Chapter 9: Color• What is color?

• Color mixtures– Intensity-distribution curves– Additive Mixing– Partitive Mixing

• Specifying colors– RGB Color – Chromaticity

What is Color?• Wavelength is a property of an electromagnetic

wave in the frequency range we call “light”

• Color is a psychological phenomenon that occurs when light waves of different wavelengths interact with the human visual system

Light wave with wavelength 650 nm

Human eye

Human visual system and brain

Color

Color Mixtures• A spectral color is a single line on this graph:

Green laser pointer532 nm

Color Mixtures• Almost everything we see is not a spectral color

but a mixture of many spectral colors.

Intensity-Distribution Curves

Both of these lights would appear white, so we can see there may be several intensity distributions that generate the same color

Additive Color Mixing

• How can we describe the sensation of a mixture of lights?

Additive Primary Colors

• Red, green and blue are called the additive primaries

• We want to select primary colors that allow us to create the largest possible number of other colors using just those primary colors

• Most colors can be described in terms of mixtures of red, green and blue

cyan

magenta

yellow

650-nm red

530-nm green

460-nm blue

Additive Mixing: Yellow

Additive Mixing: Cyan

Additive Mixing: Magenta

RGB Color Specification• RGB (red green blue) is

another way of specifying colors.

• White is made by maximum saturation of all three colors

• Instead of hue, saturation and brightness, you can specify the amount of each color

cyan

magenta

yellow

650-nm red

530-nm green

460-nm blue

RGB Colors: Example

Hue: 35°Saturation: 100%Brightness: 100%

RGB Colors: Example

Hue: 60°Saturation: 100%Brightness: 35%

RGB Colors: Example

Hue: 60°Saturation: 0%Brightness: 40%

Complementary Colors• We’ve seen that red light plus green light plus

blue light equals white light:

R + G + B = White• But we also know that red light plus green light

equals yellow light:

R + G = Y• If we do some dodgy “color math”, we get that

yellow light plus blue light equals white light:

Y + B = White

Complementary Colors• We then define blue and yellow as complementary

colors:

• When additively mixed, complementary colors produce white.

Additive mixingNOT additive mixingNOT additive mixing

Concept Question

What is the complementary color to green?

A. RedB. MagentaC. BlueD. YellowE. Cyan

cyan

magenta

yellow

650-nm red

530-nm green

460-nm blue

Recall: R + G + B = WhiteWe want: G + ?? = White

Chromaticity

This is a chromaticity diagram (see color plate 9.2 in the book)We are not going to discuss where this comes from, please read that section of the book

Chromaticity

•A chromaticity diagram has a fixed brightness or lightness for all colors

•The most saturated colors (of different hues) are around the rim (perimeter)

•Inside are the less saturated colors, including white at the interior

Saturated colors

Unsaturated colors

Chromaticity

•The colors on the curved part of the edge are wavelength colors

•The colors on the straight part are non-wavelength colors, but are still 100% saturated.

Wavelength colors

Non-wavelength colors

Chromaticity: Lightness

saturationsaturation

lightne

sslightne

ss

lightne

sslightne

ss

huehue

saturationsaturation

huehue

Uses of Chromaticity Diagrams

• To identify colors with three numbers

• To predict the results of additive mixing

• To understand complementary colors

• To find the dominant hue of a color

• To understand color gamuts and the significance of RGB mixing

Chromaticity: Identifying Colors

• The numbers that we use to identify a color are its x-value and y-value inside the diagram and a z-value to indicate its brightness or lightness

• x and y specify the chromaticity of a color• Example: Apple pickers are told

around the country that certain apples are best picked when they are a certain red (see black dot)

• x = 0.57• y = 0.28

Chromaticity: Identifying Colors

• The "purest" white is at

x = 0.33 y = 0.33

Chromaticity: Color Mixtures• An additive mixture of two

wavelength colors lies along the line joining them

• Example: The colors seen by mixing 700 nm red and 500 nm green lie along the line shown

• Where along the line is the color of the mixture?

Chromaticity: Color Mixtures• This depends on the relative

intensities of the 700 nm red and the 500 nm green.• Much more green than red

gives a green• Much more red than green

gives an orange-red• Slightly more red than green

gives a yellow

• This also works for mixing colors not at the edges

Concept Question: Chromaticity

What color would a mixture of these two colors (green and blue), equal amounts of each, look like?

A. YellowB. RedC. PinkD. CyanE. Green

Chromaticity: Complementary Colors

• Recall that a color combined with its complement produces white

• If mixtures lie on the line between the two colors, then we can find the complement by drawing a line through white to the other side

Chromaticity: Complementary Colors

• Using this diagram, we can see that the complement of 700 nm red is 490 nm cyan

• And the complement of 520 nm green is magenta (a non-wavelength color)

Concept Question: Chromaticity

What is the complementary color of blue, shown by the black dot?

A. YellowB. RedC. PinkD. CyanE. Green

Chromaticity: Dominant Hues

• Unsaturated colors are related to their saturated counterparts by the additive mixture with white

• Ex: Pink (unsaturated) = red (saturated) + white

Chromaticity: Dominant Hues

• To find the dominant hue of the color indicated by the black dot • Draw straight line from

white through the point to get dominant wavelength, and hence, hue (547 nm green)

Chromaticity: Dominant Hues

• If hue is in the non-wavelength purples, find the complementary dominant wavelength by extending backwards through white (get 495 nm cyan)

• There is no dominant wavelength color for magentas and purples

Concept Question: Chromaticity

What is the dominant hue of the color shown by the black dot?

A. YellowB. RedC. PinkD. CyanE. Green

Chromaticity: Color Gamuts

• The gamut of colors which can be reproduced by an additive mixture of red, green and blue light is inside the triangle we now draw at right.

• 530-nm green• 460-nm blue• 650-nm red

Chromaticity: Color Gamuts

• Other gamuts are generally even smaller

• Say we picked red blue and yellow, the artists primaries:

• We would not be able to make cyans or greens

Concept Question: Color Gamuts

If we selected our primaries as the three black dots shown, which colors would we NOT be able to make by combining them? In other words, which color is OUTSIDE the color gamut generated by these three primaries?

A

CD

B

E

Color Gamut of Flat Panel Displays

The intensity distribution of the primary colors used in an LED backlit display

The color gamuts of LED vs. CCFL backlit displays

Display Pixels: Partitive Mixing

• In additive mixing, we assume that the different wavelength colors reaching your eye are all coming from the same place:

Display Pixels: Partitive Mixing

• What it instead, we make the different colors coming from separate, very small, very closely spaced points

• You eye cannot see them as separate sources, so the colors mix and you see the same color. This is called partitive mixing

Partitive Mixing: Pointillism

Partitive Mixing: Pixels