Color

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Slide 33

How to compute the weights of the primaries to match any new spectral signal?

?

Given: a choice of three primaries and a target color signal

Find: weights of the primaries needed to match the color signal

p1 p2 p3

Slide 34

Computing color matches

Given primaries

Estimate their color matching functions: observer matches series of monochromatic lights, one at each wavelength.

To compute weights for new test light, multiply with matching functions.

Slide 35

Foundations of Vision, by Brian Wandell, Sinauer Assoc., 1995

Slide credit: W. Freeman

Rows of matrix C

Computing color matches

Example: color matching functions for RGB

Slide 36

Arbitrary new spectral signal is linear combination of the monochromatic sources.

t

Computing color matches

Color matching functions specify how to match a unit of each wavelength, so:

Slide 37

Why is computing the color match for any color signal for a given set of primaries useful?

Want to paint a carton of Kodak film with the Kodak yellow color.

Want to match skin color of a person in a photograph printed on an ink jet printer to their true skin color.

Want the colors in the world, on a monitor, and in a print format to all look the same.

Adapted from W. Freeman

Computing color matches

Image credit: pbs.org

Slide 38

Today: Color

Measuring color

Spectral power distributions

Color mixing

Color matching experiments

Color spaces

Uniform color spaces

Perception of color

Human photoreceptors

Environmental effects, adaptation

Using color in machine vision systems

Slide 39

Standard color spaces

Use a common set of primaries/color matching functions

Linear color space examples

RGB

CIE XYZ

Non-linear color space

HSV

Slide 40

RGB color space

Single wavelength primaries

Good for devices (e.g., phosphors for monitor), but not for perception

RGB color matching functions

Slide 41

CIE XYZ color space

Established by the commission international d’eclairage (CIE), 1931

Y value approximates brightness