Color Fundamentals

Fasih ur Rehman

Color FundamentalsWhite light consists of a continuous spectrum of colors ranging from violet to red.

Color SpectrumBand of visible light is relatively narrow in the band of frequencies in the electromagnetic spectrum.

Perception• The colors that humans perceive of an object

are determined by the nature of the light reflected from the object.– Green objects reflect light with wavelengths in the 500

to 570 nm, and absorb those at other wavelengths.• The light is visible to human eyes if its

wavelength is between 380-780 (nm). • If the light is achromatic, its only attribute is

intensity.– The term gray level refers to a scalar ranging from

black to white.

Perception (Cont.)

Primary Colors

• The cone cells in human eye can be divided into three categories, corresponding roughly to red, green and blue (Figure 6.3).

• Due to these characteristics of the human eye, colors are seen as variable combinations of the primary colors red (700 nm), green (546.1 nm), and blue (435.8 nm).– Standardized in 1931.– This standardization does not mean these three

primary colors can generate all spectrum colors.

Secondary Colors• The primary colors can

be added to produce the secondary colors of light: Cyan, Magenta, Yellow.

• The primary colors of pigments are cyan, magenta, and yellow, while the secondary colors are red, green, and blue.

More Fundamentals• The characteristics generally used to distinguish

one color from another are hue, saturation, and brightness.  – Hue: associated with color as perceived by an observer.– Saturation: relative purity or the amount of white light

mixed with a hue.– Brightness: intensity of light.

• Hue and saturation are taken together are called chromaticity; therefore, a color can be characterized by its chromaticity and brightness.

Color Diagram

Chromaticity Diagram

• Has superior performance over other color transforms especially in clustering of color distribution and estimate of color difference

• Shows color as a function of x (red) and y (green)

• Useful for color mixing• Boundary of the diagram shows fully

saturated • Equal energy point

Color Models• The color model (color space or color system) is

to facilitate the specification of colors in some standards.

• Color model is a specification of a coordinate system and a subspace within the system where a color is represented.– RGB: color monitor– CMY (cyan, magenta, yellow): color printing– HSI (hue intensity and saturation): decouple the color

and gray-scale information.

RGB Color Model

• Images represented in the RGB color model consist of three component images, one for each primary image

Color Planes

The RGB Color Model• The number of bits used to represent each pixel in RGB

space is called the pixel depth.• The term full-color image is used to denote a 24-bit RGB

color image.

The CMY Color Model• Suppose colors in RGB are normalized in [0, 1].

The RGB to CMY conversion is given by

• Instead of adding C,M, and Y to produce black, a fourth color black is added, giving rise to the CMYK color model.

BGR

YMC

111

The HSI Color Model• Human describes color in terms of hue,

saturation and brightness.– Hue: describe the pure color, pure yellow,

orange, green or red.– Saturation measures the degree to which a

pure color is diluted by white light.– Brightness is a subjective descriptor difficult to

be measured.• Comparison:

– The RGB model is ideal for image color generation.

– The HSI model is an ideal tool for developing image processing algorithms based on color descriptions

Conversion (RGB to HSI)

• Consider a color point in the RGB color cube.– Intensity: find the intersection on the intensity

axis with a perpendicular plane containing the color point.

– Saturation: The distance of the color point to the intensity axis.

• The saturation on the intensity axis is zero.– Hue: consider the triangle enclosed by white,

black, cyan. The color on this triangle is a mixture of these three colors

Conceptual Relationship RGB - HSI• All pointes contained in the plane segment are defined by the

intensity and boundary of the cube have the same hue

Hue Measurement

The HSI Color Model

Conversions• From RGB to HSI

• S=1-[3/(R+G+B)][min(R, G, B)]• I=(R+G+B)/3

GBifGBif

H

360

2/12

1

)])((()[()]()[(2/1cosBGBRGR

BRGR

Conversions (Cont.)

• RG sector (0<H<120)B = I(1-S)

G = 3I-(R+B)

)60cos(

cos1H

HSIR o

Conversions (Cont.)

• GB sector (120≤H<240)– First, let H = H -120

R=I(1-S)

B=3I-(R+G)

)60cos(

cos1H

HSIG o

Conversions (Cont.)

• BR sector (240 ≤ H ≤ 360)– First, let H = H -240

G = I(1-S)

R = 3I-(G+B)

)60cos(

cos1H

HSIB o

The HSI Color Model