Non-Photorealistic Rendering: From a general view to Suggestive contours Giorgio Conte and Massimo De Marchi EVL Creative Coding CS 488 - Computer Graphics University of Illinois at Chicago November 2014 Abstract Non-Photorealistic Rendering (NPR) is a very important branch of computer graphics. It was born in 1990s and from that time it has gained more and more popularity among the academic community. NPR con- tains applications in many different contexts which makes this field as interesting as stimulating. In this report, we have focused first our atten- tion on understanding the main steps done in the history of this discipline, then we have investigated some of the most well-known techniques and, in the end, we focused our attention on suggestive contours. After having analyzed the theoretical aspects, we have implemented one image-based algorithm to render this kind of effect. 1 Introduction ples and techniques known to artists for many centuries, that allows to get Computer Graphics is concerned with rid of unnecessary details and that bias the development of tools for generat- the user experience and perception. ing various forms of imagery. Peo- Non-Photorealistic Rendering (NPR) ple and artists are interested in these is the name often used to refer to all tools for the production of images to the techniques that follows this princi- communicate visual information and ples and in the last twenty years has the important point is that appropri- became such a popular topic that lot ate form of imagery for a given task of efforts have been spent to find new depends upon the nature of the com- algorithms. munication. Computer graphics has an amazing success story considering real- istic rendering. Conversely, there is a branch of computer graphics that seeks for something different, namely princi- 1 Photorealism is certainly good for the job of documentation, where the aim is to record scenes or objects in all their details. Crime scene photogra- phy is a perfect example. Should new evidence or theory prompt a reinves- tigation, crime scene photos may be probed for new clues. NPR, such as a sketch of the scene, could record the relative positions of the things present in the site, but any details not drawn Figure 1: NPR vs Photorealism by the artist at that time is going to be permanently lost. Long before computer graphics be- In addition, photorealistic computer came famous and popular, artists knew graphics is uniquely suited to the sit- the distinction between photorealism uation where one would like to cre- and non-photorealism. Looking at fig- ate live-action scenes (e.g. movies) ure 2 you could imagine it as a photo but where its difficult to stage the real while a painting like the one in figure scene. A possible task could be to 3 exhibits a level of abstraction that simulate a special effect such as huge remarkably departs from realism. explosions, or a flood over Chicago lakeshore requiring photorealistic com- puter graphics imagery for convincing effect. Even if we know that these effects are not real, cartoonish NPR could not be exploited to render those scenes be- cause it would destroy the illusion nec- essary for the audience to break of dis- belief and enjoy the story. Figure 3: Van Gogh, The Starry Night, 1889 Figure 2: Bouguereau, The Little Shepherdess, 1891 NPR may be the appropriate 2 means for communication for some lustrate abstract concepts using very tasks, but basically it depends on the concise image. In figure 5 the artist situation. Since computer graphics communicates an abstract idea of the has traditionally focused his atten- role of higher education in our society. tion mostly on photorealistic render- If we imagine to replace this with a ing, there are still lots of unexplored similar photoreal image, the abstract opportunities for new forms of content concept would not have been as effec- for authors. However, there are also tive as it is. various applications where NPR is rec- ommended, for example when image is used for an explanation such as in fig- ure 4. Given that, very likely the user has the real tool in front of him, a pho- tographic image contains lots of unnec- essary details; instead, the author can better explain the shape and function of the item by disambiguating differ- ent parts using distinct colors and re- move physically accurate shading that reduce the clarity of the image. Silhou- ette and feature lines are often used to increase the objects description. Figure 5: Sutter Finally, storytelling is the last ex- ample where stylized and abstract im- age can help to communicate scenes which exist only in the author's imag- ination. The effect would be totally different with comparable photoreal drawing. Moreover, children, the com- mon audience in this case, generally respond best to brightly colored illus- trations, and somehow find them more Figure 4: Technical illustration of a engaging than photos. mechanical tool Stylization is frequently used to il- 3 alism is ambiguous and graphics re- searchers and artists refer to it with different meanings. Moreover many of the techniques that are used in non- photorealistic rendering are not actu- ally rendering techniques; in fact many of the algorithms in the literature are based on image post-processing. Re- search efforts were directed to simulate hand drawn illustrations, namely at- tempt to simulate what an artist might do in order to enhance comprehensi- Figure 6: Dr. Seuss bility of visual information. However, even before the work done by Winken- In this document we would like first bach, in 1990, Takafumi Saito [17] pro- to give a brief overview of some of posed a new rendering technique that the best known techniques and algo- tries to achieve this by implement- rithms present in the literature then ing an algorithms for discontinuities, describe more in the detail an image- edges, contour lines, and curved hatch- space algorithm to render suggestive ing. contours and show the result obtained with the implementation. The re- port is structured as follows. First in section 2 we briefly outline the history of NPR and in section 3 we present some non-photorealistic algo- rithms and techniques found in the lit- erature. In section 4 we describe in de- tails how to render suggestive contours and in section 5 we presents the results obtained. Finally in section 6 we out- line the current research issues and we draw some conclusions in section 7. Figure 7: Saito 1990 2 History of NPR Subsequently many other tech- niques have been proposed among Non-photorealistic rendering is a rela- which Pen & Ink [19], watercolor [6] tively young field that emerges in the and curved brush strokes [10]. 1990s as an alternative to photoreal- ism. The term non-photorealistic ren- dering was probably coined by George Winkenbach and David Salesin in their paper dated 1994 [19] and it has been criticized for many reasons among which the fact that the term photore- 4 would be otherwise tedious to do by hand. One of the main issues was to pursue frame to frame coherence as de- scribed by Meier [15] and Litwinowicz [13] works. Figure 8: Winkenbach 1994 Figure 11: Meier 1996 Figure 9: Curtis 1997 Figure 12: Litwinowicz 1997 These result were achieved offline, but various works among which Kol- wasky [11], Gooch [8] and Praun [16], thanks also to the evolution of graph- ics hardware, opened the door for real- time animations. This was important Figure 10: Hertzmann 1998 because for the first time it was pos- sible to create interactive illustrations, All these techniques were focusing otherwise impossible to do by hand. on static imagery, but in the latter part of the decade researchers started to be interested in dynamic NPR, that is al- lowing the production of imagery that 5 Figure 13: Kowalski 1999 Figure 15: Praun 2001 Nowadays, more recent researches are trying to exploit NPR for visual- ization, especially for the production of medical illustrations useful for many applications. 3 Techniques In this section we would like to present and briefly describe some tech- niques which made the history of Non- Photorealistic rendering. 3.1 Pen and Ink A very interesting and effective tech- nique is called "pen and ink" and it was presented first by Winkenbach and Salesin in [19], 1994. This was one of the first NPR technique and it has became quite famous in the academic literature. The basic idea is to ren- der object and images as they were Figure 14: Gooch 1998 drawn by an artist just using pen and ink. The pen and ink effect has a very long story, since it first appeared in manuscripts of the Middle Ages, but 6 Figure 16: A collection of stroke textures presented by [19] . only in the last century this kind of ef- dication". Stroke textures can also be fect has been developed as an art form used to render shadows as it is depicted by itself. Pen-and-ink illustrations are in figure 17. very simple and effective, this simplic- ity makes them especially attractive. 3.2 Cel shader Moreover, each stroke could also gain more expressiveness by adding small ir- Cel shading or toon shading is a well regularities in its path and pressure. It know technique to render objects with is worth to remind that in this kind the style of cartoon or comic books. of illustrations, the drawer could only This technique is somewhat recent and vary the tone just by changing the den- it started to appear from around the sity of the strokes. beginning of twenty-first century. Ba- To render this effect, the authors have sically, there are two main techniques introduced the notion of "stroke tex- that are used to create a toon effect: tures" which are a set strokes used a tone-based technique and texture- to produce both texture and tone.