# Colour Spaces

Remember when you were in high school

The four categories below represent increasingly objective/mathematical ways of describing colour

## Based on Substances

Pigments

Iron oxide

### Greeks

#### Vermillion

• mercury sulphide
• cinnabar

## Device Dependent

#### RGYB

• R = rL
• G = gL
• B = 1 - max(r,g)

#### YIQ

• NTSC colour encoding

## Device Independent

Based on instrumental measurement

# Colour Difference

The colour spaces above give us

1. increasingly standardized colour identity
2. correct colour topology

They do not give us a measure of colour difference!

• that is, a metric

Lot's of ways to do colour difference experiments

• two colours - report
• two pairs - greater difference
• seven colours

Question 1

• How many dimensions are needed to embed the data?

Question 2.

• How should the colour arrangement be stretched or compressed?

Two answers were provided by the CIE in 1978.

1. The first answer was 3, which is incorrect; the second answer was twins, two uniform colour spaces.
2. Luv
• L = 116 (Y/Yn)^(1/3) - 16
• u = 13 L (u' - un): u',un = 4 X / (X + 15 Y + 3 Z)
• v = 13 L (v' - vn): v',vn = 9 Y / (X + 15 Y + 3 Z)
3. Lab
• L = 116 (Y/Yn)^(1/3) - 16
• a = 500 ( (X/Xn)^(1/3) - (Y/Yn)^(1/3) )
• b = 200 ( (Y/Yn)^(1/3) - (Z/Zn)^(1/3) )

Why are there two?

What does this mean about colour difference?

# Comparing Images

Here's three ways that it is done

1. Sum ( (R - R")^2 + (G-G')^2 + (B - B')^2 ) ^(1/2) over pixels
2. Sum ( (X - X")^2 + (Y-Y')^2 + (Z - Z')^2 ) ^(1/2) over pixels
3. Sum ( (L - L")^2 + (u-u')^2 + (v - v')^2 ) ^(1/2) over pixels

Why are neither of these satisfactory?

Suggestions for improvement.