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

• Brightness adaptation & Discrimination

• Image acquisition

• Image formation model

• Image sampling & Quantization

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.

Human Visual System

• Concerns how an image is perceived by human?

• Important to develop image intensity

measures.

• Required for design and evaluate DIP system.

Structure of the Human Eye

• 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

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.

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).

You should know

• Object that appear brightly colored in the daylight when seen by moon light appears colorless, why?

Human & camera vision

• 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.

• 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?

Brightness Adaptation

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.

• Brightness is a function of intensity & simultaneous contrast.

• 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.

Brightness is a function of contrast

Optical Illusions

EM Spectrum

Image Acquisition

• Using Single Sensor

• Line sensor

• Array sensor

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.

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.

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.

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

Digitizer