Henrik Wann Jensen Research Associate [Until Sep. 30, 2002] Computer Graphics Laboratory Computer Science Dept. Stanford University Gates Computer Science Building, 362B Stanford, CA 94305-4070 Phone: +1 650 725 3696 Fax: +1 650 723 0033 henrik-at-graphics.stanford.edu

My area of interest is computer graphics with a focus on realistic image synthesis in particular global illumination, rendering natural phenomena, and appearance modeling. Before joining UCSD in October 2002 I was a Research Associate in the graphics laboratory at Stanford University, and before that I was a Postdoctoral Associate in the graphics group at MIT. I received my PhD from the Technical University of Denmark.

	The simulation of all types of light scattering in a model is called global illumination. My PhD research addressed the simulation of global illumination including caustics by introducing the concept of photon mapping. The key features of the photon mapping algorithm are the use of photon tracing and the photon map. The photon map is decoupled from the scene geometry, and it can be used in models with millions of objects and complex materials. Photon mapping is a very practical technique capable of simulating color bleeding, caustics, participating media, subsurface scattering, and motion blur. Today, photon mapping is implemented in most high-end rendering software, and it is being used in architectural simulations, computer games, and movies. As an example there was a very nice caustics sequence (light focusing through a glass of whisky) in Final Fantasy. I have mostly been concerned with the caustics formed as light is focused through a glass of cognac.
	Rendering natural phenomena is a growing area of research in computer graphics. Recently, I worked on a physically based model of the night sky. The goal of this project was to simulate all important visual elements of the night sky (the moon, stars, the atmosphere, the Zodiacal light etc.) including their appearance to the eye (loss of color and blue shift). For this purpose we used multi-spectral rendering combined with an accurate simulation of light scattering in the atmosphere. Another recent project addresses the simulation and rendering of smoke and more recently we have extended this work to include fire.
	Appearance modeling is an exciting area of research in computer graphics. Understanding the incredibly complex and diverse appearance of materials is essential for rendering compelling images. At MIT I worked on projects where we wanted to understand and simulate the darkening of many wet materials. as well as the changes in the appearance of weathered stone. These projects used path tracing and photon mapping to simulate subsurface scattering. The simulation of subsurface scattering is necessary to capture the effect on the appearance due to changes in the internal structure and composition of the materials. Photon mapping is quite good at simulating subsurface scattering, but it becomes costly for highly scattering materials such as milk and skin. For these materials it is better to use a diffusion approximation. The diffusion approximation is much faster than tracing invidual photons, and it is simple enough that a BSSRDF can be formulated. The BSSRDF is particularly suited for translucent materials where the assumptions in the ubiquitous BRDF approximation break down. As an example, the BSSRDF was used to render the translucent marble bust that is shown here. Note how the BSSRDF captures the the soft and smooth appearance as well as the light diffusing through the marble - something that a BRDF cannot simulate.