My main interest is in computer graphics, especially ray tracing and global illumination. But I have also done research in computer vision and VLSI design.
I'm a principal scientist and software developer, part of Pixar's team in Seattle. We're developing a CPU+GPU version of Pixar's RenderMan renderer. RenderMan is used for rendering CG movies such as Toy Story, Cars, Coco, and Tintin and for special effects in movies such as Terminator 2, Jurassic Park, the newer Star Wars movies, Harry Potter, Lord of the Rings, Avatar, Lion King, and many more.
One of the interesting research topics for RenderMan is efficient computation of ray tracing and global illumination in extremely complex scenes. These days we're focused on path tracing. We're fortunate that the algorithms and hardware have progressed far enough that (noisy) preview images can be rendered at interactive rates. An overview of the inner workings of RenderMan can be found in our TOG paper RenderMan: An Advanced Path Tracing Architecture for Movie Rendering. We are currently working on an XPU version of RenderMan, running on CPUs, GPUs, or both.
I've done some research on well-stratified sample sequences with fast convergence for progressive rendering and adaptive sampling. A rather nerdy, but surprisingly interesting and multifaceted topic. Earlier, we introduced a multiresolution geometry cache for efficient ray tracing in scenes with very complex geometry. We've also introduced the brick map format, an efficient, tiled 3D MIP map representation of surface textures (and volume data) that does not require surface parameterization. Among their many uses, brick maps can be used as geometric primitives with built-in level-of-detail. I've also done some work on point-based computations such as approximate ambient occlusion, color bleeding, and subsurface scattering. The goal of all this was to make global illumination a useful tool for daily use in movie production.
Before Pixar, I worked in the R&D department at Square USA in Honolulu. Square USA created the Final Fantasy movie. I was mainly working on developing and optimizing the global illumination parts of the massively parallel in-house renderer Kilauea. My work focused on efficient simulation of global illumination in extremely complex scenes using a fully parallel and distributed version of the photon map method.
Prior to that, I worked at Mental Images in Berlin. Mental Images is now a part of Nvidia. Back then, the main product was the ray tracer Mental Ray (used by IBM/Dassault Systemes, Softimage, Alias, and others). My research included efficient global illumination of complex scenes and participating media using the photon map method developed by Henrik Wann Jensen (now chief scientist at Luxion, and emeritus professor at UC San Diego). We proved that the photon map method can be used in a production-strength commercial renderer. We also extended the photon map method to handle participating media by introducing a volume photon map. The resulting method is fast and simple, but nevertheless general enough to handle nonhomogeneous media and anisotropic scattering. The method can efficiently simulate effects such as multiple volume scattering, color bleeding between volumes and surfaces, and volume caustics. I also developed a shader interface for non-photorealistic contour rendering.
I am fascinated by the use of importance to make rendering more efficient. Here's a separate web page about importance in rendering.
At the University of Washington I also did research in global illumination, but using finite element methods. Most of my work was extending methods known from diffuse global illumination ("radiosity") to general global illumination. We showed how the ideas of importance-based transport and refinement, wavelet analysis, and clustering can be combined to provide an efficient solution to the radiance transport problem. Importance is used to focus the computation on the interactions having the greatest impact on the final visible solution. Wavelets are used to provide an efficient method for representing radiance, importance, and the transport operator itself. Clustering enables us to simplify light and importance transport between distant objects. My two advisors were David Salesin (now at Google) and Tony Derose (recently retired from Pixar). In addition, I was fortunate to work with Eric Stollnitz (then a grad student at the Department of Applied Mathematics, now at Adobe) and Dani Lischinski (who had a postdoc position at the time, now at the Hebrew University in Jerusalem). If you're interested in current graphics research at University of Washington, click here.
I also worked with computer vision at the University of Washington -- mostly color photometric stereo. It was an extension of Woodham's photometric stereo method, and utilized the different colors of specular and diffuse reflection to further constrain the direction of the normals. This gave higher accuracy in the determined shape. My advisor for that work was Linda Shapiro.
I wrote my first ray tracer while studying for an engineering degree at the Technical University of Denmark, with Niels Jørgen Christensen as advisor.
In a previous life, I did some research with Henrik Hulgaard on automated synthesis of asynchronous, delay-insensitive VLSI chips. This research was done at the Technical University of Denmark with Jørgen Staunstrup as advisor. Delay-insensitive circuits work without a clock and regardless of delays on wires. This has the advantage that a result can be used as soon as it is computed (rather than having to wait until the next clock cycle), and that the large areas that are occupied by clock distribution on traditional chips is eliminated. The downside is that more signal wires are necessary. This seemed like a promising idea 20+ years ago, and still does today. Our contribution was a translator from a software description of an algorithm to a design implementing the algorithm as a delay-insensitive VLSI chip.
I've been on the Eurographics Rendering Workshop/Symposium program committee nearly every year from 1999 and was co-chair with Daniel Cohen-Or in 2003. I was on the program committee for the IEEE Symposia on Interactive Ray Tracing (now merged with High Performance Graphics) and was co-chair with Alexander Keller for the 2007 symposium. I was program co-chair for the DigiPro Conference in 2023 and 2024. I've been on the SIGGRAPH papers committee as well. I regularly review articles for the SIGGRAPH, Eurographics, Graphics Interface, and Interactive 3D conferences, for the Eurographics Rendering Workshops/Symposia, and for ACM Transactions on Graphics, Computer Graphics Forum, Computers & Graphics, IEEE Transactions on Visualization and Computer Graphics, and the Journal of Computer Graphics Tools. I'm a senior member of ACM and a pioneer member of SIGGRAPH.
I've had the honor of serving on the Ph.D. committee of three outstanding researchers: Frank Suykens (KU Leuven, 2002), Wojciech Jarosz (UC San Diego, 2008), and Martin Sik (Charles University, Prague, 2019). I've also been judging the graphics course rendering competitions at Stanford (2007), UC San Diego (2007), and ETH Zurich (2013 and 2020).
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