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jgt 2004
Observations on Silhouette Sizes

Morgan McGuire

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Abstract. Silhouettes have many applications in computer graphics such as non-photorealistic edge rendering, fur rendering, shadow volume creation, and anti-aliasing. The number of edges, s, in the silhouette of a model observed from a point is therefore useful in analyzing such algorithms.

This paper examines, from a theoretical viewpoint, a menagerie of objects with interesting silhouettes (including those with minimal and maximal silhouettes). It shows that the relationship between and s and the number of triangles in a model, f, is bounded above by s = O(f) and below by s = Ω(1), and that the expected value of s over all observation points at infinity is proportional to the sum of the dihedral angles.

In practice, the models used with silhouette-based rendering algorithms are triangle meshes that are manually constructed or result from scans of human-made objects. They consist of only surface geometry with few cracks; there is no internal detail like the engine under a car's hood. Geometric and aesthetic constraints on these models appear to create an inherent relationship between f and s. Measurements of the actual silhouettes of real-world 3D models with polygon counts varied across six orders of magnitude show them to follow the relationship s ~ f 0.8. Furthermore, the expected value of s at infinity is a good approximation of the expected silhouette size for a viewer at a finite location.

Some images from the paper

@article{ mcguire04silhouette,
  author = "Morgan McGuire",
  title = "Observations on Silhouette Sizes",
  journal = "jgt",
  volume = {9},
  number = {1},
  pages = {1--12},
  year = {2004},
  URL = {http://www.cs.brown.edu/research/graphics/games/SilhouetteSize/index.html}


The following previous work is relevant: Kettner and Welz Contour Edge Analysis for Polyhedron Projections. In: W. Strasser, R. Klein, R. Rau (Eds.), Geometric Modeling: Theory and Practice, Springer, pp. 379-394, 1997. (Thanks to Aaron Hertzman for pointing out this book chapter to me).

Kettner and Welz give a sphere proof and experimental results for polyhedral models in categories like "terrain" and "anatomy." My paper follows a similar theory/practice structure and extends their results. I begins with a simpler polygonal case derivation and proofs of the upper and lower bounds on silhouette (contour) size. For experimental analysis I benefitted from the incredible speedup of PCs over the last decade. I was able to perform a much more computationally intensive task than Kettner and Welz by working with a larger, albeit heterogeneous, data set and exhaustively measuring results for infinite (orthogonal) and finite viewpoints from many angles and distances.