Fast calculation of soft shadow textures using convolution

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Abstract
The calculation of detailed shadows remains one of the most diffi- cult challenges in computer graphics, especially in the case of ex- tended (linear or area) light sources. This paper introduces a new tool for the calculation of shadows cast by extended light sources. Exact shadows are computed in some constrained configurations by using a convolution technique, yielding a fast and accurate so- lution. Approximate shadows can be computed for general con- figurations by applying the convolution to a representative "ideal" configuration. We analyze the various sources of approximation in the process and derive a hierarchical, error-driven algorithm for fast shadow calculation in arbitrary configurations using a hierar- chy of object clusters. The convolution is performed on images rendered in an offscreen buffer and produces a shadow mapused as a texture to modulate the unoccluded illumination. Light sources can have any 3D shape as well as arbitrary emission characteristics, while shadow maps can be applied to groups of objects at once. The method can be employed in a hierarchical radiosity system, or directly as a shadowing technique. We demonstrate results for various scenes, showing that soft shadows can be generated at in- In this paper, we present a new method for the calculation of soft shadows, which is able to provide pleasant, artifact-free images in a very efficient way. The method is based on the calculation of shadow maps, which are textures created from images of the light sources and occluders using a convolution technique. The convo- lution is performed with images of the light source and the set of occluders, rendered in offscreen buffers. The shadow textures are then used to modulate direct light source illumination across the re- ceiving objects. Exact images are obtained for some specific cases (parallel polygons), while for general configurations some approx- imation is necessary. We analyze the error incurred and the various sources of approximation, and show how the overall approximation can be controlled using a spatial hierarchy of object clusters. This is achieved by combining shadow maps of the sub-clusters hierar- chically. The resulting error-driven algorithm automatically computes soft shadows at interactive rates for extended light sources of arbitrary shape and exitance distribution, while avoiding excessive approxi- mation under a feature-based error metric. The method can be used in any rendering technique, with the only requirement of a hierar- chy of spatial clusters in order to use the hierarchical combination. The algorithm is naturally adaptive and eliminates the difficulties associated with light source sampling. The error-driven hierarchi- cal combination of shadow maps lets us adapt the effort to user- specified approximation tolerances.

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