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Digital Image Fundamentals

Digital Image Fundamentals. What Makes a good image?. Cameras (resolution, focus, aperture), Distance from object (field of view), Illumination (intensity of light, direction, expose time, light source), Background characteristics etc. Topics. Human vision system

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Digital Image Fundamentals

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  1. Digital Image Fundamentals

  2. What Makes a good image? • Cameras (resolution, focus, aperture), • Distance from object (field of view), • Illumination (intensity of light, direction, expose time, light source), • Background characteristics • etc

  3. Topics • Human vision system • Brightness adaptation & Discrimination • Image acquisition • Image formation model • Image sampling & Quantization

  4. Human Visual Perception • Why is it important? • Because many techniques in DIP are highly subjective. • Human analysis plays a central role • Structure of the Human eye • Human vision & Computer Vision. • Effects of brightness & contrast on human vision.

  5. Human Visual System • Concerns how an image is perceived by human? • Important to develop image intensity measures. • Required for design and evaluate DIP system.

  6. Structure of the Human Eye

  7. We can see any object because the light is reflected from that object. • The reflected light is focused on the retina. • Retina contains light receptors over its surface. These receptors are like sensors. • There are two classes of receptors: cones and rods

  8. Cones • Primarily responsible for colour perception. • Photopic (high light) vision: requires higher intensity of light. • 6-7millions, located primarily in the central part of the retina called fovea • muscles controlling the eye rotate the eyeball until the image of an object of interest falls on the fovea. • Each is connected to its own nerve, like each photoelectric cell is connected to digitizing circuit through separate wire. • human can resolve the fine detail.

  9. Rods • Scotopic (low light) vision. • 75-150millions, distributed over the retina surface. • Several rods are connected to a single nerve end so reduce the amount of details transferred. • Serve to give a general, overall picture of the field of view. • 10 times more sensitive to the light than cones. • Mainly sensitive to low illumination (low intensity of light).

  10. You should know • Object that appear brightly colored in the daylight when seen by moon light appears colorless, why?

  11. Human & camera vision

  12. Human Lens is flexible • When we see object at far distance , lens becomes flat, to capture maximum light rays reflected from the object. Same happens with zoom in feature of camera. • When we see object close to us, lens becomes thicker , to capture only rays reflected from the object. Its like zoom out in camera.

  13. When you enter a dark theater on a bright day, it takes an appreciable interval of time before you can see well enough to find an empty seat. Why is it so?

  14. Brightness Adaptation

  15. Brightness adaptation • Human vision system can adapt very wide range of brightness ranging from scotopic threshold (very dark environment) to glare limit (very bright environment). • But the human vision cannot operate over such a a range simultaneously. • Total range of distinct intensity levels it can discriminate (see separately) is smaller than total adaptation range.

  16. Brightness is a function of intensity & simultaneous contrast.

  17. It takes more time for eyes to adjust when we move from brighter to darker environment. • But it takes less time when we move from dark to light environment.

  18. Brightness is a function of contrast

  19. Optical Illusions

  20. EM Spectrum

  21. Image Acquisition • Using Single Sensor • Line sensor • Array sensor

  22. Image Sensor • Simple Image sensor is photodiode. • Constructed from CCD or CMOS • When light strikes, it gives some voltage as output based on intensity. • Filter is used with sensor for selectivity.

  23. How to Obtain 2D image with single sensor? • There must be relative displacement in both the x- and y direction between the sensor & area to be imaged. • Use rotation of a drum and lead screw technique. • Cheaper but slower method for image acquisition.

  24. Image Acquisition using Line sensor • The strip provides imaging in the one direction , while motion perpendicular to strip provided imaging in the other direction. • Used in Flat-Bed scanners • Airborn imaging application. • Also used in tomography.

  25. Image Acquisition using Sensor array • Most widely used • Entire image can be scanned without relative displacement. • Light rays reflected from the object are focused on the image plane (sensor array). • Digitizing circuitry converts output voltages of each sensor in to corresponding number. As a result digital output is obtained. • The process of digitizing of image involves 2 main steps. i.e. sampling & quantization

  26. Digitizer

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