CS488 - Introduction to Computer Graphics - Lecture 11

Comments and Questions

Local Lighting

Three components:

1. Eye
2. Surface(s)
3. Light source(s)

Eye

1. Sensitive to light per unit area: Talbot's law
   • Light per unit area depends on size of pupil

Surfaces

1. General surfaces:
   • BRDF
2. Lambertian surfaces:
   • cos(\zeta)
   • memoryless
   • pdf with more than you want to know
3. Shiny surfaces
   • delta( \zeta - \theta )
   • spread it out to get a glossy surface: cos^n (\zeta - \theta)

Light Sources

Two types

1. Ambient
   • Only amount and colour of incoming light matters
   • Geometry
     • Incoming light from all directions
       • indpt of \phi
       • sin(\theta) / sin(\theta)

       Outgoing light in all directions: BRDF

       • indpt of \eta
       • Mr Lambert: r^2 cos(\zeta) / r^2 cos(\zeta)

2. Directional
   • Take direction into account

Colour

What is needed for colour?

1. An eye.
2. A source of illumination.
3. A surface.

How is colour created?

1. Source of illumination emits light (photons of differing wavelength).
2. Surface modifies light.
   • reflectance
3. Eye compares surfaces and notices different modifications.

How do we represent colour?

• As some kind of sum of photons?
• As a distribution of photons (over wavelength)?
• As a ratio of distributions of photons?

To the rescue,

• A nineteenth century mathematician
  • Grassmann

  and a nineteenth century physicist

  • Maxwell
• All colours are subjectively the same as a linear combination of three basis colours
  • Linear combination defined in a special way
  • Is there anything special about three?
• Change of basis, etc., etc.
  • Display-dependent standard bases: RGB.
  • Display-independent standard bases: XYZ.
• Non-linear bases
  • HSV
  • Opponent colours

But,

• Only approximately correct
  • but to within 1-2% for most humans,
  • only describes matching, not appearance
• Doesn't describe non-additive colour mixture
  • CMY(K) for printing inks
• More precise requires illumination as well
  • CIELab, CIELuv

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• Bill Cowan's CS488 page for Winter 2008.
• Bill Cowan's CS488 lecture note page for Winter 2008.