Graphics Research - Abhijeet Ghosh

BRDF Acquisition with Basis Illumination

Realistic descriptions of surface reflectance have long been a topic of interest in both computer vision and computer graphics research. In this work, we describe a novel and fast approach for the acquisition of bidirectional reflectance distribution functions (BRDFs). We develop a novel theory for directly measuring BRDFs in a basis representation by projecting incident light as a sequence of basis functions from a spherical zone of directions. We derive an orthonormal basis over spherical zones that is ideally suited for this task. BRDF values outside the zonal directions are extrapolated by re-projecting the zonal measurements into a spherical harmonics basis, or by fitting analytical reflection models to the data. For specular materials, we experiment with alternative basis acquisition approaches such as measuring the response to basis defined by an anlytical model as a way of optically fitting the BRDF to such a representation. We verify this approach with a compact optical setup that requires no moving parts and only a small number of image measurements. Using this approach, a BRDF can be measured in just a few minutes.

Correlated Visibility Sampling for Direct Illumination

State-of-the-art importance sampling strategies for direct illumination take into account the importance of the incident illumination, as well as the surface BRDF. Hence, these techniques achieve low variance in unoccluded regions. However, the resulting images still have noise in partially occluded regions as these techniques do not take visibility into account during the sampling process. We introduce the notion of correlated visibility sampling, which considers visibility in partially occluded regions during the sampling process, thereby improving the quality of the shadowed regions. We aim to draw samples in the partially occluded regions according to the triple product of the incident illumination, BRDF and visibility using Monte Carlo sampling followed by Metropolis sampling.

Sequential Sampling for Dynamic Environment Map Illumination

Sampling complex illumination in the form of environment maps has received a lot of attention in computer graphics. Recent work in this area has demonstrated that drawing samples from the product of light and BRDF produces superior results to other sampling strategies. However, existing methods in this area consider only individual frames, and do not take advantage of coherence in animations. In this paper, we introduce a sequential sampling approach for dynamic environment map illumination. Our algorithm efficiently samples from the product of illumination and BRDF, while exploiting temporal coherence. We demonstrate significant performance benefits over existing methods.

Bidirectional Importance Sampling for Direct Illumination

Image-based representations for illumination can capture complex real-world lighting that is difficult to represent in other forms. Current importance sampling strategies for image-based illumination have difficulties in cases where both the illumination and the surface BRDF contain important high-frequency detail, for example, when a specular surface is illuminated by an environment map containing small light sources. We introduce the notion of bidirectional importance sampling, in which samples are drawn from the product distribution of both the surface reflectance and the light source energy. While this approach makes the sample selection process more expensive, we drastically reduce the number of visibility tests required to obtain good image quality. As a consequence, we achieve significant quality improvements over previous sampling strategies for the same compute time.

Real Illumination from Virtual Environments

We introduce a method for actively controlling the illumination in a room so that it is consistent with a virtual world. In combination with a high dynamic range display, the system produces both uniform and directional illumination at intensity levels covering a wide range of real-world environments. It thereby allows natural adaptation processes of the human visual system to take place, for example when moving between bright and dark environments. In addition, the directional illumination provides additional information about the environment in the user s peripheral field of view. We describe both the hardware and the software aspects of our system. We also conducted an informal survey to determine whether users prefer the dynamic illumination over constant room illumination in an entertainment setting.

High Dynamic Range Display Systems

The dynamic range of many real-world environments exceeds the capabilities of current display technology by several orders of magnitude. In this paper we discuss the design of two different display systems that are capable of displaying images with a dynamic range much more similar to that encountered in the real world. The first display system is based on a combination of an LCD panel and a DLP projector, and can be built from off-the-shelf components. While this design is feasible in a lab setting, the second display system, which relies on a custom-built LED panel instead of the projector, is more suitable for usual office workspaces and commercial applications. We describe the design of both systems as well as the software issues that arise. We also discuss the advantages and disadvantages of the two designs and potential applications for both systems.

Simple Blurry Reflection with Environment Maps

We present a technique which uses existing OpenGL capabilities to approximate the effect of blurry specular reflections and indirect diffuse illumination. It makes use of environment maps, mipmapping with level of detail control, and possibly texture borders. The method is extremely simple to implement, in some cases requiring just a single additional OpenGL statement.