Research Projects

Microfacet Refraction

This work extends microfacet theory (eg, Cook-Torrance models) to handle refraction through rough surfaces such as etched glass globe shown. We also discuss the Smith shadowing-masking approximation for general microfacet distributions and good importance sampling strategies (which is essential when simulating transmitted light). Published at EGSR 2007. You can download the EGSR 2007 paper (pdf) here.

Multidimensional Lightcuts

This work extends the approach and concepts of Lightcuts to scalable rendering of complex effects such as motion blur, depth of field, and volume rendering. We achieve greater scalability by holistically considering complete pixel integrals and using an implicit hierarchy called the product graph.

You can download the SIGGRAPH 2006 paper (pdf) and a movie from the

The SIGGRAPH presentation slides are available in powerpoint or pdf formats, and a separate archive of clips on the movies page. (Note: the powerpoint file was created on a Macintosh computer and some of the graphs do not seem to display properly under Windows.)

Lightcuts: A Scalable Approach to Illumination

Computing the illumination from complex sources such as area lights, HDR environment maps, and indirect, can be very expensive. Lightcuts introduces a new scalable algorithm for computing the illumination from thousands or millions of point lights. Using a perceptual metric, a binary light tree of clusters, and conservative per-cluster error bounds, we show that we can accurately compute the illumination from hundreds of point lights using only hundreds of shadow rays. A lightcut is cut through the global light tree that adaptively partitions the lights into clusters to locally control the cost vs. error tradeoff. We also introduce a related technique called reconstruction cuts that uses the lightcut framework to further reduce shading costs by intelligently interpolating illumination while preserving high frequencies details like shadow boundaries and glossy highlights.

You can download the SIGGRAPH 2005 paper (pdf) (Note: several figures were printed incorrectly in the proceedings but are correct in the PDF) and a movie from the

The SIGGRAPH presenation slides are available in powerpoint or pdf formats.

A related technical report discusses several important implementation issues for the Ward BRDF including the correct sampling weights that should be used when using it associated Monte Carlo sampling distribution.

Perceptual Illumination Components

Different components of global illumination have widely varying costs, but may not provide the same benefits to image quality. We performed a user study to examine this issue. We divided the illumination into several components based on the BRDF (indirect diffuse, indirect gloss, indirect specular), rendered images with different subsets of the illumination. We then had people rank the images and studied the results. What we found is the components had differing affects on user-ranking (though indirect diffuse was usually the most important), but that we could predict their importances quite accurately using a simple model and some simple image/material statistics. We think this can provide insight for creating more efficient rendering systems in the future.

Published in SIGGRAPH 2004. More information can be found on the first author's page.

Feature-Based Textures

By adding explicit features (eg, discontinuity edges) to a standard raster-based textures, we can increase image quality, especially during magnification, with only minimal extra storage. Features can be automatically extracted from vector format images, approximated for raster images, or hand annotated.

Published in EGSR 2004. More information can be found on this project page.

Combining Edges and Points

By combining edges (explicit discontinuities) and points (shading samples), we are able to reconstruct high quality images from relatively sparse data. By computing the exact locations of discontinuities, we are able to interpolate the sparse data without smoothing out these sharp features and we also show that knowing the subpixel location of the edges allows to compute good quality anti-aliasing almost for free and without supersampling. Our software implementation runs at interactive rates on the CPU and future versions should be able to exploit the GPU for even faster framerates.

Published SIGGRAPH 2003. More information can be found on this project page.

A GPU-based version was published at Graphics Interface 2006.

Render Cache for Interactive Rendering

During an interactive session it is not necessary (and perhaps not even desirable) to render every pixel of every frame. Especially when using more expensive rendering methods such as global illumination. In most cases, there is considerable coherence from one frame to the next which we can exploit to reduce our computational expense. Also, in an interactive session, small visual artifacts are acceptable as long as they are small, fixed quickly, and do not distract the user. In rendering methods such as ray tracing or global illlumination, the cost of renderering a pixel various greatly depending on various factors such as material, local geometry, and complexity of the local lighting. Thus rerendering every pixel would make the framerate either highly variable which is far more objectionable in an interactive session than having some minor visual artifacts would be.

The render cache enables interactive display by caching rendered results as colored 3D points, reprojecting them to estimate the current image, and automatically deciding which subset of pixels should be (re)rendered for future frames. Because the image generation and pixel (or ray) rendering happen asynchronously, we can guarantee a fast consistent framerate regardless of rendering cost (though renderering cost does affect visual quality). In our experiments we have found that we get very good visual quality when only one tenth the pixels are rendered per frame and acceptable quality when only one hundreth of the pixels are renderered per frame (the rest of the pixels are estimated by reprojecting older results).

This work was begun while I was a post-doc at the iMAGIS lab in Grenoble, France and continued at Cornell.
You can view a project page here (including a link to the Eurographics Rendering Workshop '99 paper ). (Powerpoint presentation)
A followup paper, Enhancing and Optimizing the Render Cache, appeared in EGRW 2002. (Powerpoint presentation)
A GPU-based version was published at Graphics Interface 2006.

There is also a web-downloadable binary so that you can try it out for yourself or download the C++ source code under GPL license and compile it yourself.

Using Perceptual Texture Masking for Efficient Image Synthesis

This work described a simple and fast way to precompute the perceptual masking ability of textures (stored as threshold elevation factors) and store them with the texture. This allows them to be used during interactive computations to allow higher error thresholds in textured regions with almost no runtime overhead.

Published at Eurographics 2002. You can get the paper here.

Density Estimation for Global Illumination

I investigated particle tracing and density estimation as a method to compute radiosity solutions on large environments. Traditional radiosity methods have been limited by their large memory requirements. The density estimation method uses much less memory because it can efficiently use secondary storage (e.g. disk) for most its storage needs. It can handle all transport paths (arbitrary BRDF) and it nicely decomposes the global illumination problem into three pieces: particle tracing, density estimation, and mesh decimation.

You can get my thesis here .
You can get our Transactions on Graphics 1997 paper here .
You can get our Eurographics Rendering Workshop '95 paper here .

Fitting Virtual Lights for Non-Diffuse Walkthroughs

The essence of this work is to find ways to encode non-diffuse illumination solutions for rapid display using standard 3D graphics interfaces (e.g. OpenGL) and hardware. We want to be able to use a sophisticated lighting algorithm such as global illumination with arbitrary material properties, but we also want to be able to display the results quickly enough for interactive walkthrough. Current realtime graphics interfaces and hardware only implemented a relatively primitive local shading model. Phong shading and perhaps some shadows are about the limit of their abilities. We investigate the us of these basic primitives as a set of "appearance basis" functions that can be used to approximate the appearance computed by some more accurate, complete, and general model.

You can get our SIGGRAPH '97 paper here .

Path-Buffer

The path-buffer is a modified z-buffer with two z values per pixel. The extra z value allows us to use the the path-buffer and scan conversion to trace rays in a path tracing style algorithm. This reformulated algorithm is designed to be easy to accelerate using scan conversion and z-buffer type hardware. More details can be found in:

"The Path-Buffer" - Technical Report PCG-95-4: Bruce Walter and Jed Lengyel

Here are pdf versions of the technical report and figures.

I implemented the path-buffer on the simulator for UNC's PixelFlow machine as proof of concept with the goal of interactive path tracing in simple environments. But then I got distracted by working on my dissertation and have not worked on the path buffer since.

Other projects

Iterative Adaptive Sampling

"Accurate Direct Illumination Using Iterative Adaptive Sampling" - IEEE TVCG June 2006: Michael Donikian, Bruce Walter, Kavita Bala, Sebastian Fernandez, and Donald P. Greenberg

Analyzing Ray Casting and Particle Tracing

Rays are used as a primitive in many different rendering algorithms, but unfortunately the costs of ray casting are not very well understood. In order to better understand the costs of the particle tracing phase of our density estimation radiosity algorithm, we are working on analyzing the costs of ray casting and the variance of particle tracing.

"Cost Analysis of a Monte Carlo Radiosity Algorithm" - Technical Report PCG-95-3: Bruce Walter and Peter Shirley

I don't have the source for the original report anymore, but here is a pdf version of an updated, though not quite finished, version of this technical report.

Send errors, corrections, or comments to "bjw AT graphics DOT cornell DOT edu"
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