# 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
- angle
- unusual structure in the surface

- wavelength
- time

## Subjective Effects of Coloured Surfaces

#### Area effects

Colour perception is very acute for large areas of uniform colour

How to handle it

- Carpets, shirts
- precision of colour matching

- Paint
- 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

- The usual least squares problems
- and there is no well-accepted way of doing other metrics
- Why? Because of the other problems

- Sample dependence
- You have to redo it for each new sample

- There is usually not a well-defined break point

#### Possible Remedies

- We know the dimension

## PCA of Coloured Surfaces

## PCA of Coloured Light

Return to: