Jaroslav Křivánek

Effects of Global Illumination Approximations on Material Appearance


Jaroslav Křivánek
Cornell University &
Charles University, Prague
James A. Ferwerda
Rochester Institute of Technology
 
Kavita Bala
Cornell University
 


Examples of inequivalent and equivalent VPL rendering. (a)-(b) are VPL renderings with 1k VPLs, and clamp levels C1 = 316 and C8 = 0.1, respectively, that are not equivalent to the reference (d) because they have image artifacts (a) or different perceived material appearance (b). (c) VPL rendering produces an image that is visually equivalent (≡) to the reference for 100k VPLs and clamp level C4 = 10, even though some reflections are lost where the Dragon is in contact with the pedestal and around its silhouette.


Abstract

Rendering applications in design, manufacturing, ecommerce and other fields are used to simulate the appearance of objects and scenes. Fidelity with respect to appearance is often critical, and calculating global illumination (GI) is an important contributor to image fidelity; but it is expensive to compute. GI approximation methods, such as virtual point light (VPL) algorithms, are efficient, but they can induce image artifacts and distortions of object appearance. In this paper we systematically study the perceptual effects on image quality and material appearance of global illumination approximations made by VPL algorithms. In a series of psychophysical experiments we investigate the relationships between rendering parameters, object properties and image fidelity in a VPL renderer. Using the results of these experiments we analyze how VPL counts and energy clamping levels affect the visibility of image artifacts and distortions of material appearance, and show how object geometry and material properties modulate these effects. We find the ranges of these parameters that produce VPL renderings that are visually equivalent to reference renderings. Further we identify classes of shapes and materials that cannot be accurately rendered using VPL methods with limited resources. Using these findings we propose simple heuristics to guide visually equivalent and efficient rendering, and present a method for correcting energy losses in VPL renderings. This work provides a strong perceptual foundation for a popular and efficient class of GI algorithms.


Publication & Presentation

Jaroslav Křivánek, James A. Ferwerda, and Kavita Bala. Effects of Global Illumination Approximations on Material Appearance.
ACM Trans. Graph. 29, 4 (2010) ... BibTeX

SIGGRAPH 2010 Presentation: Thursday, 29 July | 10:45 AM - 12:15 PM | Room 408 AB


Downloads

 
paper
supplementary
document
presentation
slides
fast forward
slides
stimulus
scene*
 26MB pdf | 1.8MB pdf   15MB pdf | 0.7MB pdf   20MB pptx | 5MB pdf   5.7MB ppt | 0.9MB pdf   42MB zip 

(*) This 3ds max scene served as a basis for creating the stimulus images. We exported the scene, modified the materials, and rendered the stimulus images with a custom-built renderer.


Stimulus Images

Click on an image for the corresponding VPL count vs. clamping level table.

Main Study

MS MR BS BR GS
 G0 
 G1 
 G2 
 G3 
indirect-only illumination
MS MR BS BR GS
 G0 
 G1 
 G2 
 G3 
direct-and-indirect illumination

Validation Study

MS MR BS BR GS
 Bunny 
 Dragon 
real-world geometry
MS MR
25M VPLs
G0 G1 G2 G3 Bunny Dragon
purely diffuse material

Acknowledgments

Many thanks to friends and colleagues who participated in our experiments. This work was supported by the European Commission Marie Curie Fellowship PIOF-GA-2008-221716, NSF CAREER 0644175, NSF CPA 0811680, and grants from Intel Corporation and Microsoft Corporation. The Bunny and Dragon models are courtesy of the Stanford Computer Graphics Laboratory.