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Precomputed Radiance Transfer

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Precomputed Radiance Transfer

Peter-Pike Sloan

SDE

Windows Graphics & Gaming Technologies

Microsoft Corporation

- 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

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

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

- 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

- 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

- 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)

- 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)

- 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

- 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

Radiance leaving pointpin directiond

Radiance emitted from pointpin directiond

Integral over directionsson the hemisphere aroundp

BRDF at pointpevaluated for incident directionsin outgoing directiond

Radiance arriving at pointpfrom directions (also LHS)

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

Exit radiance expressed as infinite series

Direct lighting arriving at point p – from distant environment

Direct lighting arriving at point p – from distant environment

Source Radiance – distant lighting environment

Visibility function - binary

All paths from source that take 1 bounce

L0

All paths from source that take 1 bounce

Li-1

All paths from source that take i bounces

Project Light

light

light

•

=

=

•

=

•

2D example, piecewise constant basis, shadows only

Preprocess

Rendering

.

.

.

.

.

.

Basis 16

Basis 17

illuminate

result

Basis 18

- 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

n=2

n=3

n=5

n=26

original

- 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

- 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…

- 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

- 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

- 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

- Coauthors
- John Snyder, Jan Kautz, Ben Luna, John Hart, Jesse Hall

- Samples/Artwork/Slides
- Jason Sandlin, John Steed, Shanon Drone

- Light Probes
- Paul Debevec

- 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]

- “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.