A developer's guide to silhouette algorithms for polygonal models

Abstract

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.

IRIS: Illustrative Rendering for Integral Surfaces

“…Corresponding rendering approaches can be mainly divided into the object-space, image-space, and hybrid categories (cf. [16,18]). Isenberg et al [18] recommend using an image space technique for achieving interactive frame rates with huge or animated data sets.…”

“…[16,18]). Isenberg et al [18] recommend using an image space technique for achieving interactive frame rates with huge or animated data sets. Object-space and hybrid algorithms rely on processing the mesh and thus we exclude them from consideration due to the high effort required for the large integral surfaces meshes we consider here.…”

Evaluating and Validating Non-photorealistic and Illustrative Rendering

“…One of the best examples for such techniques is the creation of sparse line drawings-lines that consist of silhouettes/contours [29] and feature lines. One question that comes up in this context is "where do people draw lines," [6][7][8] and how these lines related to the zoo of lines typically employed in NPR; another question is "how well do [such] line drawings depict shape" [6,9].…”

Interactive Generation and Modification of Cutaway Illustrations for Polygonal Models

“…Gooch et al [1] created appealing technical illustrations by enhancing functional characteristics of shape and structure though drawing of silhouette curves. Silhouette curves and contours play an important role in shape recognition as Isenberg et al [2] pointed out. We share this argument and perform a contour extraction as well.…”