CS781 - Colour for Computer Graphics - Winter 2009

Lecture 5

Surface Reflectance

Generalized Bidirectional Reflectance Distribution Functions

Reflectance varies with

• position on the surface
• angle
• wavelength
• time

Each of these has both incoming and outgoing dependence

• position on the surface
  • subsurface scattering
• angle
  • unusual structure in the surface
• wavelength
  • fluorescence
• time
  • phosphorescence

Subjective Effects of Coloured Surfaces

Area effects

Colour perception is very acute for large areas of uniform colour

How to handle it

1. Carpets, shirts
   • precision of colour matching
2. Paint
3. Bricks

Surface constancy

Principal Components Analysis of Colour

Where did it come from

• remote sensing
• paint analysis

Principal Components Analysis

Given a very large sample of data, how can you summarise it?

• e.g. spectral power distributions from many sources of light
• PQR decomposition

Weaknesses

1. The usual least squares problems
   • and there is no well-accepted way of doing other metrics
   • Why? Because of the other problems
2. Sample dependence
   • You have to redo it for each new sample
3. There is usually not a well-defined break point

Possible Remedies

1. We know the dimension
   • e.g. colour display

PCA of Coloured Surfaces

PCA of Coloured Light

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