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彬彬

Abstract

Lightcuts is an efficient illumination method for scenes with many complex lights, by hierarchical clustering of lights in a light tree. However, when the light tree is large, it is very time-consuming to traverse in the tree to get the suitable clusters of lights for illumination computation. For this, some methods proposed to use image coherence for neighboring pixels to share clusters of lights, and so save traversal cost in the light tree. However, with the image resolutions reduced, fewer and fewer coherences could be used and so their acceleration efficiency will be reduced dramatically, and they may even decrease the rendering efficiency.

This sketch proposes to exploit spatial coherences to enhance rendering by lightcuts. For the intersection points between rays and the scene, they are clustered on the fly and the points of a cluster search their respective suitable clusters of lights from a common set of clusters, called a common cut. In this way, the traversal cost from the tree root to the common cuts can be considerably saved for acceleration. Results show that our method can be faster than the methods using image coherence, and works stably with various image resolutions. And with the lights being more complex, our method can obtain more acceleration.

CR Categories: Computer Graphics [I.3.7]: Three-DimensionalGraphics and Realism—Color, shading, shadowing, and texture;

Keywords: illumination computation; lightcuts; spatial coherence

1 Introduction

Realistic rendering under complex light sources is an important topic of computer graphics. To reduce the number of lights for speeding up illumination computation, the popular method by lightcuts [Walter et al 2005] proposed to perform approximation computation with adaptive clustering of lights, where a binary tree is used to manage hierarchical clusters of lights, called a light tree. To compute the radiance L at a surface point x viewed from direction ω, it performs a top-down traversal of the global light tree to find the interior nodes whose representative lights of their corresponding clusters of lights can accurately approximate illumination from all source lights at this point. The direct illumination from a cluster, say C with a representative light j∈C, is computed by using its representative light’s material, geometric, and visibility terms Mj(x, ω), Gj(x) and Vj(x) for all of its lights (Σi∈CIi) in the following formula.

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