Computer Graphics Shading

The Physics

Local vs. Global Illumination Models Example

 Local model – direct and local interaction of Ambient each object with the Diffuse light.

 Global model: interactions and exchange of light energy between Final different objects. Specular Image

Light Sources Ambient Light  Point source (A): All light originates at a point  Assume non-directional light in the environment Rays hit planar surface at different incidence angles  Object illuminated with same light everywhere  Parallel source (B): All light rays are parallel Looks like silhouette Rays hit a planar surface at identical incidence angles May be modeled as point source at infinity  The Illumination equation I = Iaka Also called directional source Ia - ambient light  Area source (C): Light originates at finite area in intensity space. k - fraction of Inbetween the point and parallel sources a B C ambient light reflected Also called distributed source A from surface. Also defines object color

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Diffuse Light Diffused Ball  Dull surfaces such as solid matte plastic reflects incoming light uniformly in all directions. Called diffuse or Lambertian reflection  Understand intensity as the number of photons per inch2. If a flow of m photos passing each second through an inch2 window orthogonal to the flow, is hitting the red surface, how many photons hit an inch2 of the surface ?  Let θ is the angle between the direction of incoming light and normal to surface, and let L, N be corresponding unit vectors.

L N

Moon Paradox Diffuse Reflection

Reflection from a perfect mirror Specular Reflection

 Assume object is a perfect mirror. L N R  Shiny objects (e.g. metallic) reflect light in preferred θ θ α V direction R determined by surface normal N. L N R θ θ α V

 Most objects are not ideal mirrors – also reflect in the immediate vicinity of R  Lights emits the object in direction V only if V=R α=0  Phong Model – approximate attenuation by the form of cosnα (no real physical basis)

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Specular Reflection (Phong Model) Specular Reflection (cont’d)  Illumination equation:  Exponent n of cosine controls decay factor of attenuation  ks - Specular reflection coefficient function:  n - Specularity exponent  No physical basis but looks good:

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 Retroreflector sends light back where it came from regardless of the angle of insidence

More on Illumination Equation  For multiple light sources: shadingmodel

Ip of all light sources are added together Precautions should be taken from overflows

Even More on Illumination Equation Flat Shading  For distance/atmospheric attenuation sources:  Applied to piecewise linear polygonal models  Simple surface lighting approximated over polygons  Illumination value depends only on polygon normal ⇒ each polygon is colored with a dp - distance between surface and light source and/or uniform intensity distance between surface and viewer (heuristic  Looks non-smooth (worsened by Mach band effect) atmospheric attenuation)

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Normal per Vertex Gouraud Shading

 Compute illumination intensity at vertices using those normals  Interpolate intensity over polygon interior

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Gouraud Shading Phong Shading

 Interpolate (at the vertices in image space) normal vectors instead of illumination intensities  Apply the illumination equation for each interior pixel with its own (interpolated) normal

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Comments on Shading Comparison

 Phong shading is more expensive (why?) but well worth the effort

 Can achieve good looking specular highlight effects

 Both the Gouraud and Phong shading schemes are performed in the image plane and fit well into a polygonal scan-conversion fill scheme

 Both the Gouraud and Phong are view dependent  Can cause artifacts during animation as they are transformation dependent

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