Practical Filtering for Efficient Ray-Traced Directional Occlusion

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Practical Filtering for Efficient Ray-Traced Directional Occlusion

Kevin Egan  Fredo Durand Ravi Ramamoorthi

Columbia University MIT CSAIL University of California, Berkeley







Abstract

Ambient occlusion and directional (spherical harmonic) occlusion

have become a staple of production rendering because they cap-

ture many visually important qualities of global illumination while

being reusable across multiple artistic lighting iterations. How-

ever, ray-traced solutions for hemispherical occlusion require many

rays per shading point (typically 256-1024) due to the full hemi-

spherical angular domain. Moreover, each ray can be expensive

in scenes with moderate to high geometric complexity. However,

many nearby rays sample similar areas, and the final occlusion re-

sult is often low frequency. We give a frequency analysis of shadow

light fields using distant illumination with a general BRDF and nor-

mal mapping, allowing us to share ray information even among

complex receivers. We also present a new rotationally-invariant fil-

ter that easily handles samples spread over a large angular domain.

Our method can deliver 4x speed up for scenes that are computa-

tionally bound by ray tracing costs.

CR Categories: I.3.7 [Computing Methodologies]: Computer

Graphics—Three-Dimensional Graphics and Realism

Keywords: ambient occlusion, relighting, sampling, frequency

analysis

1 Introduction

Modern production rendering algorithms often compute low fre-

quency hemispherical occlusion, where the surrounding environ-

ment is approximated to either be a solid white dome (ambient oc-

clusion), or a series of low frequency spherical harmonics. Two

different bodies of work related to ambient occlusion were given

scientific Academy Awards in 2010 [AcademyAwards 2010], and

the movie Avatar used ray-traced ambient and spherical harmonic

occlusion for lighting and final rendering [Pantaleoni et al. 2010].

While fully sampling the surrounding illumination at each receiver

is the completely accurate way to compute global illumination,

these approximations of distant lighting work well in practice. An-

other advantage is that the ambient occlusion and spherical har-

monic calculations are independent of the final lighting environ-

ment and can be reused throughout the lighting process.





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