EUROGRAPHICS 2010 / H. P. A. Lensch and S. Seipel Short Paper Adapting Precomputed Radiance Transfer to Real-time Spectral Rendering Karsten Schwenk 1 , Tobias Franke 1 , Timm Drevensek 1 , Arjan Kuijper 1,2 , Ulrich Bockholt 1,2 and Dieter W. Fellner 1,2 1 Fraunhofer IGD, Germany 2 TU Darmstadt, Germany Abstract Spectral rendering takes the full visible spectrum into account when calculating light-surface interaction and can overcome the well-known deficiencies of rendering with tristimulus color models. We present a variant of the pre- computed radiance transfer algorithm that is tailored towards real-time spectral rendering on modern graphics hardware. Our method renders diffuse, self-shadowing objects with spatially varying spectral reflectance prop- erties under distant, dynamic, full-spectral illumination. To achieve real-time frame rates and practical memory requirements we split the light transfer function into an achromatic part that varies per vertex and a wavelength- dependent part that represents a spectral albedo texture map. As an additional optimization, we project reflectance and illuminant spectra into an orthonormal basis. One area of application for our research is virtual design ap- plications that require relighting objects with high color fidelity at interactive frame rates. Categories and Subject Descriptors (according to ACM CCS): I.3.7 [Computer Graphics]: Three-Dimensional Graphics and Realism—Color, shading, shadowing, and texture 1. Introduction Spectral rendering, i.e. lighting calculations that take the full visible spectrum into account, can be used to overcome the well-known deficiencies of rendering with tristimulus color models when it comes to accurate color reproduc- tion [RP98]. Most work on the topic has been carried out with offline renderers in mind, but the larger accuracy of spectral rendering can also improve the results of real-time rendering approaches based on hardware-accelerated raster- ization. In this paper, we adapt the well-known precomputed radi- ance transfer (PRT) algorithm [SKS02] to real-time spectral rendering. Existing real-time spectral rendering methods as- sume a simple lighting environment, usually consisting only of a few directional-, point-, or spotlights. Established PRT methods, on the other hand, are able to handle complex light- ing environments in real-time, but carry out their calcula- tions in a RGB color space, which negatively affects color reproduction. Our approach combines both worlds and ren- ders diffuse, self-shadowing objects with spatially varying spectral reflectance properties under distant, dynamic, full- spectral illumination. The key idea of our method is to project the data into two orthonormal bases, one for the spectral domain and one for the directional domain. With this compression scheme, the spectral reflection calculation can be approximated by two dot products of coefficient vectors. This computation can be carried out efficiently in shader programs on programmable graphics hardware. As an optimization, we split the light transfer function into an achromatic part that varies per ver- tex and a wavelength-dependent part that represents a spec- tral albedo texture map. This ensures real-time frame rates and moderate memory requirements, but limits us to diffuse objects with self-shadowing (i.e. no interreflections). The primary motivation for our work is to improve color correctness in the rendering pipeline of interactive virtual de- sign applications. Such systems often run on desktop graph- ics hardware or even on mobile devices. Rendering tech- niques for these applications require high color fidelity as well as interactive frame rates. We show that our method can significantly improve color reproduction in rendered images when compared to RGB rendering. A performance penalty arises from storing and processing spectral data, but it is tol- erable for most applications. c  The Eurographics Association 2010.