CS781 - Colour for Computer Graphics - Winter 2009
Lecture 16
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
Stone Age
Iron oxide
Egyptians
Greeks
Vermillion
- mercury sulphide
- cinnabar
Reluctance to mix pigments
Renaissance
Ultramarine
Based on Samples
Munsell
OSA
Artist's Colour Spaces
Device Dependent
Additive Primaries
RGB
HSV, HLS, HSB, HVC, etc.
RGYB
- R = rL
- G = gL
- B = 1 - max(r,g)
YIQ
Subtractive Primaries
Device Independent
Based on instrumental measurement
Tristimulus Values
Chromaticity Coordinates
Colour Difference
The colour spaces above give us
- increasingly standardized colour identity
- correct colour topology
They do not give us a measure of colour difference!
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.
- The first answer was 3, which is incorrect; the second answer was
twins, two uniform colour spaces.
- 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)
- 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
- Sum ( (R - R")^2 + (G-G')^2 + (B - B')^2 ) ^(1/2) over pixels
- Sum ( (X - X")^2 + (Y-Y')^2 + (Z - Z')^2 ) ^(1/2) over pixels
- Sum ( (L - L")^2 + (u-u')^2 + (v - v')^2 ) ^(1/2) over pixels
Why are neither of these satisfactory?
Suggestions for improvement.
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