CSE 681 Illumination continued

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CSE 681Illumination continued
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Light Material Properties Examples

• increasing diffuse

increasing ambient
increasing specular
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Material Properties
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Terminology Again

• Illumination 1. luminous flux at any point on a
  surface exposed to incident light (direct or
  indirect)
• Lighting1. The method used to provide
  artificial illumination
• Shading 1. Produce gradations of light or
  color2. Process of assigning colors to pixels

But youll hear them interchanged frequently!
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Polygonal Objects
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Face Normals
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Face Normals

• Given The vertices of a face
• Problem How can we compute the normal to the
  face?
• A face sits in an infinite plane what is the
  plane equation?
• A x B y C z D 0
• N ?(A, B, C, D), for some real scalar ? why?
• If P (x, y, z, 1) is a point on the plane then
  the planar equation is just N P 0

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Compute The Normal

• Triangle Plug and chug three points into the
  plane equation and solve for A, B, C, and D

Simple
Ugh ?
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Compute the Normal

• Take three consecutive points
• Define two vectors at the middle point
• Take the cross product (also called the curl)
• Will this always work?
• Collinear points

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Surface Normals

• How do you define normals at the vertices of a
  polygon mesh?

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Surface Normal
Take the average of the face normals incident on
the vertex???
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Normal List
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Flat Shading

• One normal per face
• Assumes that the viewer and light source are at
  an infinite distance
• Compute one color and apply it to the entire face
• I.e., compute illumination equation once per face

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Gouraud (Smooth) Shading

• One normal per vertex
• Compute illumination equation at each vertex
• Interpolate colors at the vertices across the
  face
• For triangles use barycentric coordinates for the
  interpolation

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Phong Shading

• One normal per vertex ... However
• Interpolate normals across the face
• Compute illumination equation at each internal
  point

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Directional Light Sources

• All rays have a common direction, and no point
  of origin
• The light source seems infinitely far away from
  surface
• The sun is a good example
• Light rays are parallel Þ Light vector the same
  for all surface point

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Point Light Sources

• Emits rays in radial directions
• An approximation for a light bulb
• Light vector changes across surface points

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Spotlight Effect

• Cone-shaped spotlight
• A lamp
• Defined just like a camera
• C position
• D direction
• g cut-off angle

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Light Properties

• Compute illumination separately for each
  wavelength
• Red, green, and blue components
• Compute light properties separately
• La, Ld, Ls
• Compute material properties separately
• ka, kd, ks
• Normalize these values, e.g. 0 (r, g, b) 1
• Express values as values

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Complex Reflection Functions
BRDF Bidirectional Reflectance Function
Phong Illum
Oren-Nayar
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BRDF illustrations
lumber
cement