lecture09

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David Luebke 2 Administrivia Quiz 1: Tuesday, Feb 20 Yes, I’ll have your homework graded by then (somehow) Normal written exam (oral later)
David Luebke 3 Recap: Distributed Ray Tracing Distributed ray tracing: an elegant stochastic approach that distributes rays across: Pixel for antialiasing Light source for soft shadows Reflection function for soft (glossy) reflections Time for motion blur Lens elements for depth of field Cook: 16 rays suffice for all of these

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David Luebke 4 Recap: Backwards Ray Tracing Two-pass algorithm: Rays are cast from light into scene Rays are cast from the eye into scene, picking up illumination showered on the scene in the first pass Backwards ray tracing can capture: Indirect illumination Color bleeding Caustics
David Luebke 5 Recap: Backwards Ray Tracing Arvo: illumination maps tile surfaces with regular grids, like texture maps Shoot rays outward from lights Every ray hit deposits some of its energy into surface’s illumination map Ignore first generation hits that directly illuminate surface ( Why? ) Eye rays look up indirect illumination using bilinear interpolation

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David Luebke 6 Recap: Radiosity Ray tracing: Models specular reflection easily Diffuse lighting is more difficult View-dependent, generates a picture Radiosity methods explicitly model light as an energy-transfer problem Models diffuse interreflection easily But only diffuse; no shiny (specular) surfaces View-independent, generates a 3-D model
David Luebke 7 Recap: Radiosity Basic idea: represent surfaces in environment as many discrete patches A patch, or element , is a polygon over which light intensity is constant Model light transfer between patches as a system of linear equations Solve this system for the intensity at each patch Solve for R,G,B intensities; get color at each patch Render patches as colored polygons in OpenGL

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David Luebke 8 Recap: Fundamentals Definition: The radiosity of a surface is the rate at which energy leaves the surface Radiosity = rate at which the surface emits energy + rate at which the surface reflects energy Simplifying assumptions Environment is closed All surfaces have Lambertian reflectance Surface patches emit and reflect light uniformly over their entire surface
David Luebke 9 Radiosity For each surface i : B i = E i + ρ i Σ B j F ji ( A j / A i ) where B i , B j = radiosity of patch i, j A i , A j = area of patch i, j E i = energy/area/time emitted by i i = reflectivity of patch i F ji = Form factor from j to i

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David Luebke 10 Form Factors Form factor : fraction of energy leaving the
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