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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

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

    • Lighting complexity

    • Transport complexity

    • Synergy

  • This talk focuses on techniques that enable more lighting/transport complexity


Material Complexity

  • Models how light interacts with a surface

    • Assume the “structure” of the material is below the visible scale

    • Simple variation

      • Twist maps


Meso-Scale Complexity

  • Variations at a visible scale - not geometry

    • Bump/Roughness maps

    • Parallax Mapping/BTFs extreme examples of this


Lighting Complexity

  • What kind of lighting environment is an object in?

    • Directional/point lights

    • Directional + ambient

    • “Smooth” (low frequency) lighting

    • Completely general


Lighting Complexity

  • What kind of lighting environment is an object in?

    • Directional/point lights

    • Directional + ambient

    • “Smooth” (low frequency) lighting

    • Completely general


Lighting Complexity

  • What kind of lighting environment is an object in?

    • Directional/point lights

    • Directional + ambient

    • “Smooth” (low frequency) lighting

    • Completely general


Lighting Complexity

  • What kind of lighting environment is an object in?

    • Directional/point lights

    • Directional + ambient

    • “Smooth” (low frequency) lighting

    • Completely general


Transport Complexity

  • How light interacts with objects/scene at a visible scale

    • Shadows

    • Inter-reflections

    • Caustics

    • Translucency (subsurface scattering)


Transport Complexity

  • How light interacts with objects/scene at a visible scale

    • Shadows

    • Inter-reflections

    • Caustics

    • Translucency (subsurface scattering)


Transport Complexity

  • How light interacts with objects/scene at a visible scale

    • Shadows

    • Inter-reflections

    • Caustics

    • Translucency (subsurface scattering)


Transport Complexity

  • How light interacts with objects/scene at a visible scale

    • Shadows

    • Inter-reflections

    • Caustics

    • Translucency (subsurface scattering)


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


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


PRT Teaser Demo


Terminology


Terminology


Terminology


Terminology


Rendering Equation


Rendering Equation

Radiance leaving pointpin directiond


Rendering Equation

Radiance emitted from pointpin directiond


Rendering Equation

Integral over directionsson the hemisphere aroundp


Rendering Equation

BRDF at pointpevaluated for incident directionsin outgoing directiond


Rendering Equation

Radiance arriving at pointpfrom directions (also LHS)


Rendering Equation

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


Neumann Expansion

Exit radiance expressed as infinite series


Neumann Expansion

Direct lighting arriving at point p – from distant environment


Neumann Expansion

Direct lighting arriving at point p – from distant environment


Neumann Expansion

Source Radiance – distant lighting environment


Neumann Expansion

Visibility function - binary


Neumann Expansion

All paths from source that take 1 bounce


Neumann Expansion

L0

All paths from source that take 1 bounce


Neumann Expansion

Li-1

All paths from source that take i bounces


Diffuse PRT


Diffuse PRT


Diffuse PRT


Diffuse PRT


Diffuse PRT


Diffuse PRT


Diffuse PRT


Diffuse PRT


Project Light

light

light

=

=

=

Diffuse Self-Transfer

2D example, piecewise constant basis, shadows only

Preprocess

Rendering


.

.

.

.

.

.

Precomputation

Basis 16

Basis 17

illuminate

result

Basis 18


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 Harmonics


Spherical Harmonics

n=2

n=3

n=5

n=26

original


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


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…


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


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


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


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


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]


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


© 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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