Chris Wyman and Shaun Ramsey. "Interactive Volumetric Shadows in Participating Media with Single-Scattering."
Tech Report UICS-08-01, Computer Science Department, University of Iowa. (under submission)
Technical Report: [ pdf (18 MB) ] (updated text available upon request)
Videos: [ Bouncing Ball (18.5 MB), Backlit Bunny (13.4 MB), Yeah Right (25.5 MB) ]

Abstract:
Scattering effects arising from participating media, such as smoke, haze, and fog, dramatically add to perceived realism in renderings. As shadows affect illumination throughout an environment, they significantly diminish scattering effects in umbral regions. Unlike surface shadowing, accurate volumetric shadows require simultaneously integrating illumination, scattering, and attenuation throughout the volume, which proves challenging for interactive applications. We propose a method for rendering volumetric shadows in homogeneous single scattering media that combines shadow volume and ray marching techniques, which eliminates performance deficiencies inherent in both. We extend this approach to interactively render shadows from textured lights and show results under two scattering models.

(Note: The "YeahRight" model is from Keenan Crane's model repository, and the stained-glass images for colored lights used courtesy of Light Romance Studios.)

Chris Wyman. "Hierarchical Caustic Maps."
Proceedings of ACM Symposium on Interactive 3D Graphics and Games, 163-171. (February 2008)
Paper: [ PDF (21.1 MB) ], Talk: [ PDF (2.4 MB) ]
Videos: [ Beethoven, Dragon, Bunny, Cow (DivX, 30 to 40 MB each)]

Abstract:
Interactive applications typically rely on local models for lighting, occasionally augmented by GPU-friendly methods for approximating global illumination. Caustic mapping approximates the specular focusing of light using a light-space image, akin to a shadow map, which is projected onto the scene during final rendering. Unfortunately, existing caustic map implementations must choose between quality and speed. Quickly generated maps use few photons and look extremely blurry, while sharper maps created from millions of photons only render at a few frames per second. This paper introduces a number of hierarchical enhancements to caustic mapping that allow real-time rendering with high quality caustic maps, even when using maps from multiple light sources. These techniques utilize the geometry processing stage of recent GPUs to avoid processing every photon and to render a pyramidal caustic map that allows photon splats of varying diameters without the increased costs inherent in large splats.

Qi Mo, Voicu Popescu, and Chris Wyman. "The Soft Shadow Occlusion Camera."
Proceedings of Pacific Graphics 2007, 189-198. (October 2007)
Paper Preprint: [ pdf (18.6 MB) ]
Sample Videos: [ DivX (4.4 MB), DivX (2.4 MB), DivX (3.6 MB) ]

Preliminary version presented at MIDGRAPH 2006: [ poster PDF (52 kB)].

Abstract:
A fundamental challenge for existing shadow map based algorithms is dealing with partially illuminated surfaces. A conventional shadow map built with a pinhole camera only provides a binary visibility sorting of the scene, and this all-or-nothing approach to visibility does not capture penumbral regions. We present an interactive soft shadow algorithm based on a variant of the depth discontinuity occlusion camera, a non-pinhole camera with rays that reach around blockers to sample normally hidden surfaces. Our soft shadow occlusion camera (SSOC) classifies a fragment on a continuum from fully visible to fully hidden, as seen from the light. The SSOC is used directly in fragment illumination computation without building an explicit ``soft shadow map.'' This method renders plausible soft shadows at interactive speeds under fully dynamic conditions.

Scott Davis and Chris Wyman. "Interactive Refractions with Total Internal Reflection."
Proceedings of Graphics Interface, 185-190. (May 2007)
Paper: [ pdf (6.6 MB) ]
Video: [ DivX (4.9 MB) ]

Abstract:
A requirement for rendering realistic images interactively is efficiently simulating material properties. Recent techniques have improved the quality for interactively rendering dielectric materials, but have mostly neglected a phenomenon associated with refraction, namely, total internal reflection. We present an algorithm to approximate total internal reflection on commodity graphics hardware using a ray-depth map intersection technique that is interactive and requires no precomputation. Our results compare favorably with ray traced images and improve upon approaches that avoid total internal reflection.

Chris Wyman. "Interactive Refractions and Caustics Using Image-Space Techniques," in ShaderX5: Advanced Rendering Techniques, 359-371. Edited by Wolfgang Engel. Charles River Media, 2007. ISBN: 1-58450-499-4.

(University of Iowa Library, Call No: T385 .S38 2007)
(OCLC Accession No: 74459039)

Chris Wyman and Carsten Dachsbacher. "Improving Image-Space Caustics Via Variable-Sized Splatting."
Journal of Graphics Tools (to appear).
Technical Report: [ pdf (1.0 MB) ]
Sample Videos: [ DivX (1.7 MB), DivX (24.4 MB), DivX (12.2 MB) ]
Demo: [ Windows (21 MB), additional models (44 MB), README ]

Abstract:
Interactivity requires tradeoffs to achieve the right balance between rendering quality and speed. In practice, today's applications restrict lighting to mainly direct illumination, sometimes augmented by precomputed transfer techniques for diffuse global effects. Dynamic high-frequency specular effects, such as caustics, are largely lacking due to the high costs for recomputation each frame. Recent work has introduced a variety of related caustics approximations that interactively render light-space photons into a photon buffer, gather them into a caustic map, and project this map onto the scene similar to shadow mapping. While the process is simple and straightforward, the discretization of light into a finite number of uniformly-distributed photons leads to undersampling and aliasing artifacts. This paper examines two techniques for reducing these artifacts using varying sized photon splats. Conceptually, these are similar to the variable-radius $k$-nearest neighbor search used in photon mapping, allowing noise reduction in areas of low photon density while maintaining crisp caustics at focal points. Our techniques improve image quality at a modest cost that is significantly cheaper than supersampling the photon buffer.

Dave Edwards, Solomon Boulos, Jared Johnson, Peter Shirley, Michael Ashikhmin, Michael Stark, and Chris Wyman. "The Halfway Vector Disk for BRDF Modeling."
ACM Transactions on Graphics 25(1), 1-18. (January 2006)
Paper: [ pdf (479 kB) ]

Abstract:
We present a mathematical framework for enforcing energy conservation in a BRDF by specifying halfway vector distributions in simple two-dimensional domains. Energy-conserving BRDFs can produce plausible rendered images with accurate reflectance behavior, especially near grazing angles. Using our framework, we create an empirical BRDF that allows easy specification of diffuse, specular, and retroreflective materials. We also present a second BRDF model that is useful for data fitting; although it does not preserve energy, it uses the same halfway vector domain as the first model. We show that this data-fitting BRDF can be used to match measured data extremely well using only a small set of parameters. We believe that this is an improvement over table-based lookups and factored versions of BRDF data.

Chris Wyman and Scott Davis. "Interactive Image-Space Techniques for Approximating Caustics."
Proceedings of the ACM Symposium on Interactive 3D Graphics and Games, 153-160. (March 2006)
Paper: [ pdf (727 kB) ], Talk (draft): [ PPT ]
Sample Video : [ DivX (33.2 MB) ], Demo: [ Windows (12.4 MB), README ]

Abstract:
Interactive applications require simplifications to lighting, geometry, and material properties that preclude many effects encountered in the physical world. Until recently only the most simplistic reflections and refractions could be performed interactively, but state-of-the-art research has lifted some restrictions on such materials. This paper builds upon this work, but examines reflection and refraction from the light's viewpoint to achieve interactive caustics from point sources. Our technique emits photons from the light and stores the results in image-space, similar to a shadow map. We then examine various techniques for gathering these photons, comparing their advantages and disadvantages for rendering caustics. These approaches run interactively on modern GPUs, work in conjunction with existing techniques for rendering specular materials, and produce images competitive with offline renderings using comparable numbers of photons.

(See similar, independent work here, here, and here.)

Chris Wyman. "Interactive Image-Space Refraction of Nearby Geometry."
Proceedings of GRAPHITE, 205-211. (December 2005)
Paper: [ pdf (700 kB) ], Talk: [ pdf (1.1 MB) ]
Sample Video : [ DivX (29.5 MB) ]

Abstract:
Interactive applications often strive for realism, but framerate constraints usually limit realistic effects to those that run efficiently in graphics hardware. One effect largely ignored in interactive applications is refraction. We build upon a simple, image-space approach to refraction that easily runs on modern graphics cards. This image-space approach requires two passes on a GPU, and allows refraction of distant environments through two interfaces. Our works explores extensions allowing the refraction of nearby opaque objects, at the cost of one additional pass to render nearby geometry to texture and a more complex fragment shader for computing refracted color. Like all image-based algorithms, aliasing can occur in certain circumstances, especially when a few texels are magnified to cover a sizable portion of screen space. However, our plausible refractions should suffice for many applications.

Chris Wyman. "An Approximate Image-Space Approach for Interactive Refraction."
ACM Transactions on Graphics 24(3), 1050-1053. (August 2005)
Paper: [ pdf (6.0 MB) ], Shaders (CG): [ vertex, fragment ]
Demo: [ Windows (17.7 MB), README ], Talk: [ pdf (2.6 MB) ]
Sample Videos : [ DivX (10.0 MB), DivX (7.8 MB) , DivX (13.0 MB) ]

Abstract:
Many interactive applications strive for realistic renderings, but framerate constraints usually limit realism to effects that run efficiently in graphics hardware. One effect largely ignored in such applications is refraction. We introduce a simple, image-space approach to refractions that easily runs on modern graphics cards. Our method requires two passes on a GPU, and allows refraction of a distant environment through two interfaces, compared to current interactive techniques that are restricted to a single interface. Like all image-based algorithms, aliasing can occur in certain circumstances, but the plausible refractions generated with our approach should suffice for many applications.

Chris Wyman, Steven Parker, Peter Shirley, and Charles Hansen. "Interactive Display of Isosurfaces with Global Illumination."
IEEE Transactions on Visualization and Computer Graphics 12(2), 186-196. (March/April 2006)
Paper: [ PDF (3.6 MB) ]
Sample Video: [ MP4 (41.5 MB) ]

Abstract:
In many applications, volumetric datasets are examined by displaying isosurfaces, surfaces where data, or some function of the data, takes on a given value. Interactive applications typically use local lighting models to render such surfaces. This work introduces a method to precompute or lazily compute global illumination to improve interactive isosurface renderings. The precomputed illumination resides in a separate volume and includes direct light, shadows, and interreflections. Using this volume, interactive globally illuminated renderings of isosurfaces becomes feasible while still allowing dynamic manipulation of viewpoint and isovalue

Chris Wyman, Charles Hansen, and Peter Shirley. "Interactive Caustics Using Local Precomputed Irradiance."
Proceedings of the Pacific Conference on Computer Graphics and Applications, 143-151. (October 2004)
Paper: [ pdf (538 kB) ], Talk: [ pdf (480 kB) ]
Sample Video: [ DivX (6.5 MB) ]

Abstract:
Bright patterns of light focused via reflective or refractive objects onto matte surfaces are called ``caustics''. We present a method for rendering dynamic scenes with moving caustics at interactive rates. This technique requires some simplifying assumptions about caustic behavior allowing us to consider it a local spatial property which we sample in a pre-processing stage. Storing the caustic locally limits caustic rendering to a simple lookup. We examine a number of ways to represent this data, allowing us to trade between accuracy, storage, run time, and precomputation time.

Chris Wyman. "Fast Local Approximation to Global Illumination."
PhD Thesis, University of Utah, Salt Lake City, Utah. (August 2004) (OCLC Accession No: 56020105)
Dissertation: [ pdf (4.0 MB) ] Defense Talk: [ pdf (1.4 MB) ], SH Rotation Code: [ C++ Class ]

Abstract:
Interactive global illumination remains an elusive goal in rendering, as energy from every portion of the scene contributes to the final image. Integrating over a complex scene, with a polygon count in the millions or more, proves difficulty even for static techniques. Interactive with such complex environments while maintaining high quality rendering generally requires recomputing the paths of countless photons using a small number of CPUs.

This dissertation examines a simplified approach to interactive global illumination. Observing that local illumination computations can be performed interactively even on fairly simple graphics accelerators, a reduction of global illumination problems to local problems would allow interactive rendering. A number of techniques are suggested that simplify global illumination to specific global illumination effects (e.g., diffuse interreflection, soft shadows, and caustics), which can individually be sampled at a local level. Rendering these simplified global illumination effects reduces to a few lookups, which can easily be done at interactive rates. While some tradeoffs exist between rendering speed, rendering quality, and memory consumption, these techniques show that approximating global illumination locally allows interactivity while still maintaining significant realism.

Chris Wyman and Charles Hansen. "Penumbra Maps: Approximate Soft Shadows in Real-Time."
Proceedings of the Eurographics Symposium on Rendering, 202-207. (June 2003)
Paper: [ pdf (2.3 MB) ], Video: [ DivX 5 (14 MB) ], Talk: [ pdf (370 KB) ], Examples: [ Images ]

Abstract:
Generating soft shadows quickly is difficult. Few techniques have enough flexibility to interactively render soft shadows in scenes with arbitrarily complex occluders and receivers. This paper introduces the penumbra map, which extends current shadow map techniques to interactively approximate soft shadows. Using object silhouette edges, as seen from the center of an area light, a map is generated containing approximate penumbral regions. Rendering requires two lookups, one into each the penumbra and shadow maps. Penumbra maps allow arbitrary dynamic models to easily shadow themselves and other nearby complex objects with plausible penumbrae.

Chris Wyman, Charles Hansen, and Peter Shirley. "Interactive Raytraced Caustics."
Tech Report, School of Computing, University of Utah, UUCS-03-009.
Paper: [ pdf (33.8 MB), local copy (4.6 MB) ]

Abstract:
In computer graphics, bright patterns of light focused onto matte surfaces are called ``caustics''. We present a method for rendering dynamic scenes with moving caustics at interactive rates. This technique requires some simplifying assumptions about caustic behavior allowing us to consider it a local spatial property which we sample in a pre-processing stage. Storing the caustic locally limits caustic rendering to a simple lookup. We examine a number of ways to represent this data, allowing us to trade between accuracy, storage, run time, and precomputation time.