AP 9251– DIGITAL IMAGE PROCESSING

1. AP 9251– DIGITAL IMAGE PROCESSING

2. UNIT - 1 DIGITAL IMAGE FUNDAMENTALS

3. ELEMENTS OF VISUAL PERCEPTION • Digital image processing is built on a foundation on mathematical and probabilistic formulations. • For understanding the visual perception process, first we must know the human visual mechanism (functions of human eye).

4. STRUCTURE OF THE HUMAN EYE

5. STRUCTURE OF THE HUMAN EYE • The eye is nearly a sphere with an average diameter of approx 20mm. • Three membranes enclose the eye: the cornea and sclera, the outer cover; the choroids and the retina. • The cornea is a tough, transparent section of the human eye that covers the anterior surface of the eye. • The sclera is an opaque membrane that encloses the remainder of the optic globe.

6. CHOROID • The choroid lies directly below the sclera. • This membrane contains a net work of blood vessels that serve as the major source of nutrition to the eye. • The choroid coat is heavily pigmented and hence helps to reduce the amount of extraneous light entering the eye and the backscatter within the optical globe.

7. CHOROID • The choroid is divided into the • Ciliary body • Iris • The central opening of the iris (the pupil) varies in diameter from approximately 2 to 8 mm. • The front of the iris contains the visible pigment of the eye, whereas the back contains a black pigment.

8. LENS • The lens is made up of concentric layers of fibrous cells and is suspended by fibers that attach to the ciliary body. • It contains 60 to 70% water, about 6% fat, and more protein, than any other tissue in the eye. The lens is colored by a slightly yellow pigmentation that increases with age. • The lens absorbs approximately 8% of the visible light spectrum.

9. LENS • Excessive clouding of the lens, caused by the affliction commonly referred to as cataracts, can lead to poor color discrimination and loss of clear vision. • Both infrared and ultraviolet light are absorbed appreciably by proteins within the lens structure and, in excessive amounts, can damage the eye. • The innermost membrane of the eye is the retina, which lines the inside of the wall’s entire posterior portion.

10. RETINA • When the eye is properly focused, light from an object outside the eye is imaged on the retina. • Pattern vision is afforded by the distribution of discrete light receptors over the surface of the retina.

11. RODS AND CONES • There are two classes of receptors • Cones • Rods.

12. CONE VISION • The cones in each eye number between 6 and 7 million. • They are located primarily in the central portion of the retina, called the fovea, and are highly sensitive to color. • Muscles controlling the eye rotate the eyeball until the image of an object of interest falls on the fovea. • Cone vision is called photopic or bright-light vision.

13. ROD VISION • The number of rods is much larger: Some 75 to 150 million are distributed over the retinal surface. • Objects that appear brightly colored in day- light when seen by moonlight appear as colorless forms because only the rods are stimulated. • This phenomenon is known as scotopic or dim-light vision.

14. DISTIBUTION OF ROD AND CONES

15. BLIND SPOT • The density of rods and cones for a cross section of the right eye passing through the region of emergence of the optic nerve from the eye. • The absence of receptors in this area results in the so-called blind spot. • Except for this region, the distribution of receptors is radially symmetric about the fovea.

16. Contd.., • Cones are most dense in the center of the retina (in the center area of the fovea) • The density of cones in that area of the retina is approximately 150,000 elements per mm².

17. IMAGE FORMATION IN THE EYE • The principal difference between the lens of the eye and an ordinary optical lens is that the former is flexible. • The distance between the center of the lens and the retina (called the focal length) varies from approximately 14 mm to 17 mm. • The refractive power of the lens increases from its minimum to its maximum.

18. IMAGE FORMATION IN THE EYE

19. IMAGE FORMATION IN THE EYE • The radius of curvature of the anterior surface of the lens is greater than the radius of its posterior surface. • The shape of the lens is controlled by tension in the fibers of the ciliary body. • To focus on distant objects, the controlling muscles cause the lens to be relatively flattened. Similarly, these muscles allow the lens to become thicker in order to focus on objects near the eye.

20. BRIGHTNESS ADAPTATION AND DISCRIMINATION • The digital images are displayed as a discrete set of brightness points. • Experimental evidence indicates that subjective brightness (intensity as perceived by the human visual system) is a logarithmic function of the light intensity incident on the eye. • The transition from scotopic to photopic vision is gradual over the approximate range from 0.001 to 0.1 millilambert

21. BRIGHTNESS ADAPTATION • The visual system cannot operate over the dynamic a range simultaneously. • Rather it accomplishes this large variation by changes in its overall sensitivity, a phenomenon known as brightness adaptation. • The total range of distinct intensity levels it can discriminate simultaneously is rather small when compared with the total adaptation range.

22. BRIGHTNESS ADAPTATION EXPERIMENT • The ability of the eye to discriminate between changes in light intensity at any specific adaptation level is also of considerable interest. • A subject looks at a flat, uniformly illuminated area large enough to occupy the entire field of view.

23. WEBER’S RATIO • The quantity ΔIc/I, where ΔI is the increment of illumination discriminable 50% of the time with background illumination I, is called the Weber ratio. • A small value of ΔIc/I, means that a small percentage change in intensity is discriminable. • This represents “good” brightness discrimination.

24. Contd..,

25. Contd.., • A large value of ΔIc/I, means that a large percentage change in intensity is required. This represents “poor” brightness discrimination. • A plot of log ΔIc/I, as a function of log I

26. Contd..,

27. Contd.., • Brightness discrimination is poor (the Weber ratio is large) at low levels of illumination, and it improves significantly (the Weber ratio decreases) as background illumination increases. • The two branches in the curve reflect the fact that at low levels of illumination vision is carried out by activity of the rods, whereas at high levels (showing better discrimination) vision is the function of cones.

28. MACH BAND EFFECT • The intensity of the image will be cleared, but the brightness of the image will be more, this effect is called as ‘Mach Band Effect’. • The visual system tends to undershoot or overshoot around the boundary of regions of different intensities. • It is described by Ernst Mach in 1865.

29. MACH BAND EFFECT

30. IMAGE SAMPLING AND QUANTIZATION • SAMPLING • Discretization of the image data in the spatial coordinates. • QUANTIZATION • The discretization of image intensity (gray level) values. • DISCRETIZATION • Process in which signals or data samples are considered at regular intervals.

31. Contd.., • Sampling and quantization are two important processes used to convert continuous analog image into digital image. • It deals with integer values. • Continuous image is generally represented in (x,y) coordinates and also in amplitude. • Digital conversion involves taking samples both in (x,y) coordinates and in amplitude.

32. Contd.., • The no. of pixels that can be accommodated in a unit area is called ‘resolution of an image’. • It depends on • The no. of values for N • The no. of bits m used to represent the gray levels. • Here x and y are discrete coordinates containing M Rows and N Columns.

33. Contd.., • X = 0,1,2,…… ,M-1 • Y = 0,1,2,..…. ,N-1 • The N and M are usually the integer powers of 2. i.e., M = 2^n and N = 2^k • When we discretize the gray levels, we use the integer values and the no. of integer values that can be used in denoted as G. • G = 2^m

34. Contd.., • An image of size N*M consisting of NM pixels and the number of bits required to store a digital image can be given by b= m*N*M If M=N then b=N²m m the no. of bits used to represent a gray level value in the image.

35. Contd.., • For example, The no. of bits reqd. to store an image of size 64*64 with 16 gray levels will be b = 64*64*4 = 2048 bytes or 16384 bits i.e., N= 64 and m = 4

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