# 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:
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