“Direct illumination with lazy visibility evaluation” by Hart, Dutré and Greenberg

In this paper we present a technique for computing the direct lighting in a three-dimensional scene containing area light sources. Our method correctly handles partial visibility between luminaires and receivers, and is able to efficiently generate accurate soft shadows in scenes modeled with general bidirectional reflectance distribution functions. In most current algorithms, the form factor between a light source and receiver is computed using a stochastic ray casting approach which evaluates partial visibility. Such an approach often leads to noisy artifacts or aliasing problems. Generating significantly more rays is often the only solution to improving image quality. Our approach first stores visibility information in the image plane, using lazy evaluation of the visibility function. In a second phase, illumination values for each pixel are generated, using analytic or stochastic integration. Soft shadows and other shading effects are generated with high accuracy in less time than with existing shading algorithms. Coherence in specific blocker-light source relationships across the image plane is exploited to amortize the cost of analytic form factor calculations. By storing information in the image plane, our method is currently designed for generating a single image, and is thus view-dependent.

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