cs781 - Colour for Computer Graphics - Winter 2012

Course Notes

Lecture 17 - Colour Devices: Subtractive Colour

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Subtractive Colour

Subtractive colour is defined accord to its mechanism of production. Two examples.

1. Transmission (If you prefer mathematics that looks like mathematics see this.)
   • Transmittance of a translucent filter is defined to be
     • T(\lambda) = I(\lambda) / \Phi(\lambda), where
       • I(\lambda) is the light entering the medium.
       • \Phi(\lambda) is the light exiting the medium
     • Density is a useful concept
       • Show the relationship between optical density, absorption and transmittance
2. Reflection
   • Show the similarity between the mathematics of transmission and the mathematics of reflection

It's called subtractive colour

• because light is removed from the incoming light

Characteristics of Subtractive Colour

• Secondaries are darker than primaries
  • opposite of additive colour
  • "A mixture of colours is darker than either of the mixed colours," is the generalization.
• Saturated colours are darker than desaturated colours
  • the same as subtractive colour

Calibrating Subtractive Colour

Combined with additive mixture (e.g. half-tone printing) model-based calibration seems possible, but ...

1. detailed models depend strongly on
   • ink colour
   • paper
   • screen pattern
   • screen rotation
   • lighting
   • etc.
2. approximate models, such as
   • random coverage

Laser Printers

Monochrome

Colour

Half-tone Printing

Laser Printer

process black versus separate black screen

Commercial Press

1. Solid colour,
   • 96 dots per mm
     • compare monitor, 4 dpmm
     • laser printer, early, 10 dpmm
     • laser printer, modern, 24 dpmm
   • serifs lightly curved
2. Originally printed in solid colour,
   • three inks: red, yellow, black

   Reproduced in half-tone (Why?)

   • look at the type through the loupe
   • and with the naked eye
3. B&W, half-tone colour
   • look at the eyes: through the loupe, naked eye
4. Cloth in half-tone
   • competing patterns at comparable scales
5. Compare to 3.
   • these half-tones were done by a human, not a camera
   • look at the eyes
   • look at the edge of the blue jacket

Continuous-tone Printing

Continuous tone is what you have with most additive devices

• half-tone:
  • only colour or no colour
  • intermediate colours average colour areas with no colour areas, so they are not uniform
  • pseudo-pixels must be bigger than the resolution of the device
• continuous-tone
  • `continuous' range of colours
  • no averaging needed
  • pseudo-pixels need be only as big as the number of colour channels

Examples

• rotogravure
• dye sublimation
• ink-jet

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