Geometry Shader Silhouettes

There has been a lot of research put into efficiently rendering accurate silhouettes in real-time applications. Silhouettes offer definition and detail that are important, escpecially in non-photorealistic rendering. Traditional silhouette rendering used the brute force approach or the edge buffer algorithm I briefly described in a previous post. These approaches carried out their computation on the CPU every frame and were too slow in real-time applications.

There have been many attempts to extract silhouettes on the GPU through the use of vertex shaders, many of which require extra geometry to be needlessly rendered per edge or extra information sent to the GPU per vertex. With the advent of geometry shaders there have been a few attempts at creating new geometry on the fly whenever a silhouette is found to exist.

A recent paper that's being published by a group of students at a university in Venezuela describes a single pass geometry shader that effectively extracts silhouettes in real-time. The silhouettes are created as extrustions of edges that are detected as silhouettes and are properly textured. Their paper describes their algorithm and contains the following discussion (which I summarize in this post) as history and describes their solution.

Geometry shaders have access to adjacency information which makes them perfect for silhouette extraction. At first it seems like a great idea to just extrude edges whenever a silhouette edge is detected but this can arise in discontinuities in the silhouette where two edges meet at an angle.

Another problem with this approach is generating texture coordinates. Silhouettes are often stylized in non-photorealistic renderings to match the style of the rendering. Without a continuous silhouette it is not possible to generate texture coordinates that correctly tile and rotate a texture along the silhouette.

The method described in the paper solves both the issues of the discontinuities in the silhouette edges and of the generation of the texture coordinates with a slight performance hit over another recent geometry shader method. However, the accuracy their solution provides outweighs the slight performance hit.

The undergraduates that did the research will be presenting their solution at SIACG 2009.