The document summarizes key concepts about color models. It discusses how color is perceived by the human visual system and depends on light wavelength and intensity. Common color models like RGB and CMYK are described. RGB is an additive model used in displays where combinations of red, green, and blue lights create colors. CMYK is a subtractive model where inks absorb colors to create colors by reflecting what is not absorbed. The document also discusses how images are formed through light reflection and the eye's cone receptors, and notes the RGB color model requires around 12 bits per channel to avoid aliasing effects in dark image areas on CRT displays.

1. SEMINAR
ON
“COLOR MODEL”
Submitted by:
Sadhana Singh
Shri Ram Murti Smarak College of Engg. And Tech.
Bareilly

2. 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

3. 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).

4. 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.

5. 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().

6. Image Formation

7. 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

8. 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

9. 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

10. 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.