1. 1 Introduction to Global Illumination Jack Tumblin CS 395 Advanced Computer Graphics Winter 2003

2. 2 Global Illumination Physical Simulation of Light Transport: –Accuracy account for ALL light paths conservation of energy –Prediction forward rendering calculate light meter readings –Analysis inverse rendering ! find surface properties ! –Realism? perceptually necessary?

3. 3 Local Illumination “Everything is lit by Light Sources” –Screen color = light source * surface reflectance –Refinements: reflectance = specular, diffuse, ambient, texture, … light = direct*shadow +ambient+environment maps, …

4. 4 Local Illumination “Everything is lit by Light Sources” –Refine: point light source  Area light source –Result? hard shadows  soft shadows

5. 5 Global Illumination “Everything is lit by Everything Else” –Screen color = entire scene * surface reflectance –Refinements: Models of area light sources, caustics, soft-shadowing, fog/smoke, photometric calibration, … H. Rushmeier et al., SIGGRAPH`98 Course 05 “A Basic Guide to Global Illumination”

6. 6 Global Illumination Idea: ALL POSSIBLE PATHS of light source to eye: From Jensen et al., SIGGRAPH2000 Course 20: ‘A Practical Guide To Global Illumination Using Photon Maps’

7. 7 Global Illumination Idea: ALL POSSIBLE PATHS of light source to eye: From Jensen et al., SIGGRAPH2000 Course 20: ‘A Practical Guide To Global Illumination Using Photon Maps’

8. 8Limitations Geometric Optics Only:Geometric Optics Only: –All objects, apertures >> (wavelength) –YES: Reflection, Refraction, Scattering –No: fringes, diffraction, dispersion* (see movie) Point-Based BRDF* (see Wann-Jensen et al.SIGGRAPH2001…Point-Based BRDF* (see Wann-Jensen et al.SIGGRAPH2001…

9. 9 Summary I Big Ideas: – Radiance –Measure Light: Radiance –: BRDF –Measure Light Attenuation: BRDF – The Rendering Equation (intractable: must approximate) –Light will ‘bounce around’ endlessly, decaying on each bounce:The Rendering Equation (intractable: must approximate)

10. 10 Review: Surface Properties Perfectly Specular: “Mirror” “infinite gloss” Phong Specular Model: L R cos  (  ) Andrew Glassner et al.. SIGGRAPH`94 Course 18: “Fundamentals and Overview of Computer Graphics”  Incident Light Ray Surface Normal Reflected Light

11. 11 Review: Surface Properties Slightly scattered Specular: “high gloss” Phong Specular Model: L R cos 15 (  ) L R cos 15 (  ) Incident Light Ray Surface Normal Reflected Light Andrew Glassner et al.. SIGGRAPH`94 Course 18: “Fundamentals and Overview of Computer Graphics”

12. 12 Review: Surface Properties More Scattered Specular: “medium gloss” Phong Specular Model: L R cos 5 (  ) L R cos 5 (  ) Incident Light Ray Surface Normal Andrew Glassner et al.. SIGGRAPH`94 Course 18: “Fundamentals and Overview of Computer Graphics”

13. 13 Review: Surface Properties Perfectly Diffuse “flat”, “chalky”,… Incident Light Ray Surface Normal Andrew Glassner et al.. SIGGRAPH`94 Course 18: “Fundamentals and Overview of Computer Graphics”

14. 14 Review: Surface Properties Most Materials: Combination of Diffuse and Specular Incident Light Ray Surface Normal Andrew Glassner et al.. SIGGRAPH`94 Course 18: “Fundamentals and Overview of Computer Graphics”

15. 15 Point-wise Reflectance: BRDF Bidirectional Reflectance Distribution Function  (  i,  i,  r,  r, i, r, … ) == (L r / L i ) a scalar  (  i,  i,  r,  r, i, r, … ) == (L r / L i ) a scalar  Illuminant L i  Reflected L r Infinitesimal Solid Angle

16. 16 Point-wise Light: Radiance L Radiance: The Pointwise Measure of Light Free-space light power L ==(energy/time)Free-space light power L ==(energy/time) –At least a 5D scalar function: L(x, y, z, , , …) –Position (x,y,z), Angle ( ,  ) and more (t,, …) –Power density units, but tricky…

17. 17 Radiance Units Tricky: think Hemispheres with a floor: Solid Angle (steradians) =dS = fraction of a hemisphere’s area (4  ) dA Projected Area dA cos  dA 

18. 18 Rendering Equation Radiance from point. Radiance emitted from point Radiance reflected from point (from all inward directions) (Kajiya 1986)

19. 19 Rendering Equation Opportunities Scalar operations only:  () and L(), indep. of, x,y,z, ,  …Scalar operations only:  () and L(), indep. of, x,y,z, ,  … Linearity:Linearity: –Solution = weighted sum of one-light solns. –Many BRDFs  weighted sum of diffuse, specular, gloss terms –SIGGRAPH2001 Result: reflected light = convolution(L in,  ) Difficulties Almost no notrivial analytic solutions exist; MUST use approximate methods to solveAlmost no notrivial analytic solutions exist; MUST use approximate methods to solve Verification: tough to measure real-world  () and L() wellVerification: tough to measure real-world  () and L() well Notable wavelength-dependent surfaces exist (iridescent insect wings & casing, CD grooves)Notable wavelength-dependent surfaces exist (iridescent insect wings & casing, CD grooves) BRDF doesn’t capture important subsurface scatteringBRDF doesn’t capture important subsurface scattering

20. 20 Implementation I Practical Approximations:Practical Approximations: – Radiosity Solution –Diffuse-only reflectance: Radiosity Solution Book presents old, slow, exact Gauss-Seidel… – Progressive Refinement, Path Tracing –Bounce-by-Bounce: Progressive Refinement, Path Tracing –Adaptive Meshing –Object-space Storage: Adaptive Meshing

21. 21 Remeshing Example

22. 22 Progressive Radiosity

23. 23 Implementation II Practical Approximations:Practical Approximations: – Bi-directional Tracing, –From Both Ends: Bi-directional Tracing, Trace from light to surfaces & store result, thenTrace from light to surfaces & store result, then Trace from eye to surfacesTrace from eye to surfaces – –Scattering Rays where needed: Monte-Carlo Methods,Monte-Carlo Methods, Distributed Ray TracingDistributed Ray Tracing – –Hybrids: Numerical Methods (Galerkin, etc.),Numerical Methods (Galerkin, etc.), Photon Maps,Photon Maps, Metropolis Transport,Metropolis Transport, Particles, Illumination caching,Particles, Illumination caching, 4D light volume sampling…4D light volume sampling…

24. 24 Example: Photon Maps Ideal: Trace Photon PathsIdeal: Trace Photon Paths Trouble: high compute costs (exponential)Trouble: high compute costs (exponential) ‘Photon Maps’ A Hybrid Solution‘Photon Maps’ A Hybrid Solution –‘big, sticky, aggregate photons’ –Russian Roulette (reflect, transmit, absorb?) –Trace photons outwards from light sources –Store photons only at diffuse surfaces –Scattered data interp., –Cache photons/illum. at each step.

25. 25 Example: Photon Maps Forward-tracedReverse-Traced Photon MapResult

26. 26 Photon Map Result.

27. 27Conclusion Physically accurate (geometric optics only) simulation of light transport.Physically accurate (geometric optics only) simulation of light transport. ‘Ultimate Realism’? perceptual, not physical‘Ultimate Realism’? perceptual, not physical Languished as tweak-hungry lab curiosityLanguished as tweak-hungry lab curiosity Gradual adoption for multitexturing source, for mixing real/synthetic images, Ph.Ds, theatre/architectural lighting, archaeology,…Gradual adoption for multitexturing source, for mixing real/synthetic images, Ph.Ds, theatre/architectural lighting, archaeology,… Growing interest for use in inverse rendering tasks: image-based rendering & modelingGrowing interest for use in inverse rendering tasks: image-based rendering & modeling