F or a long time, line drawings have been a part of artistic expression (for example, any pencil or pen-and-ink drawing), scientific illustrations (medical or technical), or entertainment graphics (such as in comics). Hence, computer graphics researchers have extensively studied the automatic generation of such lines. In particular, the area of nonphotorealistic rendering has focused on two main directions of research in this respect: the generation of hatching that conveys illumination as well as texture in an image and the computation of outlines and silhouettes. Silhouettes play an important role in shape recognition because they provide one of the main cues for figure-to-ground distinction. However, since silhouettes are view dependent, they need to be determined for every frame of an animation. Finding an efficient way to accomplish this is nontrivial. Indeed, a variety of different algorithms exist that compute silhouettes for geometric objects. This article provides a guideline for developers who need to choose between one of these algorithms for his or her application. Here, we restrict ourselves to discussing only those algorithms that apply to polygonal models, because these are the most commonly used object representations in modern computer graphics. (For an algorithm to compute the silhouette for free-form surfaces see, for example, Elber and Cohen. 1) Thus, we can use all algorithms discussed here to take a polygonal mesh as input and compute the visible part of the silhouette as output. Some algorithms, however, can also help compute the silhouette only, without additional visibility culling. The silhouette's representation might vary depending on the algorithm class-that is, the silhouette might take the form of a pixel matrix or a set of analytic stroke descriptions.

Stippling is an artistic rendering technique where shading and texture is given by placing points or stipples on the canvas until the desired darkness is achieved. Computergenerated stippling has focused on producing high quality 2D renditions for print media, while stippling of 3D models in animations has received little attention. After describing current advances in stippling for print media and real-time rendering, we present an approach to produce animations of 3D models using stippling as a rendering style. In our approach, we ensure frameto-frame coherence as the model moves and shading changes over time, by attaching stipple particles to the surface of the model. We present a point hierarchy, used to control the stipple density during rendering, and solutions for rendering animated and static models using conventional and vertex-programmable graphics hardware.

In this paper we show how a game engine designed to generate photorealistic images can be extended to produce non‐photorealistic and hybrid renditions. We introduce new hardware‐based methods to accomplish pen‐and‐ink illustrations. The combination of the highly optimized processing of a game engine and the use of hardware for NPR algorithms yields real‐time animation of pen‐and‐ink illustrations. The particular advance of this method is that it yields the first real‐time, frame‐coherent pen‐and‐ink animations which maintain both tone and texture.

We present an overview of the "OLPC Etoys" system, describe the intensive two-year development effort that produced the system, and discuss lessons learned.

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