1. Precomputed Radiance Transfer
3/27/ :02 PM
Precomputed Radiance Transfer
Peter-Pike Sloan
SDE
Windows Graphics & Gaming Technologies
Microsoft Corporation
© 2004 Microsoft Corporation. All rights reserved.
This presentation is for informational purposes only. Microsoft makes no warranties, express or implied, in this summary.

2. 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

3. Material Complexity Models how light interacts with a surface
3/27/ :02 PM
Material Complexity
Models how light interacts with a surface
Assume the “structure” of the material is below the visible scale
Simple variation
Twist maps
© 2004 Microsoft Corporation. All rights reserved.
This presentation is for informational purposes only. Microsoft makes no warranties, express or implied, in this summary.

4. Meso-Scale Complexity
3/27/ :02 PM
Meso-Scale Complexity
Variations at a visible scale - not geometry
Bump/Roughness maps
Parallax Mapping/BTFs extreme examples of this
© 2004 Microsoft Corporation. All rights reserved.
This presentation is for informational purposes only. Microsoft makes no warranties, express or implied, in this summary.

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

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

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

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

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

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

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

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

13. 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

14. What is Precomputed Radiance Transfer (PRT)?
3/27/ :02 PM
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
© 2004 Microsoft Corporation. All rights reserved.
This presentation is for informational purposes only. Microsoft makes no warranties, express or implied, in this summary.

15. 3/27/ :02 PM
PRT Teaser Demo
© 2004 Microsoft Corporation. All rights reserved.
This presentation is for informational purposes only. Microsoft makes no warranties, express or implied, in this summary.

16. 3/27/ :02 PM
Terminology
© 2004 Microsoft Corporation. All rights reserved.
This presentation is for informational purposes only. Microsoft makes no warranties, express or implied, in this summary.

17. 3/27/ :02 PM
Terminology
© 2004 Microsoft Corporation. All rights reserved.
This presentation is for informational purposes only. Microsoft makes no warranties, express or implied, in this summary.

18. 3/27/ :02 PM
Terminology
© 2004 Microsoft Corporation. All rights reserved.
This presentation is for informational purposes only. Microsoft makes no warranties, express or implied, in this summary.

19. 3/27/ :02 PM
Terminology
© 2004 Microsoft Corporation. All rights reserved.
This presentation is for informational purposes only. Microsoft makes no warranties, express or implied, in this summary.

20. Rendering Equation 3/27/2017 10:02 PM
© 2004 Microsoft Corporation. All rights reserved.
This presentation is for informational purposes only. Microsoft makes no warranties, express or implied, in this summary.

21. Rendering Equation Radiance leaving point p in direction d
3/27/ :02 PM
Rendering Equation
Radiance leaving point p in direction d
© 2004 Microsoft Corporation. All rights reserved.
This presentation is for informational purposes only. Microsoft makes no warranties, express or implied, in this summary.

22. Rendering Equation Radiance emitted from point p in direction d
3/27/ :02 PM
Rendering Equation
Radiance emitted from point p in direction d
© 2004 Microsoft Corporation. All rights reserved.
This presentation is for informational purposes only. Microsoft makes no warranties, express or implied, in this summary.

23. 3/27/ :02 PM
Rendering Equation
Integral over directions s on the hemisphere around p
© 2004 Microsoft Corporation. All rights reserved.
This presentation is for informational purposes only. Microsoft makes no warranties, express or implied, in this summary.

24. 3/27/ :02 PM
Rendering Equation
BRDF at point p evaluated for incident direction s in outgoing direction d
© 2004 Microsoft Corporation. All rights reserved.
This presentation is for informational purposes only. Microsoft makes no warranties, express or implied, in this summary.

25. 3/27/ :02 PM
Rendering Equation
Radiance arriving at point p from direction s (also LHS)
© 2004 Microsoft Corporation. All rights reserved.
This presentation is for informational purposes only. Microsoft makes no warranties, express or implied, in this summary.

26. 3/27/ :02 PM
Rendering Equation
Lamberts law – cosine between normal and -s = dot(Np, -s)
© 2004 Microsoft Corporation. All rights reserved.
This presentation is for informational purposes only. Microsoft makes no warranties, express or implied, in this summary.

27. Neumann Expansion Exit radiance expressed as infinite series
3/27/ :02 PM
Neumann Expansion
Exit radiance expressed as infinite series
© 2004 Microsoft Corporation. All rights reserved.
This presentation is for informational purposes only. Microsoft makes no warranties, express or implied, in this summary.

28. 3/27/ :02 PM
Neumann Expansion
Direct lighting arriving at point p – from distant environment
© 2004 Microsoft Corporation. All rights reserved.
This presentation is for informational purposes only. Microsoft makes no warranties, express or implied, in this summary.

29. 3/27/ :02 PM
Neumann Expansion
Direct lighting arriving at point p – from distant environment
© 2004 Microsoft Corporation. All rights reserved.
This presentation is for informational purposes only. Microsoft makes no warranties, express or implied, in this summary.

30. Neumann Expansion Source Radiance – distant lighting environment
3/27/ :02 PM
Neumann Expansion
Source Radiance – distant lighting environment
© 2004 Microsoft Corporation. All rights reserved.
This presentation is for informational purposes only. Microsoft makes no warranties, express or implied, in this summary.

31. Neumann Expansion Visibility function - binary 3/27/2017 10:02 PM
© 2004 Microsoft Corporation. All rights reserved.
This presentation is for informational purposes only. Microsoft makes no warranties, express or implied, in this summary.

32. Neumann Expansion All paths from source that take 1 bounce
3/27/ :02 PM
Neumann Expansion
All paths from source that take 1 bounce
© 2004 Microsoft Corporation. All rights reserved.
This presentation is for informational purposes only. Microsoft makes no warranties, express or implied, in this summary.

33. Neumann Expansion All paths from source that take 1 bounce L0
3/27/ :02 PM
Neumann Expansion L0
All paths from source that take 1 bounce
© 2004 Microsoft Corporation. All rights reserved.
This presentation is for informational purposes only. Microsoft makes no warranties, express or implied, in this summary.

34. Neumann Expansion All paths from source that take i bounces Li-1
3/27/ :02 PM
Neumann Expansion
Li-1
All paths from source that take i bounces
© 2004 Microsoft Corporation. All rights reserved.
This presentation is for informational purposes only. Microsoft makes no warranties, express or implied, in this summary.

35. 3/27/ :02 PM
Diffuse PRT
© 2004 Microsoft Corporation. All rights reserved.
This presentation is for informational purposes only. Microsoft makes no warranties, express or implied, in this summary.

36. 3/27/ :02 PM
Diffuse PRT
© 2004 Microsoft Corporation. All rights reserved.
This presentation is for informational purposes only. Microsoft makes no warranties, express or implied, in this summary.

37. 3/27/ :02 PM
Diffuse PRT
© 2004 Microsoft Corporation. All rights reserved.
This presentation is for informational purposes only. Microsoft makes no warranties, express or implied, in this summary.

38. 3/27/ :02 PM
Diffuse PRT
© 2004 Microsoft Corporation. All rights reserved.
This presentation is for informational purposes only. Microsoft makes no warranties, express or implied, in this summary.

39. 3/27/ :02 PM
Diffuse PRT
© 2004 Microsoft Corporation. All rights reserved.
This presentation is for informational purposes only. Microsoft makes no warranties, express or implied, in this summary.

40. 3/27/ :02 PM
Diffuse PRT
© 2004 Microsoft Corporation. All rights reserved.
This presentation is for informational purposes only. Microsoft makes no warranties, express or implied, in this summary.

41. 3/27/ :02 PM
Diffuse PRT
© 2004 Microsoft Corporation. All rights reserved.
This presentation is for informational purposes only. Microsoft makes no warranties, express or implied, in this summary.

42. 3/27/ :02 PM
Diffuse PRT
© 2004 Microsoft Corporation. All rights reserved.
This presentation is for informational purposes only. Microsoft makes no warranties, express or implied, in this summary.

43. Diffuse Self-Transfer
3/27/ :02 PM
Diffuse Self-Transfer
2D example, piecewise constant basis, shadows only
Preprocess
Project Light
light
Rendering • = = •
light = •
© 2004 Microsoft Corporation. All rights reserved.
This presentation is for informational purposes only. Microsoft makes no warranties, express or implied, in this summary.

44. Precomputation . Basis 16 Basis 17 Basis 18 illuminate result
3/27/ :02 PM
Precomputation .
Basis 16
Basis 17
illuminate
result
Basis 18
© 2004 Microsoft Corporation. All rights reserved.
This presentation is for informational purposes only. Microsoft makes no warranties, express or implied, in this summary.

45. Spherical Harmonics Spherical analog to Fourier transform
3/27/ :02 PM
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
© 2004 Microsoft Corporation. All rights reserved.
This presentation is for informational purposes only. Microsoft makes no warranties, express or implied, in this summary.

46. Spherical Harmonics 3/27/2017 10:02 PM
© 2004 Microsoft Corporation. All rights reserved.
This presentation is for informational purposes only. Microsoft makes no warranties, express or implied, in this summary.

47. Spherical Harmonics n=2 n=3 n=5 n=26 original 3/27/2017 10:02 PM
© 2004 Microsoft Corporation. All rights reserved.
This presentation is for informational purposes only. Microsoft makes no warranties, express or implied, in this summary.

48. Spherical Harmonics Rotation invariance
3/27/ :02 PM
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
© 2004 Microsoft Corporation. All rights reserved.
This presentation is for informational purposes only. Microsoft makes no warranties, express or implied, in this summary.

49. 3/27/ :02 PM
PRT Demo
© 2004 Microsoft Corporation. All rights reserved.
This presentation is for informational purposes only. Microsoft makes no warranties, express or implied, in this summary.

50. 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…

51. 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

52. 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

53. 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

54. 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

55. 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

56. 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

57. © 2005 Microsoft Corporation. All rights reserved.
3/27/ :02 PM
© 2005 Microsoft Corporation. All rights reserved.
This presentation is for informational purposes only. Microsoft makes no warranties, express or implied, in this summary.
© 2004 Microsoft Corporation. All rights reserved.
This presentation is for informational purposes only. Microsoft makes no warranties, express or implied, in this summary.