Basic Illumination

1
Basic Illumination
2
Light Source Independent Models

• Depth Shading
• Color or intensity determined solely by
  elevation "depth" of polygon.
• Darker colors or intensities at lower
  elevations.
• Effective in modeling terrain or surface data
• Avoids complex calculations of lighting
  dependent models
• Simulates realism
• Depth Cueing
• Reduce intensity of pixel as the distance from
  the observer increases
• Simulates reduction in clarity as distances
  from the observer increases
• Image fades in the distance
• Often used in medical imaging

3
Light Source Dependent Models

• What an object looks like depends on
• Properties of the light source such as color,
  distance from object, direction from object,
  intensity of source
• Surface characteristics of object such as
  color and reflectance properties
• Location of the observer
• Light striking a surface of an object can be
• Reflected (Diffuse reflection Specular
  reflection)
• Absorbed
• Transmitted (Translucent or transparent)
• Combination of all three

4
Ambient Illumination

• Ambient light is the illumination of an object
  caused by reflected light from other surfaces.
  To calculate this exactly would be very
  complicated. A simple model assumes ambient
  light is uniform in the environment.
• Let
• Ia Ambient light intensity
• ka Ambient light reflected
• Then intensity on the surface is described by
• I Iaka

5
Diffuse Reflection using Lambert's Law

• Lambert's Law - The intensity of light reflected
  from a surface is proportional to the cosine of
  the angle between the vector L to the light
  source and the normal vector N perpendicular to
  the surface.

The amount of reflected light is dependent on the
position of the light source and the object but
independent of the observer's position.
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Simple Illumination Model

• Let
• I Illumination intensity
• Ip Point light source intensity (white light)
• kd Surface reflection coefficient (0?kd ? 1)
• A simple illumination model I Ipkd(cosß)
• Since cosß (LN)/(L N), then if L and N
  have unit length then we can use
• Adding diffuse reflectance to
• our illumination model
• I Iaka Ipkd (LN)

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Light-source Attenuation

• Thus far we have ignored the inverse square law
  energy decays with the inverse square of the
  distance dL to the light source. Including this
  term we get
• I Iaka Ipkd (LN)/dL2
• However, due to our previous assumptions of a
  point light source and uniform ambient light,
  using the dL2 term gives too rapid of a decrease
  in illumination intensity to look realistic. The
  dL2 term is usually replaced by 1/fatt where
• fatt MIN (1/(c1 c2dL c3dL2), 1)
• I Iaka Ipkd (NL)fatt

8
Specular Reflection

• Light bounces off a glossy surface maintaining
  the color of the light source.
• Visible when the angle of incidence of the light
  from the point light source is equal to the angle
  of reflection toward the observer.
• For a nonperfect reflector, intensity of
  reflected light decreases rapidly as angle to
  observer increases beyond the angle of incidence.

Position of
V Observer Position N Normal Vector L Light
Src Vector
Max Specular
N
L
Reflection
R
V
ø
ø
ß
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Phong's Highlighting Term

• Ipfatt W(ø) cosnß
• Ip Point light source intensity
• fatt Light-source Attenuation
• W(ø) Fraction of specularly reflected light
  (usually a constant, ks)
• n Specular reflection exponent
• cos ß R V (if R and V are of unit length)
• I Iaka Ipkd (NL)fatt Specular Component
• Specular Component Ipfatt W(ø) cosnß
• This term represents the amount of the light
  sources color that should be added!

10
Illumination and Color Models

• The illumination equation for many sources of
  white light is
• I Iaka ?sources(Ip fatt (kd (NL)
  ks(RV)n))
• Colored lights have different intensities for
  different wavelengths.
• A colored object reflects light of some
  wavelengths more than others.
• Handling the R, G, and B wavelengths separately
  gives a rough (incorrect, but acceptable)
  approximation of this
• Ambient color (IaR, IaG, IaB), Diffuse color of
  light source (IpR, IpG, IpB)
• Objects color (OR, OG, OB)
• IR IaRka OR ?sources(IpR fatt (kdOR(NL)
  ks(RV)n))
• IG IaGkaOG ?sources(IpG fatt (kdOG(NL)
  ks(RV)n))
• IB IaBkaOB ?sources(IpB fatt (kdOB(NL)
  ks(RV)n))

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Too Intense

• With multiple light sources, it is easy to
  generated values of I gt 1
• One solution is to set the color value to be
  MAX(I, 1)
• An object can change color, saturating towards
  white
• Ex. (0.1, 0.4, 0.8) (0.5, 0.5, 0.5) (0.6,
  0.9, 1.0)

• Another solution is to renormalize the
  intensities to vary from 0 to 1 if one I gt 1.
• Requires calculating all Is before rendering
  anything.
• No over-saturation, but image may be too bright,
  and contrasts a little off.
• Image-processing on image to be rendered (with
  original Is) will produce better results, but is
  costly.