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
  3. Three stage process
  4. Paper moves continuously in high volume press

Continuous-tone Printing

Technologies

Photography

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


Colour Printing

Do it three or four times

Offset

Half-tone

Calculating colour

A pixel is covered by several differently coloured areas

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

A smart printer could receive PostScript with calibrated colour

Continuous Tone

Dry printing versus wet printing

Dry printing

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

Commercial printing is different.


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