cs781 - Colour for Computer Graphics - Winter 2012

Course Notes

Lecture 18 - Calibrating Subtractive Colour

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  1. Projects
  2. Tables of useful colour data

Continuous Tone Subtractive Devices

Digital

Continuous tone is what you have with most additive devices

Examples

Craft

Photography

How is it different?

Nostalgia for Kodachrome

Calibrating Subtractive Colour

Definition. Given

you can create a mapping from CMYK --> XYZ. With a little trouble, and a few workarounds, you can make it invertible. The problem is ensuring that it will be the same tomorrow. Then you can use

Adobe put something like this into PostScript under the name `colour model'.

Making the Mapping

The stability assumption is the hard one.

Using Brute Force

  1. Print a calibration sheet with 8x8x8 (=512) colours using the printer's CMY --> CMYK conversion function
  2. Using the standard illumination measure XYZ for each sample.
  3. Usually assuming monotonicity, fit the data to something smooth.
  4. Rebuild the tabular function.
  5. Use trilinear interpolation to interpolate in the table.
  6. Use interpolation to build the reverse table.

Using a Model

Think about the actual process of making colour. For example,

  1. The CMY you give an ink-jet printer is converted internally to CMYK.
  2. Each pass adds some pigment to the paper.
  3. Measure how much pigmented ink is added to the paper in each channel
  4. Using an ink-mixing model like Kubelka-Munk to find the result. (You can find many recent papers tweaking such models for the properties of inks on paper.)

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

When the gamuts don't fit,


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