A Survey of Pen-and-Ink Illustration in Non-photorealistic

Transcription

1 A Survey of Pen-and-Ink Illustration in Non-photorealistic Rendering CS361 Computer Science Department GWU Page 1 of 25

2 Table of Contents A Survey of Pen-and-Ink Illustration in Non-photorealistic Rendering... 1 Introduction... 3 Background... 4 Terminologies in hand-drawn pen illustration... 4 Principles... 5 Classification... 6 Object-based Approaches... 8 Algorithm Algorithm Algorithm Algorithm Algorithm Algorithm Image-based Approaches Algorithm Algorithm Algorithm Application Future Research References Page 2 of 25

3 Abstract This paper presents a survey of the work done in the pen-and-ink illustration field of nonphotorealistic rendering. Non-photorealistic rendering(npr), as a relatively new area is getting more and more attention from the computer graphics community. Pen-and-Ink illustration, as one of the styles of NPR, received much research in the recent fifteen years. In this paper, important terminologies and principles are first introduced. Then the general pen-and-ink illustration techniques are categorized and examined with certain focus. Afterwards one important application field is discussed. The paper also suggests important areas for future research. Introduction Computer graphics research has focused on photorealistic rendering, which attempts to create images of physical scenes with ever-increasing realism, that looks just like the real world. However it is not as effective and efficient as non-photorealistic rendering in a lot of situations. Non-photorealistic rendering (NPR) is any technique that produces images of simulated 3d world in a style other than realism[1]. There are many styles of NPR, for example, water color[2], impressionistic[3,4,32], pen-and-ink illustration, engraving[5], etching[1], etc. Because of the broad spectrum of the area, the focus of the paper is put on the pen-and-ink illustration style. Pen-and-ink is an extremely limited medium, allowing only individual monochromatic strokes of the pen. However beautiful pen-and-ink illustrations incorporating a wealth of textures, tones, and styles[18] can be created. Indeed, because of their simplicity and economy, there are a lot of applications and advantages of this illustration technique. With only a simple form, image creators are able to use expressive Page 3 of 25

4 abstraction to emphasize the area of interest and focus the viewer s attention without being forced to depicting every detail. Therefore it is very effective and efficient for purpose of illustrating and expressing(i.e. effectively conveying the meaning or feeling[28]), especially if the underlying models are too complex to render realistically. Second, in architectural design and industrial design, stylized illustrations are often more appropriate, especially in the initial design phase, than photorealistic pictures and CAD because they give the impression of approximation and incompleteness and therefore are more stimulating[30,27,25]. Third, because the files created using this method usually consume less storage, they are reproduced more easily and transmitted more quickly and especially convenient for laser printers. Forth, the illustrations made in this style are blended well with texts, using the same ink as texts. Because of these characteristics and advantages, pen-and-ink illustrations are widely used in textbooks, repair manuals, advertising and many other forms of media. First we will discuss some fundamental terminologies, essential principles in penand-ink illustration and a general classification of the general algorithms described here will also be presented. Background Terminologies in hand-drawn pen illustration For further discussion and instruction, interested reader should consult Guptil[6], a classic comprehensible text on pen and ink illustration. There are two different kinds of marks or strokes in pen-and-ink illustration: outlines, hatching. Outlines are the external boundaries and internal edges, used to define shapes. Outlines are exceptional in that they may be long and individually significant. Page 4 of 25

5 The choice of whether or not to use outlines is largely an aesthetic one. Hatching is used to indicate shading but should also follow the surface curvature. Pen-and-Ink limitation in being monochromatic is overcome to some extent through the use of tone, style and texture. Tone is the darkness of a section of a drawing. The perceived gray level or tone in an illustration depends largely on how dense the strokes are in a region. Style is the brokenness of a line, for example solid line, dashed line, dotted line. Texture is a tactile impression of a surface as rough, sandy, smooth.[7] Texture is the collective result of many pen strokes, each individual stroke is not critical and need not be drawn precisely. Indeed a certain amount of irregularities in each stroke is desirable to keep the resulting texture from appearing mechanical. The most commonly used textures include: hatching formed by roughly parallel lines; cross-hatching formed by overlapped hatching in several directions; and stippling formed by small dots or very short lines. Principles These are some fundamental principles of illustrating in pen and ink, useful for purely computer generated illustrations. Interested readers are referred to [6], [8] and [15]. Strokes must look natural, not mechanical, the thickness of a line should vary along its length, wavy lines are a good way to indicate that a drawing is schematic and not yet completely resolved. It is not necessary to depict each individual tone accurately, however presenting the correct arrangement of tones among adjacent regions is essential, to disambiguate objects it is sometimes important to force tone by enhancing contrast or inventing shadows. And the character of strokes and outlines is important for conveying texture, as well as geometry as lighting, e.g. straight lines are good for glass. Page 5 of 25

6 S. Strassmann[9] Hairy brush proposed the path-and-stroke metaphor to emulate brush using four different objects: brush, stroke, dip and paper to get a variety of strokes[25]. S. Hsu and I. Lee[12] extended the metaphor by using general objects like textures, images and recursively defined fractals that are drawn along a given path. Haeberili[4] also focused on manipulating individual stroke by inspecting a collection of attributes of the stroke, including location, color, size, direction and shape, and a path is defined and physically simulated brush is used to generate the stroke. Classification There are three distinct types of input for stylized depiction processes[1]: (1) 3D scenes (described in terms of geometry, color, lighting, etc.) for rendering (2) images for processing (3) brush strokes from a user ( like the input to a paint system) Depending on the scene input(1,2), all the techniques presented here can be classified into two classes, object-based and image-based[10]. Image-based systems produce their illustrations directly from grayscale images. In object-based category, there are two different kinds of image rendering algorithms, the image-centered and the scenecentered algorithms[11]. Scene-centered algorithms project un-occluded objects onto the image. Such techniques are qualified for polygonal scenes, while for other object kinds special treatment is necessary, especially for free form surfaces. Image-centered algorithms typically have a post-processing phase. Depending on (3), these techniques can be categorized along the axis from interactive to fully automatic[1]. Below is a classification of the general techniques that we will examine afterwards, represented by tags. Page 6 of 25

7 Object-based Image-based Object-centered Image-centered automatic Algorithm4, Algorithm2, Algorithm9 Algorithm5, Algorithm3 Algorithm6 interactive Algorithm1 Algorithm7, Algorithm8 The advantage of geometry-based(object-based) systems is because of the availability of the 3D geometry and viewing information they can produce illustrations whose strokes not only convey the tone and texture of the surfaces in the scene, but also convey the 3D forms of the surfaces by placing strokes along the natural contours of surfaces. Existing image-based systems, on the other hand, until now have been able to convey 3D information only by having a user draw individual strokes or specify directions for orienting particular strokes. Whereas the ability to generate illustrations with an image-based system offers several advantages. First, using an image-based system greatly reduces the tasks of geometric modeling and of specifying surface reflectance properties. Second, an image-based system provides the flexibility of using any type of physical photograph, computer-generated image, or arbitrary scalar, vector or tensor field as input, allowing visualization of data that is not necessarily physical in nature. Finally, image-based systems offer more direct user control by providing the ability to modify tone, texture, or stroke orientation[10]. General techniques of pen-and- Page 7 of 25

8 ink illustration are discussed in the following sections with an emphasis on the advantages and disadvantages of the system, important steps and key features of the specific system, the generation and placement of strokes( including rendering of outlines, strokes, textures and tones) and applications of the algorithm. Object-based Approaches Algorithm1 Dooley and Cohen[13] proposed a system to enhance a traditional shading images with illustration techniques. Their system is able to handle the difficulties arising from triangulating complex objects when there are many surfaces with common borders present. Objects in the scene are attached to a set of semantic attributes that are interactively determined. The rendering algorithm works with illustration rules that are applied to the projected lines according to their attributes. They attempt a taxonomy of line styles and semantics. They showed how line and surface qualities could be customized by the user to create more effective images. For example, lines can vary thickness, transparency and style and line attributes are described by a matrix on the values of importance, line type and hiddenness. Algorithm2 To produce outlined and contoured drawings instead of shaded image, T. Saito et al. [14] propose an enhancement technique for 3D shapes that conceptualizes geometric properties. This is an automatic image-centered approach, because all operations are realized with 2D image processing operations and with no user interaction. The problems with this method contain: aliasing and reflected or transparent images can not be enhanced because each pixel can represent only one surface; inefficiency in both Page 8 of 25

9 execution time and memory space because all images are preserved as floating point data in order to avoid digitization errors. The technique can be divided into 3 separate processes: 1.geometric process based on geometric factors such as object shapes and camera parameters(projection and hidden surface removal); 2.physical process based on physical factor such as reflectance, colors, textures(shading, texture mapping); 3.artificial process, such as outline enhancement. The authors introduce a special G-buffer for geometric information used repetitively by the algorithms. The contents of the buffer include: object/patch identifier, parametric patch coordinates, Z-buffer, world coordinates and normal vector for the visible patch. G-buffer is formed during the geometric process and used by the physical and artificial processes. The basic enhancement operations, i.e. the drawing of discontinuity lines, contour lines and curved hatching, are done using G-buffer contents during post-processing. It is separated from geometric and physical processes. Edge, Contour and Hatching Drawing Edge contains profile and internal edge. Profile is the border line of an object on the screen; internal edge is a line where two faces meet. Profile and internal edge are the 0 th and first order discontinuity of the depth image respectively. Discontinuity can be extracted with a first order differential operator and discontinuity of the first order differential of an image can be extracted with a second order differential operator. The artifact resulted from these operations, such as confusing real discontinuities from rapid depth change can be corrected using the minimum and maximum of neighboring differential values. So both profile and internal edges can be drawn. Page 9 of 25

10 Using image processing technique, contour lines are generated as raster data and consists of homogeneous calculations on each pixel and its neighbors. Curved hatching rendering of a surface is performed using the contour method above in combination with a technique that thins out and disappears the contours when they become too dense and adds lines when the lines become too thin; the purpose is to have an overall periodic set of lines in pixel space. Application This algorithm is commonly used in hand drawn illustrations in industrial design, line illustration, topographical maps, medical imaging and surface analysis. The method is also useful for photorealistic rendering. Piranesi[19] system proposed by Landsown and Schofield also uses NPR to create illustrations from 3D models. Piranesi uses a standard graphics pipeline to create a 2D reference image akin to a G-buffer. The user is then allowed to select specific regions of the image and apply textures that emulate natural media interactively or automatically. Algorithm3 Leiser[11] presents a technique to emulate cooper-plate rendering, an engraving technique used for old system of printing. The goal is to render a copper plate, drawing that consists of lines of varying thickness and of single points for 3d scene. A ray-tracing approach is used to render curves on free-form objects. An advantage of this approach is that it easily handles reflections and shadowing because ray tracing is used. The method uses a kind of volume texturing in connection with image processing algorithms and is suitable for implementation in a ray tracing algorithm. Shading is done by regular hatchings in several thickness and distances. Copper plates can be generated Page 10 of 25

11 from 2D pixel images with filter algorithms and image processing techniques. In the postprocessing step an edge detection algorithm is applied. Edges are sudden changes in the image domain, that are difficult to determine with an image-centered method. This technique is able to deal with not only polygonal scene but also freeform surfaces. Experience shows that this method is especially interesting for illustration in books and for generating icons on user interface. Algorithm4 Winkenbach et al.[15] propose an automatic object-centered rendering system. Compared to [Algorithm2 and Algorithm6] the use of strokes are more expressive. And this method is resolution-independent while a lot of other computer drawing programs do not scale well. The limitation is that it can only illustrate polyhedral models and can not be used for curved surfaces. Also flat-shaded surfaces are assumed. Main Steps Firstly, the system computes the visible surfaces and the shadow polygons. Secondly, it projects the polygons to NDC space to generate certain data structure. Thirdly, each visible surface is then rendered. The procedural texture attached to each surface is invoked to generate the strokes conveying the correct texture and tone for the surface. Then all the strokes are clipped to the visible portion of the surface. Finally the outlines are drawn by extracting form the structure generated in step2. Key Features Their rendering system is a basic graphics pipeline with a few notable changes. Some of changes include rendering of texture and tone, clipping and outline. Because polygons are no long scan converted, both texture and tone must be conveyed with hatching. And Page 11 of 25

12 because the stroke is now distorted or displaced in some way, it is important to allow strokes to sometimes stray slightly outside of the clipped region. To achieve this, we clip the straight-line path of our strokes prior to adding in the function for waviness. And boundary and interior outlines must be drawn in a way that takes into account both the textures of the surrounded regions and the adjacency information. Rendering Texture and tone To render the texture and tone, we use prioritized stroke textures. A prioritized stroke texture is a set of strokes each with an associated priority. When rendering a prioritized stroke texture, all of the strokes of highest priority are drawn first; if the rendered tone is still too light, the next highest priority strokes are added until the proper tone is achieved. For example for a brick texture, the outlines of the individual brick elements have highest priority, the strokes for shading individual bricks have medium priority, and the hatching strokes that go over the entire surface have lowest priority. In the cross-hatching texture, vertical strokes have priority over horizontal strokes, which have priority over the various diagonal stroke direction. We express texture with outline. Each stroke texture has associated with it a boundary outline. Outline drawing The interior outlines are used within polygons to suggest shadow directions or to give view-dependent accents to the stroke texture. And for the sake of principles of pen-andink illustration, we minimize outline by drawing it only if the tones of two neighboring faces are not sufficiently different for disambiguation. Accented outlines, i.e., thickening edges is a technique for providing subtle but important cues about the 3d aspects of an illustrated scene. For example, the interior outlines of each brick in the brick stroke Page 12 of 25

13 texture are drawn according to their relationship with the directin of the light source: brick edges that cast shadows are rendered with thickened edges while illuminated brick edges are not drawn at all. In addition to the light source direction, the viewing direction is another important parameter that should be taken into account when drawing outline strokes. Algorithm5 Winkenbach et al.[16] extend the previous method[algorithm4], and propose one algorithm that can handle curved surfaces formulated parametrically, such as B-spline surfaces, NURBS and surfaces of revolution. [Algorithm2] mainly uses image processing technique in the post-processing stage for outline and hatching generation. While this method integrates aspects of 2D and 3D rendering. In addition, traditional texture mapping techniques can be used to extend the range of effects that can be achieved with pen-and-ink rendering. The biggest limitation is that it deals only with surfaces possessing a global parameterization. One possible solution is to parameterize such surfaces. Generation of Stroke Textures The progress compared to their last implementation is in the generation of the stroke texture. Firstly they use a grid of lines, which consists of parallel lines, running in one or more user-specified directions in the parameter domain, to orientate hatching strokes along a surface. Secondly, they introduce a technique called controlled-density hatching. This technique allows strokes to gradually disappear in light areas of a surface or in areas where too many strokes converge together and allows new strokes to gradually Page 13 of 25

14 come into existence in dark areas or areas in which the existing strokes begin to diverge too much. Specifically a recursive algorithm is used. Figure 1. Illustration with Texture Map Controlled-density hatching allows fine grain control over the tone of a pen-and-ink illustration. With this new capability, we can use traditional texture mapping to vary the tone on the surface of an object. Algorithm6 Markosian et al.[17] build a system to deliberately trade accuracy for speed. In contrast Winkenbach s pen-and-ink rendering system produces decidedly finer images, but takes several minutes to do so. This system is very quick and can be easily extended to other styles. But there is no shadow created. Key Features This real-time NPR technique is based on economy of line the idea that a great deal of information can be effectively conveyed by very few strokes. This algorithm only renders silhouettes, certain user-chosen key features and some minimal shading of surface regions. One obstacle to achieving real-time NPR is the problem of determining visibility, since a straightforward use of z-buffer may give incorrect results. So the key idea is rapid identification of silhouette edges using interface coherence of silhouette edges and fast visibility determination using improvements and simplification in Appel s hidden-line algorithm. Page 14 of 25

15 Main Steps The overall structure of the algorithm is: 1.determine the silhouette curves in the model 2.determine the visibility of silhouette and other feature edges by a modified Appel s algorithm 3.render the silhouette and feature edges. Generating Strokes World-space polylines to be rendered are first projected into the film plane. Artistic or expressive strokes are then generated by modifying the resulting 2D polylines. There are three techniques for generating expressive strokes: drawing the polylines directly with slight enhancements such as variations in line width or color; adding offsets to the polylines to define high-resolution artistically perturbed strokes; and texture-mapped strokes which follow the shape of the polyline. In the second case the polylines are parameterize by arc length. A new parametric curve Q(t) is based on the original parametric curve p(t) by adding a vector offset v(t) defined in the tangent-normal basis, i.e. Q(t) =p(t) + vx(t)p (t) + vy(t)n(t). The third method builds a texture-mapped mesh using the polyline as a reference spine. Each texture map represents a single brush stroke. The textures are repeated along the reference spine, approximately preserving its original aspect ratio. Placement of Strokes Shading strokes (particles) are put in world space (not the surface) rather than define them in screen space. This is the approach used by Meier in her painterly rendering [3]. The advantage of this approach is it maintains frame-to-frame coherence. Stroke directions are defined by the cross product of local surface normal and the ray from the Page 15 of 25

16 camera to the stroke location and the ray from the camera to the stroke location, so that stroke line up with silhouette lines. Applications Haro[24] implements a non-photorealistic renderer for producing pictures that look like sketches using this technique. Interested reader may refer to [24] for details. Image-based Approaches Algorithm7 Salisbury et al.[18] present an algorithm for rendering subdivision surface models of complex scenes using an interactive editable particle system, with 2D grayscale images as a starting point. Interaction Strassmann [9] presents interactive simulation of traditional artist tool. Compared to his approach, rather than focus on individual strokes, this new system tries to directly support the higher-level cumulative effect that the strokes can achieve: texture, tone and shape. The interaction between the user and system is high-level in that the user paints using textures and tones, and the computer draws the individual strokes. Exceptions are outlines that have individual significance; in addition an artist might occasionally need to touch up fine details of textured work. Texture generation Different from Winkenbach al. s approach, a combination of non-procedural and procedural stroke textures are used. Non-procedural texture is that the textures tiled the plane and the stroke selected for drawing at a point was the one that happened to pass through that point. A library of user-defined stored stroke textures is supported, as well as Page 16 of 25

17 several built-in procedural stroke textures. A stored texture is simply a collection of strokes. Drawing a texture at a given darkness is a matter of choosing from the collection a subset that has enough strokes to reach the desired tone. For some textures such as scribbles, the choice of strokes to draw for a given tonality is not critical. In these cases the system simply selects strokes from the texture in a random sequence, generating candidate strokes and testing the tonal effect of candidate strokes. For other textures, the system supports a predefined priority for each stroke. In addition to modifying individual strokes, the user can edit collections of strokes, like the lighting operation which incrementally removes strokes. Outline Drawing The system allows scanned, rendered or painted images to be used as a reference for tone and shape. The reference image is used for several purposes: as a visual reference for the artist, as a tone reference for painting, as a source image from which the outline is extracted, as a reference for determining stroke and texture orientation. Algorithm8 P. Salisbury et al. [10] are the first to use orientable textures for image-based pen-and-ink illustration. This is also an improvement of the previous method[algorithm7]. This system is able to render strokes and stroke textures according to a vector field in such a way that they also produce the proper texture and tone; and estimate tones as new oriented strokes are progressively applied. Interaction Editor allows the user to specify the three components of a layer (tone, direction, and texture), the system does the tedious work of placing all the strokes. Page 17 of 25

18 Figure 2. The User Input(from left to right) tone, direction and a stroke example In their previous work, the textures tiled the plane and the stroke selected for drawing at a point was the one that happened to pass through that point. By contrast, in this new system the placement of strokes on the final illustration is independent of their relative position in the texture. Spacing between strokes is instead maintained indirectly by the rendering system. Placement of Strokes Dynamic placement of stroke is import to maintain the density. Dynamic is implemented by drawing in order of importance, the fraction of its intended darkness that has not yet been accumulated at that point. Rendering consists of looking for the location with greatest importance, placing a stroke, update an image that records the importance and repeat until the importance everywhere is below a termination threshold. The whole process of matching the illustration to the tone image is recorded in a difference image, updated after each stroke is drawn, whose value at each pixel is the difference between the tone image and the blurred version of the illustration. The importance image is derived from the difference image, its value at each point is the current difference derived by the initial value of the difference. This algorithm is called Difference Image Algorithm. Page 18 of 25

19 After knowing where to put the stroke, orient and bend the stroke. Map the stroke into the direction field so that at every point along its length, the stroke s new angle relative to the direction field is the same as the prototype stroke s angle with respect to the vertical direction. Algorithm9 Curtis[26] implements a loose and sketchy filter to automatically draw the visible silhouette edges using image processing and a stochastic, physically-based particle system. The only input is a depth map of the model and it will be converted into two images: template image and force field. Template image represents the amount of ink needed for each pixel. And the force field pushes particles along the silhouette edges. The particles are generated randomly initialized by the template image. Acceleration of each particle is based on the force field. Particle generates ink and remove ink during its traveling, a technique used in [10, 31] Application In the following text, one application area of pen-and-ink illustration, the rendering of trees or complex natural objects, is discussed.the advantage of art-based illustration is evident in the kind of application where the underlying model is so complex that it is very time-consuming to model and render while the whole thing can be rendered with a few strokes which evoke the impression of complexity using pen-and-ink illustration. Trees are one of those complex objects. Kowalski et al. [20] suggest an algorithm to render fur, grass, trees etc. by penand-ink illustration. This approach is image-centered and interactive. Their approach is to generate abstract sketches of trees by using geometric primitives like spheres for defining Page 19 of 25

20 rough approximations of a tree s foliage. These primitives were rendered conventionally to achieve gray-scale images. In the second step, the images were used to procedurally place graftals small objects representing leaves or hair on the surfaces by applying the difference image algorithm proposed earlier by Salisbury[Algorithm8]. Their algorithm controls the density of hatching strokes in order to match the gray tones of the target image. And this method enables different textures assigned to different regions, while in the previous version, [Algorithm6] all the surfaces are assigned with textures uniformly. It is accomplished by dividing models into one or more regions (patches). The particle placing is a hybrid of screen and object. To meet the requirement that graftals appear to stick to surfaces in the scene, graftals are placed in the scene after converting the 2D screen position to a 3D position on some surface[3]. Figure 4,5. scene rendered without graftal texture(left) and with graftal texture(right) Fraftals are based on a flat tapering shape by gradually reducing width about a central spine, which is a planar polyline. After being placed with the DIA, each graftal determines how to orient and draw itself. The drawbacks of the algorithm[20] are that each new graftal texture requires a procedural implementation that included writing code. Also graftals are regenerated in each frame in a way that leads to excessive introduction and elimination of graftals even for small changes in camera parameters. Thirdly graftals choose from among a small Page 20 of 25

21 number of discrete levels of detail at which to draw, making transition between levels noticable. Markosian et al.[21] propose a new system which overcome these drawbacks by a more expressive interface, static placement and continuous levels of detail. The new approach is to use a static placement scheme, where graftals are distributed onto surfaces during the modeling phase. Certain graftals have a multiresolution structure, so that a single graftal, seen from larger distance, transforms into several graftals, when viewed from nearby. Further, these transformations are carried out in a continuous manner by smoothly varying the shape, size, position and orientation. Deussen et al.[22] take a very different approach which models detailed tree trunk and leaves while the goal of the previous approaches is to avoid complex modeling. The lines drawn are the result of visually combining many drawing primitives instead of placing graftal objects on some large geometries. A drawback of this approach is that they potentially have to deal with more input data. The solution to this problem is to represent a tree at several levels of detail, for example if the current model has too many leaves a much simpler model can be used instead. The advantage of this method is the ability to draw both an abstract tree and a specific plant which will be not very different from its realistic image and the ability to make use of existing tree libraries. To draw the trunk, the method raised by Markosian in [17][Algorithm6] to render the outline of the trunk can be used. The skeleton can be crosshatched using difference image algorithm by Salisbury[19][Algorithm7]. The direction of the strokes is similar to what is used in [17] [Algorithm6] either at random or affected by the normal vector of the Page 21 of 25

22 stem geometry. Leaves are drawn using abstract drawing primitives. For example, each leaf can be represented as the outline of a disk. Future Research 1. Improve the procedural stroke textures and automate further methods for creating them 2. Incorporate other illustration effects 3. Add more interactive controls to help designing 3d illustrations 4. Render other natural objects 5. Create animation 6. Explore other forms of illustration besides pen-and-ink, including traditional forms like water color and air brushing Page 22 of 25

23 References [1] Craig Reynolds. Stylized Depiction in Computer Graphics. [2] Cassidy J. Curtis, Sean E. Anderson, Kurt W. Fleischer, David H. Salesin. In SIGGRAPH 97 Conference Proceedings. [3] Barbara J. Mier. Painterly Rendering for Aniamtion. In SIGGRAPH 96 Conference Proceedings. [4] Paul Haeberli. Paint by Numbers: Abstract Image Representation. In SIGGRAPH 90 Conference Proceedings, Volume 24, Number 4, August [5] Victor Ostromoukhov. Digital Facial Engraving. In SIGGRAPH 99 Conference Proceedings. [6] Arthur L. Guptil. Rendering in Pen and Ink. Watson-Guptil Publications, New York, [7] Tom Jazen. Ten Papers on Non-Photorealistic Rendering. [8] Frank Lohan. Pen and Ink Techniques. Contemporary Books, inc., Chicago, [9] Steve Strassmann. Hairy Brush. In SIGGRAPH 86 Conference Proceedings, Volume 20, Number 4, August [10] M. Salisbury, M. Wong, J.F. Hughes, and D. Salesin. Orientable textures for image-based pen-and-ink illustration. In SIGGRAPH 97 Conference Proceedings. [11] Wolfgang Leister. Computer Generated Copper Plates. Computer Graphics Forum, Volume 13, Number 1, March 1994, pp , Blackwell, ISSN [12] S. Hsu and I. Lee. Drawing and animation using skeletal strokes. In SIGGRAPH 94 Conference Proceedings, pages [13] Debra Dooley, Michael F.Cohen. Automatic Illustration of 3D Geometric Models: Lines. In SIGGRAPH 90 Conference Proceedings, Volume 24, Number 2, pages 77-82, March [14] Takafumi Saito, Tokiichiro Takahashi. Comprehensible Rendering of 3D Shapes. In SIGGRAPH 90 Conference Proceedings, Volume 24, Number 4, pages , August [15] G. Winkenbach and D. Salesin. Computer-generated pen-and-ink illustration. In SIGGRAPH 94 Conference Proceedings, pages [16] G. Winkenbach and D. Salesin. Rendering parametric surfaces in pen and ink. In SIGGRAPH 96 Conference Proceedings, pages Page 23 of 25

24 [17] Lee Markosian, Michael A. Kowalski, Sam Trychin, Lubomir Bourdev, Daniel Goldstein, John F. Hughes. Real-Time Non-Photorealistic Rendering. In SIGGRAPH 97 Conference Proceedings. [18] Michael P. Salisbury, Sean E. Anderson, Ronen Barzel, David H. Salesin. Interactive Pen-and-Ink Illustration. In SIGGRAPH 94 Conference Proceedings. [19] Piranesi. [20] Michael A. Kowalski, Lee Markosian, J.D.Northrup, Loring S. Holden, John F. Hughes. Art-based Rendering of Fur, Grass, and Tree. In SIGGRAPH 99 Conference Proceedings. [21] Lee Markosian, Barbara J. Meier, Michael A. Kowalski, Loring S. Holden, J.D. Norhthrip, John F. Hughes. Art-based Rendering with Continuous Levels of Detail. In NPAR 00 Conference Proceedings. [22] Oliver Deussen, Thomas Strothotte. Computer-generated Pen-and-ink Illustration of Trees. In SIGGRAPH 00 Conference Proceedings. [23] Thomas Strothotte, Bernhard Preim, Andreas Raab, Jutta Schumann, David R. Forsey. How to Render Frames and Influence People. In Computer Graphics Forum (13) 3, Proceedings of eurographics 1994, pages ,1994. [24] Antonio Haro. A nonphotorealistic render. [25] Stefan Schlechtweg. Lines and How to Draw Them. Magdeburg.DE/~stefans/pubi/norsigd2.html [26] Cassidy Curtis. Loose and Sketchy Animation. [27] Paul Bourke. Computer Sketching. [28] Matthew Kalplan, Bruce Gooch, Elaine Cohen. Interactive Artistic Rendering. In NPAR 00 Conference Proceedings. [29] Maic Masuch, Stefan Schlechtweg, Bert Schonwalder. DaLi Drawing Animated Lines!. In Proceedings of Simulation and Animtation 97, S.87-96, SCS Europe, [30] Jutta Schumann, Thomas Strothotte, Stefan Laser. Assessing the Effect of Non-Phototealistic Rendered Images in CAD. dings/papers/schumann/chi96fi.html [31] Greg Turk and David Banks. Image-Guided Streamline Placement. In SIGGRAPH 96 Conference Proceedings. Page 24 of 25

25 [32] Peter Litwinowicz. Processing Images and Video for an Impressionist Effect. In SIGGRAPH 97 Conference Proceedings. Page 25 of 25