Color models

Color Models (Color Space, Color System)

Presentation withDR . NAHLA El HAGAR

Prepared by Mohamed Sweelam .

References

• R.C. Gonzales and R.E. Woods, Digital Image Processing, Addison-Wesley, Reading, 1992.

• I. Newton, Opticks, 4th Edition, Dover, New York, 1704/1952.

• Gur, E., Zalevsky, Z., 2007, Single-Image Digital Super-Resolution A Revised Gerchberg Papoulis Algorithm, international Journal of Computer Science.

• WWW of course ^_^ .

Over view

Color image processing is a very importantfield with a long history :-• – What are color models?• – What are color spaces and how are they

used?• – What new techniques are possible by

working with color pixels directly?• What’s hue, saturation, and brightness?

The Meaning

• The color model also called ( Color system or space color ) is a specification of a coordinate system and subspace within the system where each color is represented by single point .

S = T( f(r) ) .

History of Colors

Colors Type

• RGB : is the most use now days and called 24-bits color Image .

• Gray Scale : this type has a range [0-255] and called 8-bits color Image.

• Binary : knows as ( White and Black ) and has a range [ 0 , 1 ] and called 1-bits Image.

Light And The Electromagnetic Spectrum

•Light is just a particular part of the electromagnetic spectrum that can be sensed by the human eye•The electromagnetic spectrum is split up according to the wavelengths of different forms of energy.

Human with Colors

•The lens focuses light from objects onto the retinaThe retina is covered with light receptors called cones (6-7 million) androds (75-150 million)•Cones are concentrated around the fovea and are very sensitive to colour•Human eyes have three types of cone receptors that detect light in Red , Green and Blue .

RGB representation

• We can imagine the RGB as a 3-dimensions .• RGB is our Goal from Now to the end.• Actually RGB is three layers behind each other but just for imagining .

Hue, Saturation and Brightness

• Hue, saturation, and brightness are aspects of color in the red, green, and blue ( RGB ) scheme. These terms are most often used in reference to the color of each pixel in a cathode ray tube ( CRT ) display. All possible colors can be specified according to hue, saturation, and brightness (also called brilliance ), just as colors can be represented in terms of the R, G, and B components.

Hue

• Most sources of visible light contain energy over a band of wavelengths. Hue is the wavelength within the visible-light spectrum at which the energy output from a source is greatest. This is shown as the peak of the curves in the accompanying graph of intensity versus wavelength. In this example, all three colors have the same hue, with a wavelength slightly longer than 500 nanometers, in the yellow-green portion of the spectrum.

• • Hue is the actual color. It is measured in angular degrees counter-clockwise around the cone starting and ending at red = 0 or 360 (so yellow = 60, green = 120, etc.).

Hue … Cont.

• You also perceive color changing along two other dimensions. One of the dimensions is lightness-darkness. How light or dark a color is is referred to either as a colors lightness or value. In terms of a spectral definition of color, value describes the overall intensity or strength of the light. If hue can be thought of as a dimension going around a wheel, then value is a linear axis like an axis running through the middle of the wheel

Saturation• The last dimension of color that describes our response to color

is saturation. Saturation refers to the dominance of hue in the color. On the outer edge of the hue wheel are the 'pure' hues. As you move into the center of the wheel, the hue we are using to describe the color dominates less and less. When you reach the center of the wheel, no hue dominates. These colors directly on the central axis are considered desaturated. These desaturated colors constitute the grayscale; running from white to black with all of the intermediate grays in between. Saturation, therefore, is the dimension running from the outer edge of the hue wheel (fully saturated) to the center (fully desaturated), perpendicular to the value axis (Figure 9). In terms of a spectral definition of color, saturation is the ratio of the dominant wavelength to other wavelengths in the color. White light is white because it contains an even balance of all wavelengths.

Saturation Cont.

• Saturation is the purity of the color, measured in percent from the center of the cone (0) to the surface (100). At 0% saturation, hue is meaningless.• That is mean if you reach to the pure color there is no Conflict of the saturate of this Color .Like Green , Blue and Red .

Brightness

• • Brightness is measured in percent from black (0) to white (100). At 0% brightness, both hue and saturation are meaningless.

• Brightness is a relative expression of the intensity of the energy output of a visible light source. It can be expressed as a total energy value (different for each of the curves in the diagram), or as the amplitude at the wavelength where the intensity is greatest (identical for all three curves). In the RGB color model, the amplitudes of red, green, and blue for a particular color can each range from 0 to 100 percent of full brilliance. These levels are represented by the range of decimal numbers from 0 to 255, or hexadecimal numbers from 00 to FF.

Summary.HSBStart with the foreground color HSB panel (lower left), with the default setting: saturation and brightness 0%, hue 0° (black).Move the Saturation slider from 0% to 100% and back down to 0%. Nothing happens, since brightness is still 0%.Do the same with the Hue control: move it around the circle and again nothing happens without brightness. Return it to the 0° position or simply type 0 in the Hue text box.Now raise Brightness gradually from 0% to 100%. Hue is still irrelevant, since there is no saturation; you are watching a grey scale as the color (center panel) goes from black to white. Notice that the RGB sliders (top panel) are moving together, since grey or white is produced by equal amounts of red, green, and blue light. Leave brightness at full value.