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How to make a PIXAR movie

How to make a PIXAR movie. Global Illumination Effects. Motivation. Realistic illumination of the scene. Motivation. Soft shadows. Motivation. Subsurface scattering. Motivation. Many algorithms exist Photon mapping Ambient Occlusion …

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How to make a PIXAR movie

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  1. Howtomake a PIXAR movie Global Illumination Effects

  2. Motivation • Realisticilluminationofthescene

  3. Motivation • Soft shadows

  4. Motivation • Subsurfacescattering

  5. Motivation • Manyalgorithmsexist • Photon mapping • AmbientOcclusion • … • Common goal: Solvingpartsofthe Rendering Equation

  6. Problems • Scene changes -> New computation • Still not possible in real-time • UsesRaytracingorRadiosity Alreadyexplained Goingtobeexplainednow

  7. Photons • Haveenergy • h: Planck constant • v: Frequencyoflight

  8. RadiometricQuantities

  9. Radiance θ: angle betweensurface‘s normal andω cosθ: Lambertianlaw Constant along a ray

  10. Irradiance

  11. BRDF Bidirectionalreflectancedistributionfunction Howmuchlightisreflected

  12. ReflectionEquation Integrateoverthehemisphere

  13. Rendering Equation Radiance BRDF Geometryfactor Visibility Emittedlight Surfaces

  14. Radiosity • Ideal diffuse reflectioncanbesimulatedwithRadiosity • Uses finite elements • IntroducedbyGoral et al.

  15. Radiosity • Origin: Thermal heattransfer • Developed in 1984, still in use • Modellingof diffuse lighting • Doesn‘taccountforspecularlighting • Independent ofviewer • Therefore: Staysconstant in constantscene

  16. RadiosityEquation constant Emissivity Radiosity Reflectivity Form factor

  17. Form Factors

  18. Form Factors

  19. Nusselt Analog • Simple geometric analog forcalculating form factors B A

  20. HemicubeAlgorithm Hemicubeinsteadofhemisphere

  21. HemicubeAlgorithm • Idea: • Precomputedelta form factorsanalytically • Count coveredpixels • Sumupcovereddelta form factorstothetrue form factor

  22. HemicubeExample

  23. HemicubeAlgorithm on GPU • Useprojectioncenterasviewport • Usecurrentfaceasviewing plane • Do therendering • Grab thecolourbuffer (IDs ofpatches) • Count colouredpixels • Visibilitytestperformedbydepthtest

  24. RadiosityAlgorithm • Compute form factors • Solve linear equationsystem for i = 1, … , n

  25. Von Neumann Series 0 Bounces 1 Bounce 2 Bounces 3 Bounces

  26. Jacobi Iteration

  27. Shooting / Gathering

  28. RadiosityResult

  29. Radiosity vs. Ray Tracing

  30. RadiosityConclusion • Old, but still in use • Usedforsimulating diffuse lighting • Resultcanbeused in combinationwithother GI algorithms

  31. AmbientOcclusion Motivation • Ambienttermconstant in Phong model • Not veryrealistic • Idea: Computeocclusionofeachface

  32. AmbientOcclusion • Result: Occludedareasappeardarkerthanbrigtherones • MultiplywithusualPhong model • 2 possibilities: • Screen space • Objectspace

  33. Screen Space AmbientOcclusion • Can becompletelydone on GPU • Nopreprocessing • Independent ofscenecomplexity • Idea: Insteadofperformingfullraytracinguseocclusioninformationfrom z-buffer

  34. Screen Space AmbientOcclusion • Take 3D samplesaroundeachpoint • Determineocclusionofeachpointbytestingagainstthedepthbuffer • Farsampleswithlessinfluence • Useblurringfor smooth results

  35. Screen Space AmbientOcclusion

  36. Object Space AmbientOcclusion • Definesurfaceelementas an orienteddisk • UseHeron‘sformula , • Store position, normal andarea in textureforpixelshader

  37. Object Space AmbientOcclusion • Computeaccessibilityvalueat eachelement (% ofhemisphere) • Approximation based on the solid angle of an orienteddisk • Stronglydependent on scene complexity

  38. Object Space AmbientOcclusion

  39. AmbientOcclusionResults

  40. AmbientOcclusionResults

  41. AmbientOcclusionConclusion • Can bepreprocessedforeachobject • Used in thecurrentversionofPIXAR‘sRenderMan

  42. Outlook • Fastercomputation • Cheaper • Artistscanseeresultsfaster • More realisticlighting

  43. Conclusion • Veryimportantforanyanimatedmovie • Computation time not tooimportant

  44. Thanksforyourattention!

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