Three-Receptor Model

• Designing a system that can individually display thousands of colors is very difficult

• Instead, colors can be reproduced by mixing an appropriate set of three primary colors– It has been discovered that there are three different types of

cone cells in the human retina. When light falls on the retina, it excites the cone cells. The excitation of different types of cone cells determines the color seen by the observer

– See http://colorvisiontesting.com/ for more information on color-blindness

• The RGB color model is an additive color model in which red, green, and blue light are added together in various ways to reproduce a broad array of colors. The name of the model comes from the initials of the three additive primary colors, red, green, and blue

• RGB is a device-dependent color model: different devices detect or reproduce a given RGB value differently, since the color elements (such as phosphors or dyes) and their response to the individual R, G, and B levels vary from manufacturer to manufacturer, or even in the same device over time. Thus an RGB value

• Each of the three beams is called a component of that color, and each of them can have an arbitrary intensity, from fully off to fully on, in the mixture.

Additive model

• The RGB color model is additive in the sense that the three light beams are added together, and their light spectra add, wavelength for wavelength, to make the final color's spectrum

• The zero intensity for each component gives the darkest color (no light, considered the black), and full intensity of each gives a white;

• When the intensities for all the components are the same, the result is a shade of gray, darker or lighter depending on the intensity. When the intensities are different, the result is a colorized hue, more or less saturated depending on the difference of the strongest and weakest of the intensities of the primary colors employed

• The main purpose of the RGB color model is for the sensing, representation, and display of images in electronic systems, such as televisions and computers, though it has also been used in conventional photography

Perceptual Attributes of Color• Brightness

– Perceived luminance

• Saturation– Human’s impression of how different the color is from

an achromatic (white or gray) color.

Hue• In an RGB color space, hue can be thought of as

an angle φ in standard position. • To calculate φ, let R, G, B be the color coordinates

in RGB space, defined on a scale from zero to one. Then, after obtaining the brightness μ and the saturation σ, the hue could be obtained from

• Ranges from 0° to 359° when measured in degrees

Tint

• If you tinted a color, then you've added white to the original color, A tint is lighter than the original color.

• When used as a dimension of a color space, tint can be the amount of white added to an original color. In such a color space a pure color would be non-tinted.

Shade

• When used as a dimension of a color space, shade can be the amount of black added to an original color. In such a color space a pure color would be non-shaded.

Saturation

• From a perceptional point of view saturation influences the grade of purity or vividness of a color/image. A desaturated image is said to be dull, less colorful or washed out but can also make the impression of being softer.

Lightness

• Lightness defines a range from dark (0%) to fully illuminated (100%). Any original hue has the average lightness level of 50%.

What is the deference between device dependent anddevice independent color space?

• A device dependent color space is a color space where the resultant color depends on the equipment and the set-up used to produce it.– rgb = (250,134,67) will be altered as you vary the

brightness and contrast on your display.– Changing the red, green and blue phosphors of your

monitor will have slightly deferent characteristics and the color produced will change.

Thus RGB is a color space that is dependent on the system being used, it is device dependent.

Device independent color space

• A device independent color space is one where the coordinates used to specify the color will produce the same color wherever they are applied. An example of a device independent color space is the CIE L*a*b* color space ((known as CIELAB)

What is a color gamut ?• A color gamut is the area

enclosed by a color space in three dimensions. It is usual to represent the gamut of a color reproduction system graphically as the range of colors available in some device independent color space.

Color Matching

• The science of color measurement is known as colorimetry.

• Some laws for color matching– Any color can be matched by mixing at most three

colored lights– The luminance of a color mixture = sum of the

luminance of its components– Color Addition: If colors A & B match with colors C &

D, respectively, then color (A+B) matches color (C+D).– Color Subtraction: If color (A+B) matches color

(C+D), and color A matches color C, then color B matches color D.

Additive Color Mixing

400 500 600 700 nm

400 500 600 700 nm

red

gree

n

Red and green make…

400 500 600 700 nm

yell

ow

Yellow!

When colors combine by adding the color spectra (w.r.t. cones excited). Example color displays that follow this mixing rule: CRT phosphors, multiple projectors aimed at a screen, Polachrome slide film.