CS781 - Colour for Computer Graphics - Winter 2009

Lecture 15

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Creating Colour: Illuminated Devices

Illuminated devices make colour by subtracting light from a source of illumination.

Subtractive Colour

It's called subtractive colour, but the simplest examples all involve multiplication.

Laser Printer

Charging the photoconductor

Adding toner

The fuser

The PostScript engine

Half-tone Printing

From a laser printer

From a printing press

How a printing press works

1. Laser printer, or film recorder, writes a transparency at very high resolution (2400 dpi/100dpmm or higher)
2. Metal plates are cast, cylindrical for high volume printing
   • flat for letterpress
3. Three stage process
   • cleaning
   • inking
   • pressing to paper
4. Paper moves continuously in high volume press
   • page at a time for letterpress

Continuous-tone Printing

Technologies

• rotogravure: variable depth ink wells
• dye sublimation
• ink-jet

Photography

• random sizes of dots
• random locations of dots

`Random' is likely to have a not-yet-understood connection to hyperacuity

Colour Printing

Do it three or four times

• three means process black
  • too much ink on the paper
• four uses black ink
  • black usually printed last
  • under-colour removal

Offset

Half-tone

• Each colour pass is offset with respcet to the other colour passes

Calculating colour

A pixel is covered by several differently coloured areas

• eight for three colour printing
  1. white paper
  2. cyan
  3. magenta
  4. yellow
  5. cyan + yellow = green
  6. magenta + yellow = red
  7. cyan + magenta = blue
  8. cyan + magenta + yellow = process black
• nine for four colour printing
  • added colour is black

The resulting colour

1. Measure the colour of each
2. Sum the fractional area covered times the colour
3. Random coverage is usually used as an assumption
4. E.g. if cyan = magenta = 0.5 yellow = 1.0 fractions are
   1. white 0
   2. cyan 0
   3. magenta 0
   4. yellow 1/4
   5. green 1/4
   6. red 1/4
   7. blue 0
   8. process black 1/4

Compare this to how we make colours with additive mixture

• Model the difference using spectral power distributions
  • filter models
  • relationship to Beer's law
• Saturated colours are darker with subtractive colour
• Nature is like this. Why?

A smart printer could receive PostScript with calibrated colour

• Use an internal calibration to print
• Internal calibration normally uses exhaustive measurement
  • plus tri-linear interpolation.

Continuous Tone

Dry printing versus wet printing

• wet printing is unpredictable without a lot of standardization

Dry printing

• slower
• more expensive
• relatively predictable

Even so, filter models are not good enough for colour quality

• exhaustive measurement is the normal procedure

Commercial printing is different.

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