SEMINAR ON

“COLOR MODEL”

Submitted by: Sadhana Singh

Shri Ram Murti Smarak College of Engg. And Tech.Bareilly

Color Color is a psychophysical concept depending both upon the spectral distribution of the radiant energy of the illumination source and the visual sensations perceived by the viewer Color perception depends mainly upon the physics of light and the physiology of the visual system, which results in the following psychological color sensations:

•hue: the color sensation associated with different parts of the spectrum such as red, yellow, or blue•saturation: the color sensation corresponding to the degree of hue in a color•brightness is the primary visual sensation

Color Science Light is an electromagnetic wave. Its color is characterized by the

wavelength content of the light. Laser light consists of a single wavelength: e.g., a ruby

laser produces a bright, scarlet-red beam. Most light sources produce contributions over many

wavelengths. However, humans cannot detect all light, just

contributions that fall in the “visible wavelengths”. Short wavelengths produce a blue sensation, long

wavelengths produce a red one. Visible light is an electromagnetic wave in the range 400 nm

to 700 nm (where nm stands for nanometer, 109 meters).

Human Vision The eye works like a camera, with the lens focusing

an image onto the retina (upside-down and left-right reversed). The retina consists of an array of rods and three kinds of cones. The rods come into play when light levels are low and produce an

image in shades of gray For higher light levels, the cones each produce a signal. Because of

their differing pigments, the three kinds of cones are most sensitive to red (R), green (G), and blue (B) light.

Image Formation

Surfaces reflect different amounts of light at different wavelengths, and dark surfaces reflect less energy than light surfaces.

Fig. shows the surface spectral reflectance from (1) orange sneakers and (2) faded bluejeans. The reflectance function is denoted S().

Image Formation

Image Formation

So image formation involves an illuminant with SPD E() reflects off a surface with spectral reflectance function S() and is filtered by the eyes’ cone functions q() as shown in figure 4.5

The function C() is called the color signal and is formed by the product of the illuminant and the reflectance

Color Systems

Combinations of three primary colors can match any unknown color for observers with normal color vision

Often, we choose red, green, and blue as the three primary colors, and we can then represent some color C by a mixture of red, green, and blue:

C = rCR + gCG + bCB

RGB is the color model (a conceptual system for specifying colors numerically) used in computer monitors

This model is additive

Color Systems

CMYK is the color model used by printing presses This model is subtractive

Light is absorbed, or subtracted by cyan, magenta, and yellow ink

In process-color printing, layers of translucent inks are used, each subtracting certain colors of light Colors that are not absorbed pass through to the paper below which reflects all

color For example, magenta ink looks magenta because it allows magenta light to

pass through but absorbs all other colors

RGB Color Model for CRT Displays We expect to be able to use 8 bits per color channel

for color that is accurate enough. However, in fact we have to use about 12 bits per

channel to avoid an aliasing effect in dark image areas - contour bands that result from gamma correction.

For images produced from computer graphics, we store integers proportional to intensity in the frame buffer. So should have a gamma correction LUT between the frame buffer and the CRT.

If gamma correction is applied to floats before quantizing to integers, before storage in the frame buffer, then in fact we can use only 8 bits per channel and still avoid contouring artifacts.