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Precomputed Radiance Transfer. Peter-Pike Sloan SDE Windows Graphics & Gaming Technologies Microsoft Corporation. Challenges in Rendering. Generating realistic images interactively is hard Many dimensions of complexity Geometric complexity Material complexity Meso-scale complexity

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Precomputed Radiance Transfer

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Precomputed radiance transfer l.jpg

Precomputed Radiance Transfer

Peter-Pike Sloan

SDE

Windows Graphics & Gaming Technologies

Microsoft Corporation


Challenges in rendering l.jpg

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


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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


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Meso-Scale Complexity

  • 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


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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


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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


Lighting complexity8 l.jpg

Lighting Complexity

  • 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)


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Transport Complexity

  • 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)


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Transport Complexity

  • 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


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What is Precomputed Radiance Transfer (PRT)?

  • Parameterize an object’s 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


Terminology l.jpg

Terminology


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Terminology


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Terminology


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Terminology


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Rendering Equation


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Rendering Equation

Radiance leaving pointpin directiond


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Rendering Equation

Radiance emitted from pointpin directiond


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Rendering Equation

Integral over directionsson the hemisphere aroundp


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Rendering Equation

BRDF at pointpevaluated for incident directionsin outgoing directiond


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Rendering Equation

Radiance arriving at pointpfrom directions (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

L0

All paths from source that take 1 bounce


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Neumann Expansion

Li-1

All paths from source that take i bounces


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Diffuse PRT


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Diffuse PRT


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Diffuse PRT


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Diffuse PRT


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Diffuse PRT


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Diffuse PRT


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Diffuse PRT


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Diffuse PRT


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Project Light

light

light

=

=

=

Diffuse Self-Transfer

2D example, piecewise constant basis, shadows only

Preprocess

Rendering


Precomputation l.jpg

.

.

.

.

.

.

Precomputation

Basis 16

Basis 17

illuminate

result

Basis 18


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Spherical Harmonics

  • Spherical analog to Fourier transform

  • Represents complex functions on the sphere, real form used in graphics

  • Polynomials in R3

    • Full basis through O has O2 coeffs

  • Projection/Evaluation/Rotation are fairly straightforward

  • Small number of bands implies “low frequency” lighting


Spherical harmonics46 l.jpg

Spherical Harmonics


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Spherical Harmonics

n=2

n=3

n=5

n=26

original


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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


Prt demo l.jpg

PRT Demo


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PRT Limitations/Extensions

  • Rigid objects

    • “Local, Deformable Precomputed Radiance Transfer”, Siggraph 2005

  • Raw form unwieldy

    • 6th 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


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PRT in the SDK

  • Precomputation for diffuse objects only

    • Includes inter-reflections and subsurface scattering

    • Glossy isn’t 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


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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


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DirectX Community Resources

  • http://msdn.com/directx

  • “Windows Game Development” MSDN Forums

    • http://forums.microsoft.com/msdn/default.aspx

    • 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

  • [email protected]


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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 2005

  • http://research.microsoft.com/~ppsloan


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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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