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Introduction to Global Illumination

This introduction covers the concepts of global and local illumination in computer graphics, discussing the physical simulation of light transport and the accuracy needed for all light paths. It delves into topics such as forward and inverse rendering, surface properties, and the importance of realism in rendering. Limitations, principles of BRDF, and the rendering equation are also explained. The text highlights key ideas in global illumination, the behavior of light, and various surface properties like perfect specular, Phong specular models, and diffused surfaces. The challenges and practical implementations of global illumination methods, such as progressive radiosity and bi-directional tracing, are explored. This comprehensive overview provides insights into the complexities and opportunities in achieving realistic lighting effects in computer graphics.

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Introduction to Global Illumination

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

  2. Global Illumination Physical Simulation of Light Transport: • Accuracyaccount for ALL light pathsconservation of energy • Predictionforward renderingcalculate light meter readings • Analysisinverse rendering! find surface properties ! • Realism?perceptually necessary?

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

  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. 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. 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. Limitations • 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…

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

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

  11. Review: Surface Properties Slightly scattered Specular: “high gloss” Phong Specular Model: L R cos15() Incident LightRay SurfaceNormal ReflectedLight Andrew Glassner et al.. SIGGRAPH`94 Course 18: “Fundamentals and Overview of Computer Graphics”

  12. Review: Surface Properties More Scattered Specular: “medium gloss” Phong Specular Model: L R cos5() Incident LightRay SurfaceNormal Andrew Glassner et al.. SIGGRAPH`94 Course 18: “Fundamentals and Overview of Computer Graphics”

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

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

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

  16. Point-wise Light: Radiance L Radiance: The Pointwise Measure of Light • 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. Radiance Units Tricky: think Hemispheres with a floor: Solid Angle (steradians) =dS = fraction of a hemisphere’s area (4) Projected Area cos  dA  dA dA

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

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

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

  21. Remeshing Example

  22. Progressive Radiosity

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

  24. Example: Photon Maps • Ideal: Trace Photon Paths • Trouble: high compute costs (exponential) • ‘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. Example: Photon Maps Forward-traced Reverse-TracedPhoton Map Result

  26. Photon Map Result • .

  27. Conclusion • Physically accurate (geometric optics only) simulation of light transport. • ‘Ultimate Realism’? perceptual, not physical • Languished as tweak-hungry lab curiosity • 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 & modeling

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