 
Summary: On Shadow Volume Silhouettes
Tomas AkenineM¨oller and Ulf Assarsson
Chalmers University of Technology
Abstract
In shadow volume rendering, the shadow volume silhouette edges
are used to create primitives that model the shadow volume. A com
mon misconception is that the vertices on such silhouettes can only
be connected to two silhouette edges, i.e., have degree two. Fur
thermore, some believe that the degree of such a vertex can have
any degree. In this short note, we present a geometric proof that
shows that the degree of a silhouette vertex must be even, and not
necessarily two.
1 Introduction
The shadow volume (SV) algorithm [4] has become a very popular al
gorithm for realtime rendering [5] of hard shadows. Recently, the SV
algorithm has been extended to handle soft shadow as well [1, 2]. In
all these algorithms the shadow volume silhouette (SVS) of the shadow
casting objects are found. An edge of such an SVS is connected to two
polygons, where one is frontfacing and the other is backfacing as seen
from the light source. The degree of an SVS vertex is the number of SVS
