ACM Reference Format Sun, X., Zhou, K., Chen, Y., Lin, S., Shi, J., Guo, B. 2007. Interactive Relighting with Dynamic BRDFs. ACM Trans. Graph. 26, 3, Article 27 (July 2007), 10 pages. DOI = 10.1145/1239451.1239478 http://doi.acm. org/10.1145/1239451.1239478. Copyright Notice Permission to make digital or hard copies of part or all of this work for personal or classroom use is granted without fee provided that copies are not made or distributed for profit or direct commercial advantage and that copies show this notice on the first page or initial screen of a display along with the full citation. Copyrights for components of this work owned by others than ACM must be honored. Abstracting with credit is permitted. To copy otherwise, to republish, to post on servers, to redistribute to lists, or to use any component of this work in other works requires prior specific permission and/or a fee. Permissions may be requested from Publications Dept., ACM, Inc., 2 Penn Plaza, Suite 701, New York, NY 10121-0701, fax +1 (212) 869-0481, or permissions@acm.org. © 2007 ACM 0730-0301/2007/03-ART27 $5.00 DOI 10.1145/1239451.1239478 http://doi.acm.org/10.1145/1239451.1239478 Interactive Relighting with Dynamic BRDFs Xin Sun ∗ Kun Zhou † Yanyun Chen † Stephen Lin † Jiaoying Shi ∗ Baining Guo † ∗ State Key Lab of CAD&CG, Zhejiang University † Microsoft Research Asia Figure 1: Image sequence of Venus rendered with a dynamic BRDF. Ground truth renderings of the corresponding materials are shown on spheres at the bottom-right of the images. The changes in Venus’ reflectance properties produce corresponding indirect lighting effects on the surrounding walls. Here, precomputation of 12.3 hours with 389 Mbytes memory is used for the 39.7K vertices. The rendering performance is 2.62 fps whenthe viewpoint, BRDF, and lighting are all simultaneously changing; 10.94 fps when only the viewpoint and BRDF are dynamic; and 57.09 fps when only the viewpoint changes. Abstract We present a technique for interactive relighting in which source radiance, viewing direction, and BRDFs can all be changed on the fly. In handling dynamic BRDFs, our method efficiently accounts for the effects of BRDF modification on the reflectance and inci- dent radiance at a surface point. For reflectance, we develop a BRDF tensor representation that can be factorized into adjustable terms for lighting, viewing, and BRDF parameters. For incident radiance, there exists a non-linear relationship between indirect lighting and BRDFs in a scene, which makes linear light transport frameworks such as PRT unsuitable. To overcome this problem, we introduce precomputed transfer tensors (PTTs) which decom- pose indirect lighting into precomputable components that are each a function of BRDFs in the scene, and can be rapidly combined at run time to correctly determine incident radiance. We additionally describe a method for efficient handling of high-frequency specular reflections by separating them from the BRDF tensor representa- tion and processing them using precomputed visibility information. With relighting based on PTTs, interactive performance with in- direct lighting is demonstrated in applications to BRDF animation and material tuning. Keywords: precomputed radiance transfer, bidirectional re- flectance distribution function, relighting, global illumination, ten- sor factorization † e-mail: {kunzhou,stevelin,bainguo}@microsoft.com 1 Introduction The visual quality of a computer-generated scene can be apprecia- bly enhanced by simulating the natural propagation of light. Before reaching the viewer, light emitted from illumination sources may undergo a number of physical interactions in a scene, giving rise to rich visual effects such as soft shadows and interreflections. Ac- counting for the myriad paths of light through a scene and their complex interactions with objects, however, involves a substantial expense in computation. For interactive relighting, much of this global illumination com- putation may be performed in advance. Although arbitrary envi- ronment lighting is possible at run time, the exit radiance at a sur- face point may be efficiently evaluated using precomputed radiance transfers (PRT) [Sloan et al. 2002]. Different transfer functions are precomputed for different levels of reflectance effects. For view- independent diffuse reflections, source lighting is transformed by a transfer vector into exit radiance from a surface point. For view- dependent glossy reflections, a transfer matrix is needed to con- vert source illumination into a distribution of exit radiance that can be evaluated in the direction of the viewer. These radiance trans- fer functions are precomputed with respect to given BRDFs in the scene, such that these material properties are essentially baked into the transfer mechanism and cannot be changed at run time. In this work, we introduce an interactive relighting technique that more generally accommodates changes in reflectance in addition to lighting conditions and viewing directions. The consideration of dynamic BRDFs with indirect lighting brings special challenges. The BRDFs at viewed surface points need to be efficiently evalu- ated with respect to changing incident light distributions, viewing directions, and reflectance parameters. For this, we present a BRDF tensor representation that can be factorized into adjustable compo- nents for each of these dynamic quantities. A more challenging issue is that incoming radiance at a surface point has a non-linear relationship to BRDFs in a scene [Forsyth and Zisserman 1991]. This non-linearity arises from the fact that distinct light rays may interact with a surface different numbers of times, such that they become transformed dissimilarly by a sur- ACM Transactions on Graphics, Vol. 26, No. 3, Article 27, Publication date: July 2007.