Color MixingThere are two ways to control how much red, green, and blue light reaches the eye:

“Additive Mixing” Starting with black, the right amount of red, green, and blue light are ‘added’ to an image.

“Subtractive Mixing” Starting with white, the right amount of red, green, and blue light are ‘subtracted’ from an image.

Additive Color Mixing

Mixing the three color sources is known as “additive mixing” to distinguish it from mixing paints or dyes (“subtractive mixing”).

By exciting the red, green, and blue sensitive cones, any color can be produced by adding together the three additive primaries (R,G,B).

Additive Color Mixing

For example, when blue and green lights overlap, the blue and green cones are illuminated, and we perceive cyan

green + blue = cyan red + blue = magenta

red + green = yellow

Additive Color Mixing

red + green + blue = white

red + green = yellow

Additive Color Mixing

red + green + blue = white

red + green/2 = orange red/2 + green = lime

red + green + blue = grayred + green + blue = gray

Additive Color Reproduction

Color video projectors use additive color mixing—Projected red, green, and blue images contribute

RGB components to create color images

R

G B

In addition to the superposition method described above, there are two other methods of mixing R, G, & B primaries.

- Spatial mixing (as in color TV)

- Temporal mixing (as in digital cinema)

Both rely on limitations of the visual system;

Additive Color Mixing Methods

Because the visual system has limited spatial resolution, small areas of different colors are mixed perceptually.

Spatial Mixing (Video Monitor)

x

y

Spatial addressability of typical monitors goes from (640 x 480) to (1600 x 1280) pixels.

Because the visual system has limited temporal resolution, rapidly changing colors are mixed perceptually.

Temporal Mixing (Digital Cinema)

time

time

time

Color Monitors A number of color monitors exist in most digital color

document systems.—Different color monitors are likely to display the same

digital file differently.

Subtractive Color Mixing

Color hardcopy devices can’t use additive mixing because they aren’t sources of light; they can’t add Red, Green, or Blue components.

Instead, they use subtractive mixing. Starting with white light reflected by the substrate, they subtract the unwanted red, green, and blue components using cyan, magenta, and yellow colorants.

Subtractive Color Mixing

cyan colorant“minus red”

b+r =

m

White light

magenta colorant“minus green”

g+b =

c

White light

White light

r+g =

y

yellow colorant“minus blue”

The goal is the same; to control the amount of Red, Green, and Blue light getting to the eyes’ three cone types

Each colorant absorbs 1/3 and transmits 2/3 of white light

white substrate

Subtractive Color Mixing

Other colors are made by varying the amount of colorant in each layer.

yellow magentayellow

+ magenta/2orange

yellow & magenta = red

White light

r+g/

2 = or

ange

White light

White light

+ cyan

black

Subtractive Color Reproduction

Color printing uses subtractive color mixing.

Adding black allows more accurate grays, and conserves the more expensive CMY colorants.

C Y

M K

Subtractive Color Imaging

Colors are rendered by different mixtures of cyan, magenta, and yellow inks printed.—Gradations in each channel can be achieved by

halftone marking.

Contonegrayscale

Halftonegrayscale

Subtractive Color Imaging

Process color printing is an example of subtractive color mixing—The spatial addressability of typical printers

goes from 400 spots/in to 3,600 spots/in.

C Y

M K

Subtractive Color Imaging

Assumptions:—White substrate (or paper) is used

It reflects all red, green, and blue light

—Process inks are semi-transparent Each ink absorbs ~1/3 of the visible spectrum

cyan subtracts red, transmits green and bluemagenta subtracts green, transmits red and blueyellow subtracts blue, transmits red and green