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Precomputed Radiance Transfer Peter-Pike Sloan SDE Windows Graphics & Gaming Technologies Microsoft Corporation Peter-Pike Sloan SDE Windows Graphics & Gaming Technologies Microsoft Corporation

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Challenges in Rendering Generating realistic images interactively is hard Many dimensions of complexity Geometric complexity Material complexity Meso-scale complexity Lighting complexity Transport complexity Synergy This talk focuses on techniques that enable more lighting/transport complexity Generating realistic images interactively is hard Many dimensions of complexity Geometric complexity Material complexity Meso-scale complexity Lighting complexity Transport complexity Synergy This talk focuses on techniques that enable more lighting/transport complexity

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Material Complexity Models how light interacts with a surface Assume the structure of the material is below the visible scale Simple variation Twist maps Models how light interacts with a surface Assume the structure of the material is below the visible scale Simple variation Twist maps

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Meso-Scale Complexity Variations at a visible scale - not geometry Bump/Roughness maps Parallax Mapping/BTFs extreme examples of this Variations at a visible scale - not geometry Bump/Roughness maps Parallax Mapping/BTFs extreme examples of this

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Lighting Complexity What kind of lighting environment is an object in? Directional/point lights Directional + ambient Smooth (low frequency) lighting Completely general What kind of lighting environment is an object in? Directional/point lights Directional + ambient Smooth (low frequency) lighting Completely general

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Lighting Complexity What kind of lighting environment is an object in? Directional/point lights Directional + ambient Smooth (low frequency) lighting Completely general What kind of lighting environment is an object in? Directional/point lights Directional + ambient Smooth (low frequency) lighting Completely general

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Lighting Complexity What kind of lighting environment is an object in? Directional/point lights Directional + ambient Smooth (low frequency) lighting Completely general What kind of lighting environment is an object in? Directional/point lights Directional + ambient Smooth (low frequency) lighting Completely general

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Lighting Complexity What kind of lighting environment is an object in? Directional/point lights Directional + ambient Smooth (low frequency) lighting Completely general What kind of lighting environment is an object in? Directional/point lights Directional + ambient Smooth (low frequency) lighting Completely general

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Transport Complexity How light interacts with objects/scene at a visible scale Shadows Inter-reflections Caustics Translucency (subsurface scattering) How light interacts with objects/scene at a visible scale Shadows Inter-reflections Caustics Translucency (subsurface scattering)

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Transport Complexity How light interacts with objects/scene at a visible scale Shadows Inter-reflections Caustics Translucency (subsurface scattering) How light interacts with objects/scene at a visible scale Shadows Inter-reflections Caustics Translucency (subsurface scattering)

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Transport Complexity How light interacts with objects/scene at a visible scale Shadows Inter-reflections Caustics Translucency (subsurface scattering) How light interacts with objects/scene at a visible scale Shadows Inter-reflections Caustics Translucency (subsurface scattering)

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Transport Complexity How light interacts with objects/scene at a visible scale Shadows Inter-reflections Caustics Translucency (subsurface scattering) How light interacts with objects/scene at a visible scale Shadows Inter-reflections Caustics Translucency (subsurface scattering)

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Some of All of This Real scenes have all of these forms of complexity Extreme realism in one form of complexity is not necessarily that interesting Incredible material models that are completely homogenous and lit by a single directional light Great lighting environments for diffuse surfaces with no shadows Real scenes have all of these forms of complexity Extreme realism in one form of complexity is not necessarily that interesting Incredible material models that are completely homogenous and lit by a single directional light Great lighting environments for diffuse surfaces with no shadows

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What is Precomputed Radiance Transfer (PRT)? Parameterize an objects response to lighting, expressed in some basis Partition into two processes Offline transport simulator precomputes spatially varying linear operators that map lighting in given basis to exit radiance Run time render the object using the current viewing/lighting environment using precomputed data Parameterize an objects response to lighting, expressed in some basis Partition into two processes Offline transport simulator precomputes spatially varying linear operators that map lighting in given basis to exit radiance Run time render the object using the current viewing/lighting environment using precomputed data

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PRT Teaser Demo

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Terminology

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Rendering Equation

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Radiance leaving point p in direction d

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Rendering Equation Radiance emitted from point p in direction d

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Rendering Equation Integral over directions s on the hemisphere around p

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Rendering Equation BRDF at point p evaluated for incident direction s in outgoing direction d

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Rendering Equation Radiance arriving at point p from direction s (also LHS)

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Rendering Equation Lamberts law – cosine between normal and - s = dot(Np, - s )

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Neumann Expansion Exit radiance expressed as infinite series

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Neumann Expansion Direct lighting arriving at point p – from distant environment

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Neumann Expansion Direct lighting arriving at point p – from distant environment

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Neumann Expansion Source Radiance – distant lighting environment

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Neumann Expansion Visibility function - binary

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Neumann Expansion All paths from source that take 1 bounce

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Neumann Expansion All paths from source that take 1 bounce L0L0

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Neumann Expansion All paths from source that take i bounces L i-1

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Diffuse PRT

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light 2D example, piecewise constant basis, shadows only Diffuse Self-Transfer PreprocessProject Light light Rendering = = =

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Precomputation Basis 16 Basis 17 Basis 18 illuminateresult......

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Spherical Harmonics Spherical analog to Fourier transform Represents complex functions on the sphere, real form used in graphics Polynomials in R 3 Full basis through O has O 2 coeffs Projection/Evaluation/Rotation are fairly straightforward Small number of bands implies low frequency lighting Spherical analog to Fourier transform Represents complex functions on the sphere, real form used in graphics Polynomials in R 3 Full basis through O has O 2 coeffs Projection/Evaluation/Rotation are fairly straightforward Small number of bands implies low frequency lighting

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Spherical Harmonics

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n=2n=3n=5 n=26original

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Spherical Harmonics Rotation invariance No temporal wobbling of projection Low frequency is also strength Reduces necessary surface sampling rate Addresses lighting that is most difficult with traditional techniques Global support is a limitation Rotation invariance No temporal wobbling of projection Low frequency is also strength Reduces necessary surface sampling rate Addresses lighting that is most difficult with traditional techniques Global support is a limitation

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PRT Demo

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PRT Limitations/Extensions Rigid objects Local, Deformable Precomputed Radiance Transfer, Siggraph 2005 Raw form unwieldy 6 th order would require 108 coefficients/vertex Siggraph 2003 paper compresses both data and computation, small number (4-12) coefficiens/vertex, much simpler shaders This is what makes it work in games… Rigid objects Local, Deformable Precomputed Radiance Transfer, Siggraph 2005 Raw form unwieldy 6 th order would require 108 coefficients/vertex Siggraph 2003 paper compresses both data and computation, small number (4-12) coefficiens/vertex, much simpler shaders This is what makes it work in games…

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PRT Limitations/Extensions Glossy surfaces Still to heavyweight for games, EGSR 2004 papers most applicable (separable BRDF approximations) All Frequency, using other light basis functions Ng et al Siggraph 2003, two EGSR 2004 papers Dramatically increases storage location per point, and spatial sampling rates over surfaces Glossy surfaces Still to heavyweight for games, EGSR 2004 papers most applicable (separable BRDF approximations) All Frequency, using other light basis functions Ng et al Siggraph 2003, two EGSR 2004 papers Dramatically increases storage location per point, and spatial sampling rates over surfaces

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PRT in the SDK Precomputation for diffuse objects only Includes inter-reflections and subsurface scattering Glossy isnt practical for games Compression Siggraph 2003 paper, what made it actually doable in a game Run time code for efficient evaluation of lighting models, projection and rotation Precomputation for diffuse objects only Includes inter-reflections and subsurface scattering Glossy isnt practical for games Compression Siggraph 2003 paper, what made it actually doable in a game Run time code for efficient evaluation of lighting models, projection and rotation

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Conclusions Precomputed Radiance Transfer enables interactive rendering of complex global illumination effects Appropriate for low frequency lights, can be mixed with traditional techniques to model high frequency lighting Precomputed Radiance Transfer enables interactive rendering of complex global illumination effects Appropriate for low frequency lights, can be mixed with traditional techniques to model high frequency lighting

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Acknowledgments Coauthors John Snyder, Jan Kautz, Ben Luna, John Hart, Jesse Hall Samples/Artwork/Slides Jason Sandlin, John Steed, Shanon Drone Light Probes Paul Debevec Coauthors John Snyder, Jan Kautz, Ben Luna, John Hart, Jesse Hall Samples/Artwork/Slides Jason Sandlin, John Steed, Shanon Drone Light Probes Paul Debevec

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DirectX Community Resources Windows Game Development MSDN Forums Forums: General, Graphics, Tools, Performance, and Audio Fully Moderated Discussions Notification Alerts, Messages and RSS Feeds Integrated with Visual Studio 2005 Advanced Search FAQs Answered/Unanswered Questions Windows Game Development MSDN Forums Forums: General, Graphics, Tools, Performance, and Audio Fully Moderated Discussions Notification Alerts, Messages and RSS Feeds Integrated with Visual Studio 2005 Advanced Search FAQs Answered/Unanswered Questions

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References Precomputed Radiance Transfer for Real-Time Rendering in Dynamic, Low-Frequency Lighting Environments, Sloan, Kautz and Snyder Siggraph 2002 Clustered Principal Components for Precomputed Radiance Transfer, Sloan, Hall, Hart and Snyder Siggraph 2003 All-Frequency Shadows Using Non-linear Wavelet Lighting Approximation, Ng, Ramamoorthi and Hanranan Siggraph 2003 All-Frequency Precomputed Radiance Transfer for Glossy Objects, Liu, Sloan, Shum and Snyder, Eurographics Symposium on Rendering 2004 All-Frequency Relighting of Non-Diffuse Objects using Separable BRDF Approximation, Wang, Tran and Luebke, Eurographics Symposium on Rendering 2004 Local, Deformable Precomputed Radiance Transfer, Sloan, Luna and Snyder Siggraph Precomputed Radiance Transfer for Real-Time Rendering in Dynamic, Low-Frequency Lighting Environments, Sloan, Kautz and Snyder Siggraph 2002 Clustered Principal Components for Precomputed Radiance Transfer, Sloan, Hall, Hart and Snyder Siggraph 2003 All-Frequency Shadows Using Non-linear Wavelet Lighting Approximation, Ng, Ramamoorthi and Hanranan Siggraph 2003 All-Frequency Precomputed Radiance Transfer for Glossy Objects, Liu, Sloan, Shum and Snyder, Eurographics Symposium on Rendering 2004 All-Frequency Relighting of Non-Diffuse Objects using Separable BRDF Approximation, Wang, Tran and Luebke, Eurographics Symposium on Rendering 2004 Local, Deformable Precomputed Radiance Transfer, Sloan, Luna and Snyder Siggraph 2005

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© 2005 Microsoft Corporation. All rights reserved. This presentation is for informational purposes only. Microsoft makes no warranties, express or implied, in this summary.

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