Transcript MACHINE VISION

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
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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