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computer graphics & visualization Global Illumination Effects

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computer graphics & visualization Global Illumination Effects Christian A. Wiesner Motivation Realistic illumination of the scene

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computer graphics & visualization Global Illumination Effects Christian A. Wiesner Motivation Soft shadows

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computer graphics & visualization Global Illumination Effects Christian A. Wiesner Motivation Subsurface scattering

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computer graphics & visualization Global Illumination Effects Christian A. Wiesner Motivation Many algorithms exist – Photon mapping – Ambient Occlusion – … Common goal: Solving parts of the Rendering Equation

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computer graphics & visualization Global Illumination Effects Christian A. Wiesner Problems Scene changes -> New computation Still not possible in real-time Uses Raytracing or Radiosity Already explained Going to be explained now

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computer graphics & visualization Global Illumination Effects Christian A. Wiesner Photons Have energy h: Planck constant v: Frequency of light

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computer graphics & visualization Global Illumination Effects Christian A. Wiesner Radiometric Quantities Radiant energyJ Radiant powerW IrradianceW/m² RadiosityW/m² Radiant intensityW/sr

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computer graphics & visualization Global Illumination Effects Christian A. Wiesner Radiance θ: angle between surface‘s normal and ω cosθ: Lambertian law Constant along a ray

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computer graphics & visualization Global Illumination Effects Christian A. Wiesner Irradiance

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computer graphics & visualization Global Illumination Effects Christian A. Wiesner BRDF Bidirectional reflectance distribution function How much light is reflected

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computer graphics & visualization Global Illumination Effects Christian A. Wiesner Reflection Equation Integrate over the hemisphere

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computer graphics & visualization Global Illumination Effects Christian A. Wiesner Rendering Equation Radiance Emitted light Surfaces BRDF Visibility Geometry factor

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computer graphics & visualization Global Illumination Effects Christian A. Wiesner Radiosity Ideal diffuse reflection can be simulated with Radiosity Uses finite elements Introduced by Goral et al.

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computer graphics & visualization Global Illumination Effects Christian A. Wiesner Radiosity Origin: Thermal heat transfer Developed in 1984, still in use Modelling of diffuse lighting – Doesn‘t account for specular lighting – Independent of viewer – Therefore: Stays constant in constant scene

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computer graphics & visualization Global Illumination Effects Christian A. Wiesner Radiosity Equation RadiosityEmissivity Reflectivity Form factor constant

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computer graphics & visualization Global Illumination Effects Christian A. Wiesner Form Factors

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computer graphics & visualization Global Illumination Effects Christian A. Wiesner Form Factors

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computer graphics & visualization Global Illumination Effects Christian A. Wiesner Nusselt Analog Simple geometric analog for calculating form factors B A

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computer graphics & visualization Global Illumination Effects Christian A. Wiesner Hemicube Algorithm Hemicube instead of hemisphere

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computer graphics & visualization Global Illumination Effects Christian A. Wiesner Hemicube Algorithm Idea: – Precompute delta form factors analytically – Count covered pixels – Sum up covered delta form factors to the true form factor

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computer graphics & visualization Global Illumination Effects Christian A. Wiesner Hemicube Example

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computer graphics & visualization Global Illumination Effects Christian A. Wiesner Hemicube Algorithm on GPU Use projection center as viewport Use current face as viewing plane Do the rendering Grab the colour buffer (IDs of patches) Count coloured pixels Visibility test performed by depth test

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computer graphics & visualization Global Illumination Effects Christian A. Wiesner Radiosity Algorithm Compute form factors Solve linear equation system for i = 1, …, n

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computer graphics & visualization Global Illumination Effects Christian A. Wiesner Von Neumann Series 0 Bounces 1 Bounce 2 Bounces 3 Bounces

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computer graphics & visualization Global Illumination Effects Christian A. Wiesner Jacobi Iteration

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computer graphics & visualization Global Illumination Effects Christian A. Wiesner Shooting / Gathering

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computer graphics & visualization Global Illumination Effects Christian A. Wiesner Radiosity Result

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computer graphics & visualization Global Illumination Effects Christian A. Wiesner Radiosity vs. Ray Tracing

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computer graphics & visualization Global Illumination Effects Christian A. Wiesner Radiosity Conclusion Old, but still in use Used for simulating diffuse lighting Result can be used in combination with other GI algorithms

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computer graphics & visualization Global Illumination Effects Christian A. Wiesner Ambient Occlusion Motivation Ambient term constant in Phong model Not very realistic Idea: Compute occlusion of each face

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computer graphics & visualization Global Illumination Effects Christian A. Wiesner Ambient Occlusion Result: Occluded areas appear darker than brigther ones Multiply with usual Phong model 2 possibilities: – Screen space – Object space

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computer graphics & visualization Global Illumination Effects Christian A. Wiesner Screen Space Ambient Occlusion Can be completely done on GPU No preprocessing Independent of scene complexity Idea: Instead of performing full raytracing use occlusion information from z-buffer

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computer graphics & visualization Global Illumination Effects Christian A. Wiesner Screen Space Ambient Occlusion Take 3D samples around each point Determine occlusion of each point by testing against the depth buffer Far samples with less influence Use blurring for smooth results

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computer graphics & visualization Global Illumination Effects Christian A. Wiesner Screen Space Ambient Occlusion

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computer graphics & visualization Global Illumination Effects Christian A. Wiesner Object Space Ambient Occlusion Define surface element as an oriented disk Use Heron‘s formula, Store position, normal and area in texture for pixel shader

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computer graphics & visualization Global Illumination Effects Christian A. Wiesner Object Space Ambient Occlusion Compute accessibility value at each element (% of hemisphere) Approximation based on the solid angle of an oriented disk Strongly dependent on scene complexity

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computer graphics & visualization Global Illumination Effects Christian A. Wiesner Object Space Ambient Occlusion

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computer graphics & visualization Global Illumination Effects Christian A. Wiesner Ambient Occlusion Results

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computer graphics & visualization Global Illumination Effects Christian A. Wiesner Ambient Occlusion Results

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computer graphics & visualization Global Illumination Effects Christian A. Wiesner Ambient Occlusion Conclusion Can be preprocessed for each object Used in the current version of PIXAR‘s RenderMan

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computer graphics & visualization Global Illumination Effects Christian A. Wiesner Outlook Faster computation – Cheaper – Artists can see results faster More realistic lighting

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computer graphics & visualization Global Illumination Effects Christian A. Wiesner Conclusion Very important for any animated movie Computation time not too important

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computer graphics & visualization Global Illumination Effects Christian A. Wiesner Thanks for your attention!

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Computer Graphics (Spring 2008) COMS 4160, Lecture 20: Illumination and Shading 2

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