Computational Aesthetics in Graphics, Visualization and Imaging (2005)L. Neumann, M. Sbert, B. Gooch, W. Purgathofer (Editors)Computer Aided Color Appearance Designusing Environment Map Based LightingC. Shimizu and G.W. MeyerDepartment of Computer Science and Engineering and Digital Technology Center, University of MinnesotaAbstractA BRDF approximation is developed that is suitable for interactive color appearance design in direct lightingprovided by environment maps. The BRDFs are approximated as a linear combination of cosine lobes with a fixedset of specular exponents. A nonlinear optimization routine is used to fit the cosine lobes to BRDFs appropriatefor a specific color appearance design application: automotive paint. Modification and rendering of the BRDF ismade possible by linearly combining prefiltered environment maps for each cosine lobe in real time.Categories and Subject Descriptors (according to ACM CCS) : I.3.7 [Computer Graphics]: Three-DimensionalGraphics and Realism-Color, shading, shadowing, and texture1. IntroductionThe recent development of pixel shading hardware and soft-ware has dramatically increased the control that a user hasover the color appearance of a computer graphic object.Rather than being limited to a single simple hardwired re-flectionmodel,theshadingmodelappropriateforthedesiredcolor appearance can be programmed, and it will automati-cally be evaluated at each pixel on the object’s surface. Inaddition, it is possible to change the parameters of this re-flection model in real time. The ability to interactively ad-just the simulation of light reflection from a surface makesit possible to design the color appearance of an object. If thereflection model accurately simulates a real paint or surfacecoating then a new type of CAD, computer aided color ap-pearance design or CACAD, becomes possible [Mey].The evaluation of an object’s color appearance is im-proved by making the lighting as realistic as possible. Cor-rectly simulating the direct illumination provided by dis-tant area light sources, including the sky, can have a majorimpact on an object’s overall color appearance. This morerealistic lighting can be introduced, while still maintaininginteractive frame rates, by using pre-filtered environmentmaps [KM] or spherical harmonics [KSS]. Although it in-volves a time-consuming offline computation for each set ofreflection model parameters, the extra overhead isn’t a prob-lem for many computer graphic applications because the re-flection model doesn’t change during the simulation. How-ever, it presents a major obstacle for the interactive CACADprogram described above.The goal of the research described in this paper is to de-velop a CACAD program that combines the interactive con-trol of a reflection model, implemented as a pixel shader,with the complex real-time lighting provided by an environ-ment map. The approach begins with the assumption that thefinal tool is intended to solve a specific problem: the designof new automotive finishes [SMW03]. This leads to the se-lectionofareflectionmodelappropriateforautomotivepaintin which just a few BRDF measurements are taken and a loworderpolynomialisusedtointerpolatethedata[WM].Giventhese important but practical restrictions, it is assumed thatthe space of bi-directional reflection distribution functions(BRDFs) generated by the automotive paint reflection modelcan be spanned by a set of basis BRDFs. If a basis BRDFcan be rendered with global illumination interactively, thenit is possible to render images of the combination of basisBRDFs interactively. If it is possible to rapidly compute howto combine the basis BRDFs to approximate a target BRDF,the target BRDF can be continually changed while generat-ing images of it.1.1. BRDF Design SuiteAutomotive styling is an important aesthetic design prob-lem. Because of the sculptural nature of vehicles and thec ? The Eurographics Association 2005.
