CS781 - Colour for Computer Graphics - Winter 2009

Lecture 17

Colour Gamuts

Important Features

Neutral Axis

Tone Reproduction Curves (TRCs)

What is a TRC

  1. Neutral
  2. Exaggerate shadows
  3. Exaggerate highlights
  4. Exaggerate mid-tones
  5. Minimize mid-tones

Highly Saturated Colours

Additive Devices

Overall Shape of Gamut

Colour Spaces

RGB

HSV

HLS

RGYB

YIQ

NTSC colour encoding

Subtractive Devices

Paints

Printing Inks

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.

Colour Reproduction (aka 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

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