CS488 - Introduction to Computer Graphics - Lecture 17

Comments and Questions

Review

1. Hierarchical models

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

Lighting Models

Must incorporate geometry in addition to colour

Goals:

• the right colour at every pixel
• fast enough
  • either simple or GPU-calculable

Actual illumination

• area light sources with direction dependent photon distributions

Actual surfaces

• complex geometry
  • surface reflection
  • body reflection
• complex physics
  • You don't want to know all the details

For computer graphics

Start simple, only make it more complex `when we need to', which means `when somebody powerful complains'.

Lambertian Surfaces

Model of body reflection

• good model of most artificial surfaces
  • paint
  • plastic
  • cloth (most types)

  and many natural surfaces

  • hunman skin (sort of)
  • plants (most parts)
  • animals (most parts)
  • etc.
• light direction completely randomized
  • outgoing direction independent of incoming direction
• completely specified by reflectance function

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