1 Chapter 5: Sources, Shadows, Shading Light source: Anything emits light that is internally generated Exitance: The internally generated power per unit area around a point

2 The geometry (e.g., point, line, area) of the source has profound effects on the distribution of light around the source.

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Area Sources (e.g., overcast sky) -- Modeled as surface patches whose radiance is independent of direction, i.e.,

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8 The radiosity at scene point P due to source point Q

Line Sources (e.g., fluorescent light bulb) -- Modeled as a thin cylinder with diameter (2) The radiosity of patches changes as the reciprocal of distance to the source rather than the square of the distance (1) The length of the source does not change, but the width does as Remarks:

Local Shading Model -- Describes radiosity at a scene point due to light sources Object Mesh Model Shaded Model

11 ○ Shadows occur when the patch can not see sources point source area source

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Ambient Illumination ○ Strategies for dealing with ambient illumination i) Add a constant term ii) Model the world as polygons of constant radiosity

Application: Photometric Stereo -- Determines depths of pixels from a number of images taken under different illuminations. The variations in brightness code the shapes of scene surfaces Basketball 。 Example:

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19 。 Monge curve -- A 2D curve can be formulated by an explicit function, i.e.,

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21 where C: ending at h: representing the height of the surface at the start point Derivatives give changes in surface height Reconstructing the surface by summing derivatives along some path C

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Interreflections, Global Shading Models Interreflection: A surface patch is illuminated by light reflected off other patches Global Shading Model: A shading model incorporates interreflection effects

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Solving for Radiosity Subdivide the world into small, flat patches with constant radiosity The radiosity at P on patch i due to the radiosity of patch j is

Qualitative Effects of Interreflections Local shading model describes the world poorly Global shading model involves unknown terms Assume the exitance of each patch is constant

28 ○ Qualitative effects of interreflection Example 1: Observed Predicted

29 Example 2: PredictedObserved

30 Example 3: ObservedPredicted

31 Example 4:

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