art generated by the same software system, where the notion of progression was explored; by describing a planned exhibition, where autonomy and independence in the system will be highlighted; and by providing a partial roadmap for progress towards autonomously creative software in the visual arts.
Consistent Stylization and Painterly Rendering of Stereoscopic 3D Images
(The Eurographics Association, 2012) Northam, Lesley; Asente, Paul; Kaplan, Craig S.; Paul Asente and Cindy Grimm
We present a method for stylizing stereoscopic 3D images that guarantees consistency between the left and right views. Our method decomposes the left and right views of an input image into discretized disparity layers and merges the corresponding layers from the left and right views into a single layer where stylization takes place. We then construct new stylized left and right views by compositing portions of the stylized layers. Because the left and right views come from the same source layers, our method eliminates common artifacts that cause viewer discomfort. We also present a stereoscopic 3D painterly rendering algorithm tailored to our layerbased approach. This method uses disparity information to assist in stroke creation so that strokes follow surface geometry without ignoring painted surface patterns. Finally, we conduct a user study that demonstrates that our approach to stereoscopic 3D image stylization leads to images that are more comfortable to view than those created using other techniques.
Active Strokes: Coherent Line Stylization for Animated 3D Models
(The Eurographics Association, 2012) Benard, Pierre; Lu, Jingwan; Cole, Forrester; Finkelstein, Adam; Thollot, Joelle; Paul Asente and Cindy Grimm
This paper presents a method for creating coherently animated line drawings that include strong abstraction and stylization effects. These effects are achieved with active strokes: 2D contours that approximate and track the lines of an animated 3D scene. Active strokes perform two functions: they connect and smooth unorganized line samples, and they carry coherent parameterization to support stylized rendering. Line samples are approximated and tracked using active contours (''snakes'') that automatically update their arrangment and topology to match the animation. Parameterization is maintained by brush paths that follow the snakes but are independent, permitting substantial shape abstraction without compromising fidelity in tracking. This approach renders complex models in a wide range of styles at interactive rates, making it suitable for applications like games and interactive illustrations.
Towards effective evaluation of geometric texture synthesis algorithms
(ACM, 2013) AlMeraj, Zainab; Kaplan, Craig S.; Asente, Paul; Forrester Cole and Cindy Grimm
In recent years, an increasing number of example-based Geometric Texture Synthesis (GTS) algorithms have been proposed. However, there have been few attempts to evaluate these algorithms rigorously. We are driven by this lack of validation and the simplicity of the GTS problem to look closer at perceptual similarity between geometric arrangements. Using samples from a geological database, our research first establishes a dataset of geometric arrangements gathered from multiple synthesis sources. We then employ the dataset in two evaluation studies. Collectively these empirical methods provide formal foundations for perceptual studies in GTS, insight into the robustness of GTS algorithms and a better understanding of similarity in the context of geometric texture arrangements.
Preface and Table of Contents
(ACM, 2013) Forrester Cole and Cindy Grimm
ELASTIFACE: Matching and Blending Textured Faces
(ACM, 2013) Zell, Eduard; Botsch, Mario; Forrester Cole and Cindy Grimm
In this paper we present ELASTIFACE, a simple and versatile method for establishing correspondence between textured face models, either for the construction of a blend-shape facial rig or for the exploration of new characters by morphing between a set of input models. While there exists a wide variety of approaches for inter-surface mapping and mesh morphing, most techniques are not suitable for our application: They either require the insertion of additional vertices, are limited to topological planes or spheres, are restricted to near-isometric input meshes, and/or are algorithmically and computationally involved. In contrast, our method extends linear non-rigid registration techniques to allow for strongly varying input geometries. It is geometrically intuitive, simple to implement, computationally efficient, and robustly handles highly non-isometric input models. In order to match the requirements of other applications, such as recent perception studies, we further extend our geometric matching to the matching of input textures and morphing of geometries and rendering styles.
Rasterizing and antialiasing vector line art in the pixel art style
(ACM, 2013) Inglis, Tiffany C.; Vogel, Daniel; Kaplan, Craig S.; Forrester Cole and Cindy Grimm
Pixel artists rasterize vector shapes by hand to minimize artifacts at low resolutions and emphasize the aesthetics of visible pixels. We describe Superpixelator, an algorithm that automates this process by rasterizing vector line art at a low resolution pixel art style. Our technique successfully eliminates most rasterization artifacts and draws smoother curves. To draw shapes more effectively, we use optimization techniques to preserve shape properties such as symmetry, aspect ratio, and sharp angles. Our algorithm also supports ''manual antialiasing, the style of antialiasing used in pixel art. Professional pixel artists report that Superpixelator's results are as good, or better, than hand-rasterized drawings by artists.
Semi-Automatic Digital Epigraphy from Images with Normals
(The Eurographics Association, 2015) Berkiten, Sema; Fan, Xinyi; Rusinkiewicz, Szymon; David Mould and Pierre Bénard
We present a semi-automated system for converting photometric datasets (RGB images with normals) into geometry-aware non-photorealistic illustrations that obey the common conventions of epigraphy (black-and-white archaeological drawings of inscriptions). We focus on rock inscriptions formed by carving into or pecking out the rock surface: these are characteristically rough with shallow relief, making the problem very challenging for previous line drawing methods. Our system allows the user to easily outline the inscriptions on the rock surface, then segment out the inscriptions and create line drawings and shaded renderings in a variety of styles. We explore both constant-width and tilt-indicating lines, as well as locally shape-revealing shading. Our system produces more understandable illustrations than previous NPR techniques, successfully converting epigraphy from a manual and painstaking process into a user-guided semi-automatic process.
The Markov Pen: Online Synthesis of Free-Hand Drawing Styles
(The Eurographics Association, 2015) Lang, Katrin; Alexa, Marc; David Mould and Pierre Bénard
Learning expressive curve styles from example is crucial for interactive or computer-based narrative illustrations. We propose a method for online synthesis of free-hand drawing styles along arbitrary base paths by means of an autoregressive Markov Model. Choice on further curve progression is made while drawing, by sampling from a series of previously learned feature distributions subject to local curvature. The algorithm requires no useradjustable parameters other than one short example style. It may be used as a custom ''random brush'' designer in any task that requires rapid placement of a large number of detail-rich shapes that are tedious to create manually.
Texture-Aware ASCII Art Synthesis with Proportional Fonts
(The Eurographics Association, 2015) Xu, Xuemiao; Zhong, Linyuan; Xie, Minshan; Qin, Jing; Chen, Yilan; Jin, Qiang; Wong, Tien-Tsin; Han, Guoqiang; David Mould and Pierre Bénard
We present a fast structure-based ASCII art generation method that accepts arbitrary images (real photograph or hand-drawing) as input. Our method supports not only fixed width fonts, but also the visually more pleasant and computationally more challenging proportional fonts, which allows us to represent challenging images with a variety of structures by characters. We take human perception into account and develop a novel feature extraction scheme based on a multi-orientation phase congruency model. Different from most existing contour detection methods, our scheme does not attempt to remove textures as much as possible. Instead, it aims at faithfully capturing visually sensitive features, including both main contours and textural structures, while suppressing visually insensitive features, such as minor texture elements and noise. Together with a deformation-tolerant image similarity metric, we can generate lively and meaningful ASCII art, even when the choices of character shapes and placement are very limited. A dynamic programming based optimization is proposed to simultaneously determine the optimal proportional-font characters for matching and their optimal placement. Experimental results show that our results outperform state-of-the-art methods in term of visual quality.
Hybrid-Space Localized Stylization Method for View-Dependent Lines Extracted from 3D Models
(The Eurographics Association, 2015) Cardona, Luis; Saito, Suguru; David Mould and Pierre Bénard
We propose a localized stylization method that combines object-space and image-space techniques to locally styl- ize view-dependent lines extracted from 3D models. In the input phase, the user can customize a style and draw strokes by tracing over view-dependent feature lines such as occluding contours and suggestive contours. For each stroke drawn, the system stores its style properties as well as its surface location on the underlying polygonal mesh as a data structure referred as registered stroke. In the rendering phase, a new attraction field leads active contours generated from the registered strokes to match current frame feature lines and maintain the style and path coordinates of strokes in nearby viewpoints. For each registered stroke, a limited surface region referred as influence area is used to improve the line matching accuracy and discard obvious mismatches. The proposed styl- ization system produces uncluttered line drawings that convey additional information such as material properties or feature sharpness and is evaluated by measuring its usability and performance.
Drawing Characteristics for Reproducing Traditional Hand-Made Stippling
(The Eurographics Association, 2015) Martín, Domingo; Sol, Vicente del; Romo, Celia; Isenberg, Tobias; David Mould and Pierre Bénard
We contribute an in-depth analysis of the characteristics of traditional stippling and relate these to common practices in NPAR stippling techniques as well as to the abilities and limitations of existing printing and display technology. In our work we focus specifically on the properties of stipple dots and consider the dimensions and attributes of pens and paper types used in artistic practice. With our analysis we work toward an understanding of the requirements for digital stippling, with the ultimate goal to provide tools to artists and illustrators that can replicate the stippling process faithfully in the digital domain. From the results of our study we provide a dataset for use in new example-based stippling techniques, derive a taxonomy of characteristics and conditions for the reproduction of stippling, and define future directions of work.
A Benchmark Image Set for Evaluating Stylization
(The Eurographics Association, 2016) Mould, David; Rosin, Paul L.; Pierre Bénard and Holger Winnemöller
The non-photorealistic rendering community has had difficulty evaluating its research results. Other areas of computer graphics, and related disciplines such as computer vision, have made progress by comparing algorithms' performance on common datasets, or benchmarks. We argue for the benefits of establishing a benchmark image set to which image stylization methods can be applied, simplifying the comparison of methods, and broadening the testing to which a given method is subjected. We propose a preliminary set of benchmark images, representing a range of possible subject matter and image features of interest to researchers, and we describe the policies, tradeoffs, and reasoning that led us to the particular images in the set.
Interactive NPAR: What Type of Tools Should We Create?
(The Eurographics Association, 2016) Isenberg, Tobias; Pierre Bénard and Holger Winnemöller
I argue that we need to increase our consideration of the interaction that is possible and/or needed for the NPAR algorithms we develop. Depending on the application domain of a given algorithmic contribution, different degrees of interaction are required to make it practically useful and, thus, relevant. The spectrum of interactivity ranges from (almost) fully automatic processing to levels of control that are similar to those of traditional tools-some of the approaches even needing to support the full spectrum. Only if these considerations are first-class members of the NPAR development process can we expect others to want to work with our tools and to use them on a regular basis.

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