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1

LECTURE NOTES

ON

DIGITAL IMAGE PROCESSING

PREPARED BY

DR. PRASHANTA KUMAR PATRA

COLLEGE OF ENGINEERING AND TECHNOLOGY, BHUBANESWAR 2

Digital Image Processing

UNIT-I

DIGITAL IMAGE FUNDAMENTALS AND TRANSFORMS

1. ELEMENTS OF VISUAL PERCEPTION

1.1 ELEMENTS OF HUMAN VISUAL SYSTEMS

ͻ The following figure shows the anatomy of the human eye in cross section ͻ There are two types of receptors in the retina

The rods are long slender receptors

The cones are generally shorter and thicker in structure ͻ The rods and cones are not distributed evenly around the retina.

ͻ Rods and cones operate differently

Rods are more sensitive to light than cones.

At low levels of illumination the rods provide a visual response called scotopic vision Cones respond to higher levels of illumination; their response is called photopic vision 3

Rods are more sensitive to light than the cones.

Digital Image Processing

ͻ There are three basic types of cones in the retina ͻ These cones have different absorption characteristics as a function of wavelength with peak absorptions in the red, green, and blue regions of the optical spectrum.

ͻ is blue, b is green, and g is red

Most of the cones are at the fovea. Rods are spread just about everywhere except the fovea There is a relatively low sensitivity to blue light. There is a lot of overlap 4

Digital Image Processing

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1.2 IMAGE FORMATION IN THE EYE

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Digital Image Processing

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1.3 CONTRAST SENSITIVITY

ͻ The response of the eye to changes in the intensity of illumination is nonlinear ͻ Consider a patch of light of intensity i+dI surrounded by a background intensity I as shown in the following figure ͻ Over a wide range of intensities, it is found that the ratio dI/I, called the Weber fraction, is nearly constant at a value of about 0.02. ͻ This does not hold at very low or very high intensities ͻ Furthermore, contrast sensitivity is dependent on the intensity of the surround.

Consider the second panel of the previous figure.

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1.4 LOGARITHMIC RESPONSE OF CONES AND RODS

ͻ The response of the cones and rods to light is nonlinear. In fact many image processing systems assume that the eye's response is logarithmic instead of linear with respect to intensity. ͻ To test the hypothesis that the response of the cones and rods are logarithmic, we examine the following two cases: ͻ If the intensity response of the receptors to intensity is linear, then the derivative of the response with respect to intensity should be a constant. This is not the case as seen in the next figure. ͻ To show that the response to intensity is logarithmic, we take the logarithm of the intensity response and then take the derivative with respect to intensity. This derivative is nearly a constant proving that intensity response of cones and rods can be modeled as a logarithmic response. ͻ Another way to see this is the following, note that the differential of the logarithm of intensity is d(log(I)) = dI/I. Figure 2.3-1 shows the plot of dI/I for the intensity response of the human visual system. ͻ Since this plot is nearly constant in the middle frequencies, we again conclude that the intensity response of cones and rods can be modeled as a logarithmic response. 9

Digital Image Processing

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