cs781 - Colour for Computer Graphics - Winter 2012

Course Notes

Lecture 19 - Colour Spaces

And Now from our Sponsor

  1. Projects
  2. Tables of useful colour data

Note about Mappings

Mappings from input coordinates to amounts of ink, or toner, or coloured wax, etc. are a required feature of every printer, and are an important aspect of creating satisfied customers. Calibration services for any sort of colour device have not been successful outside commercial colour reproduction, where they are usually done in-house.

Gamut Mapping

Talk about fire engines: NTSC phosphor chromaticities

Here are the heuristics used by printers

  1. Get the neutral colours neutral
  2. Get the TRC of the white axis right
  3. Maximize luminance contrast
  4. Keep most colours within the destination gamut
  5. Minimize shifts in hue and saturation
  6. Maximize the saturation

Manipulating TRCs

What is a TRC?

Ways of adjusting TRCs

Adjusting a TRC

Could different primaries have different TRCs?

In Summary

  1. Computers are far from being able to match the performance of humans when it comes to high-quality gamut mapping.
  2. It's still true that one-off printing of no particular quality, is still better done by humans than computers.
  3. Computers, however, provide tools that take over much of the routine work previously done by humans.
  4. Digital cameras do a large amount of routine gamut mapping without human intervention, assisted by very simple and regular colour gamuts. This is device-to-device colour mapping, opposed to image-to-device colour mapping.

Colour Spaces

Suppose you are on the beach.

The four categories below represent increasingly objective/mathematical ways of describing how you have arranged the colours.

1. Based on Substances

A Short History of Pigments

Stone Age


Desire for a nice bright red. `Bright' means two things

  1. high in luminance compared to the objects that surround it
  2. high in saturation compared to the objects that surround it

If you ask observers to equalize the brightness of differently hued colours, they do not give you equal luminance.


Reluctance to mix pigments

Mixing pigments that are dull in colour makes colours that are more dull.



Early Nineteenth Century Inorganic Chemistry


defined by the vivid colour of chlorphyll

Three greens that co-existed into the coal tar revolution

  1. Emerald green
  2. Viridian
  3. Prussian blue

Late Nineteenth Century Organic Chemistry


At present almost all pigments are manufactured from organic raw materials.

The colours are

Colour spaces built on artist's colours

Colour wheels with pigment names are common in the late nineteenth century.

2. Based on Samples

These have a variety of different purposes

  1. Colour specification: Munsell, OSA
  2. Measurement of colour differences: Munsell, OSA
  3. Creation of colour harmonies: Goethe, Ostwald, OSA



Artist's Colour Spaces

3. Device Dependent

Additive Primaries


HSV, HLS, HSB, HVC, etc.


Coordinates are r, g & L

R+G+B = (1 + min(r,g))L. Do I believe this?


Subtractive Primaries



4. Device Independent

Based on instrumental measurement

Tristimulus Values

Chromaticity Coordinates

5. 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!

Lot's of ways to do colour difference experiments

Question 1

Question 2.

Answers to these two questions 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
  3. Lab

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.

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