Greg Nichols :: research

research

Ph.D. Thesis:

Multiresolution Image-Space Rendering for
Interactive Global Illumination
Greg Nichols
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Abstract: Global illumination adds tremendous visual richness to rendered images. Unfortunately, such illumination proves quite costly to compute, and is therefore often coarsely approximated by interactive applications, or simply omitted altogether. Global illumination is often quite low-frequency, aside from sharp changes at discontinuities. This thesis describes three novel multiresolution image-space methods that exploit this characteristic to accelerate rendering speeds. These techniques run completely on the GPU at interactive rates and require no precomputation, allowing fully dynamic lighting, geometry, and camera.
The first approach, multiresolution splatting, is a novel multiresolution method for rendering indirect illumination. This work extends reflective shadow maps, an image space method that splats contributions from secondary light sources into eye-space. Splats are refined into multiresolution patches, rendering indirect contributions at low resolution where lighting changes slowly and at high resolution near discontinuities; this greatly reduces GPU fill rate and enhances performance.
The second method, image space radiosity, significantly improves the performance of multiresolution splatting, introducing an efficient stencil-based parallel refinement technique. This method also adapts ideas from object-space hierarchical radiosity methods to image space, introducing two adaptive sampling methods that allow much finer sampling of the reflective shadow map where needed. These modifications significantly improve temporal coherence while maintaining performance.
The third approach adapts these techniques to accelerate the rendering of direct illumination from large area light sources. Visibility is computed using a coarse screen-space voxelization technique, allowing binary visibility queries using ray marching. This work also proposes a new incremental refinement method that considers both illumination and visibility variations. Both diffuse and non-diffuse surfaces are supported, and illumination can vary over the surface of the light, enabling dynamic content such as video screens.
Ph.D. Thesis, University of Iowa, July 2010
Downloads: [PDF, 37MB]

Conference, Journal, and Book Publications:

Sorted Deferred Shading for Production Path Tracing
Christian Eisenacher, Gregory Nichols, Andrew Selle, Brent Burley
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Abstract: Ray-traced global illumination is becoming widespread. However, incoherent ray traversal and shading has traditionally limited ray tracing to scenes that fit in memory. To combat these issues, we introduce a sorting strategy for large, potentially out-of-core ray batches, and we sort and defer shading of ray hits. As a result, we achieve perfectly coherent shading and texture access, removing the need for a shading cache.
Accepted to the Eurographics Symposium on Rendering, June 2013
Downloads: [Paper PDF, 7.3MB]
Interactive, Multiresolution Image-Space Rendering for Dynamic Area Lighting
Greg Nichols, Rajeev Penmatsa, and Chris Wyman
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Abstract: Area lights add tremendous realism, but rendering them interactively proves challenging. Integrating visibility is costly, even with current shadowing techniques, and existing methods frequently ignore illumination variations at unoccluded points due to changing radiance over the light's surface. We extend recent image-space work that reduces costs by gathering illumination in a multiresolution fashion, rendering varying frequencies at corresponding resolutions. To compute visibility, we eschew shadow maps and instead rely on a coarse screen-space voxelization, which effectively provides a cheap layered depth image for binary visibility queries via ray marching. Our technique requires no precomputation and runs at interactive rates, allowing scenes with large area lights, including dynamic content such as video screens.
Accepted to the Eurographics Symposium on Rendering, June 2010
Downloads: [Preprint PDF, 7.3MB] [Additional Discussion, 1MB]
[DivX AVI, 96MB]
Interactive Indirect Illumination Using Adaptive Multiresolution Splatting
Greg Nichols and Chris Wyman
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Abstract: Global illumination provides a visual richness not achievable with the direct illumination models used by most interactive applications. To generate global effects, numerous approximations attempt to reduce global illumination costs to levels feasible in interactive contexts. One such approximation, reflective shadow maps, samples a shadow map to identify secondary light sources whose contributions are splatted into eye-space. This splatting introduces significant overdraw that is usually reduced by artificially shrinking each splat's radius of influence. This paper introduces a new, multi-resolution approach for interactively splatting indirect illumination. Instead of reducing GPU fill rate by reducing splat size, we reduce fill rate by rendering splats into a multi-resolution buffer. This takes advantage of the low-frequency nature of diffuse and glossy indirect lighting, allowing rendering of indirect contributions at low resolution where lighting changes slowly and at high resolution near discontinuities. Because this multi-resolution rendering occurs on a per-splat basis, we can significantly reduce fill rate without arbitrarily clipping splat contributions below a given threshold - those regions simply are rendered at a coarse resolution.
IEEE Transactions on Visualization and Computer Graphics, in press.
Downloads: [Preprint PDF, 22.5MB]
Fast, Stencil-Based Multiresolution Splatting for Indirect Illumination
Chris Wyman, Jeremy Shopf, and Greg Nichols.
GPU Pro: Advanced Rendering Techniques, pg. 199-214. Edited by Wolfgang Engel. AK Peters, 2010. ISBN: 978-1-56881-472-8.
Hierarchical Image Space Radiosity for Interactive Global Illumination
Greg Nichols, Jeremy Shopf, and Chris Wyman
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Abstract: We introduce image-space radiosity and a hierarchical variant as a method for interactively approximating diffuse indirect illumination in fully dynamic scenes. As oft observed, diffuse indirect illumination contains mainly low-frequency details that do not require independent computations at every pixel. Prior work leverages this to reduce computation costs by clustering and caching samples in world or object space. This often involves scene preprocessing, complex data structures for caching, or wasted computations outside the view frustum. We instead propose clustering computations in image space, allowing the use of cheap hardware mipmapping and implicit quadtrees to allow coarser illumination computations. We build on a recently introduced multiresolution splatting technique combined with an image-space lightcut algorithm to intelligently choose virtual point lights for an interactive, one-bounce instant radiosity solution. Intelligently selecting point lights from our reective shadow map enables temporally coherent illumination similar to results using more than 4096 regularly-sampled VPLs.
Presented at the Eurographics Symposium on Rendering, June 2009
Downloads: [Paper PDF, 3.6MB] [DivX AVI, 36MB]
Multiresolution Splatting for Indirect Illumination
Greg Nichols and Chris Wyman
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Abstract: Global illumination provides a visual richness not achievable with the direct illumination models used by most interactive applications. To generate global effects, numerous approximations attempt to reduce global illumination costs to levels feasible in interactive contexts. One such approximation, reective shadow maps, samples a shadow map to identify secondary light sources whose contributions are splatted into eye-space. This splatting introduces significant overdraw that is usually reduced by artificially shrinking each splat's radius of inuence. This paper introduces a new, multiresolution approach for interactively splatting indirect illumination. Instead of reducing GPU fill rate by reducing splat size, we reduce fill rate by rendering splats into a multi-resolution buffer. This takes advantage of the low-frequency nature of diffuse and glossy indirect lighting, allowing rendering of indirect contributions at low resolution where lighting changes slowly and at high resolution near discontinuities. Because this multi-resolution rendering occurs on a per-splat basis, we can signicantly reduce fill rate without arbitrarily clipping splat contributions below a given threshold - those regions simply are rendered at a coarse resolution.
Presented at the ACM Symposium on Interactive 3D Graphics and Games, February 2009
Awards: Best Paper Finalist (one of three)
Downloads: [Paper PDF, 10.2MB] [XviD AVI, 33.5MB]
Adaptive Caustic Maps Using Deferred Shading
Chris Wyman and Greg Nichols
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Abstract: Caustic maps provide an interactive image-space method to render caustics, the focusing of light via reection and refraction. Unfortunately, caustic mapping suffers problems similar to shadow mapping: aliasing from poor sampling and map projection as well as temporal incoherency from frame-to-frame sampling variations. To reduce these problems, researchers have suggested methods ranging from caustic blurring to building a multiresolution caustic map. Yet these all require a xed photon sampling, precluding the use of importance-based photon densities. This paper introduces adaptive caustic maps. Instead of densely sampling photons via a rasterization pass, we adaptively emit photons using a deferred shading pass. We describe deferred rendering for refractive surfaces, which speeds rendering of refractive geometry up to 25% and with adaptive sampling speeds caustic rendering up to 200%. These benets are particularly noticable for complex geometry or using millions of photons. While developed for a GPU rasterizer, adaptive caustic map creation can be performed by any renderer that individually traces photons, e.g., a GPU ray tracer.
Computer Graphics Forum 28(2), 309-318, April 2009
Downloads: [Paper PDF, 2MB]

Posters:

Direct Illumination from Dynamic Area Lights With Visibility
Greg Nichols, Rajeev Penmatsa, and Chris Wyman
Poster presented at the ACM Symposium on Interactive 3D Graphics and Games, February 2010
Downloads: [Abstract PDF, 100 KB]
Voxel-Space Ambient Occlusion
Rajeev Penmatsa, Greg Nichols, and Chris Wyman
Poster presented at the ACM Symposium on Interactive 3D Graphics and Games, February 2010
Downloads: [Abstract PDF, 100 KB]
Direct Illumination from Dynamic Area Lights
Greg Nichols and Chris Wyman
Poster presented at SIGGRAPH, August 2009.
Awards: 2009 SIGGRAPH Student Research Competition Finalist
Downloads: [Abstract PDF, 240 KB] [DivX AVI 31MB]
Exploring Volume Rendering With Path Tracing
Scott Davis, Xiaoqian Jiang, Greg Nichols, and James Cremer
Poster presented at SIGGRAPH 2005
Downloads: [Abstract PDF, 78KB]

Page last modified on March 12, 2018