CS321 Dr. Mark L. Hornick

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Illumination

• Natural lighting effects
• Surface characteristics
• Shadows
• Reflections
• Mathematical models
• Theoretical empirical

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Terminology

• Illumination (lighting) model
• Calculating light intensity
• At each point on a surface
• Surface rendering
• Apply lighting model
• Obtain pixel intensities of projected surface
  positions

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Light Sources Emitters
Point source Area small compared to scene
Distributed source Area large compared to scene
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Light Emitters Reflectors
Specular (shiny)
Diffuse (dull)
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Ambient Light

• General brightness of scene
• Light sources, reflections, etc.
• Not spatial or directional
• Constant for all surfaces
• Independent of direction
• Viewing direction, surface orientation
• Reflected light depends on surface

Ia
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Ambient Diffuse Reflection
Incident light from all directions
Reflected light scattered to all directions
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Directional Diffuse Reflection
Incident light from one direction
But spread over varying areas
Reflected light scattered equally in all
directions Intensity depends on angle of incidence
Unit vectors N (normal) and L (to light source
position)
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Flat Shading

• Polygon surface rendering
• Each polygon in the surface is shaded according
  to the intensity calculations based on the
  polygons surface normal

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Gouraud Shading

• Vertex normals are calculated as the average of
  the normals of the polygons that share the vertex
  
• The vertex normal is an approximation to the true
  normal of the surface at that point.
• Determine vertex intensities based on vertex
  normal
• Linearly interpolate across surface

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Gouraud Shading
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Specular Reflection
Reflected light not diffused Angle of incidence
angle of reflection
For perfect reflector, no reflection visible at
any other angle
f
For imperfect reflectors, some reflection visible
at angle f from R
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Phong Model
Empirical model of specular reflection
Specular reflection parameter, ns, is large
(gt100) for shiny surfaces, small (1) for dull
ones
Specular reflection coefficient, W(q), is
relatively constant for many opaque materials
i.e. modeled by constant ks
W(q)
q
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Multiple Light Sources

• We assume linear superposition of effects of all
  light sources.
• It may be necessary to scale to avoid intensity
  saturation.

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Other Lighting Issues

• Not all sources are points
• Control intensity by direction (Warn)
• Intensity falls off at distance
• Attenuation functions
• Empirical rather than exact models
• Color
• Adjust reflection coefficients

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