Download ENT 273 Lecture 2

Machine Vision
ENT 273
Lecture 2.
Ms. HEMA C.R.
Image Geometry and Acquisition
Road Map
 Image Formation
 Image Geometry
 Image Sampling
 Image Quantization
 Image Acquisition
 Image Definitions
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Image Formation
Image formation in the eye and the
camera
 Understanding function of the human
eye provides insight into machine vision
solutions
 Biological vision is the process of using
light reflected from the surrounding
world as a way of modifying behavior
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Image Formation in the eye
 Light enters through cornea
 Passes through aqueous humor, the
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lens and vitreous humor
Finally forms an image on the retina
Lens adjusts and focus image
directly on retina
Retina is a complex tiling of
photoreceptors known as rods and
cones
When stimulated by light they
produce electrical signals that are
transmitted to the brain by the optic
nerve
Refer: http://homepages/inf.ed.ac.uk/rbf/CVonline/
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Image formation :Pin hole camera
 Camera is
analogous to
the eye
 Pin hole camera
has a small hole
through which
light enters before forming an inverted image
 Pin hole cameras are modeled by placing
image plane between focal point of the camera
and the object so that image is not inverted
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Image Geometry
 Image Formation has two divisions
– Geometry of image formation
– Physics of light [brightness of point]
 Image geometry determines where a
world point is projected on the image
plane
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Image Geometry
Object Point
(x,y,z)
y
f
r
x`
r`
(x`, y`)
z
y
x
y`
 Object point is represented by x, y and
z 3D co-ordinates.
 Image plane is parallel to x and y axis
[world] at a distance f [focal length]
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r  x2  y2
r   x 2  y  2
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Image Geometry
Object Point
(x,y,z)
y
f
r
x`
r`
(x`,y`)
y`
f
r

z
r
x
y
r


x
y
r
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y
x
z
f
z
f
z

x

y


x
x
y
y
f
z
f
z
x
y
Mapping of three dimensions onto two dimension is
called perspective projection
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Image Sampling
 Continuous images are sampled to convert them to
digital form
 Each image sample is called a pixel [picture
element]
 Sampling is the process of representing a continuous
signal by a set of samples taken at discrete intervals
of time [sampling interval]
Sampling Frequency f s  1
T 
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Continuous Signal
Sampled Signal
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Image Sampling
Original Image
Sampled Image
Reconstructed Image
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Image Quantization
 Quantization is the process of
converting
analog
pixel
intensities to discrete valued
integer numbers
 Quantization
involves
assigning a single value to
each sample values in such a
way
that
the
image
reconstructed from quantized
values is good
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Image Quantization
550 x 413 pixels
49868 colors
8 colors
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Image Quantization
From 600x400, 32 bits image
To 600x400, 4 bits image
(16 colors)
To 600x400, 2 bits image
(4 colors)
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Image Acquisition
 Image acquisition is the first stage of a vision
system
 Acquired Image is dependent on
– Nature of sensing device
 Vidicon, CCD, infra red , grayscale , color
– Properties of the device
 Sensitivity, resolution, lenses, stability , focus
– The lighting of the scene
 Shadows, excessive reflection, poor contrast
– The environment
 Dust, fog, humidity
– The reflective properties of the objects
 Texture, color, specularity
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Image Acquisition
 Two Dimensional Images
 Three Dimensional Images
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Image Acquisition
 Acquisition [capture] of 2D Images
– Monochrome or Color
Analog Cameras
Digital CCD Cameras
Digital CMOS Cameras
Video Cameras [Analog and
Digital]
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Image Acquisition
 Methods of acquisition for 3D
– Laser Ranging Systems
– Structured Lighting Methods
– Moire Fringe Methods
– Shape from Shading Methods
– Passive Stereoscopic Methods
– Active Stereoscopic Methods
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Image Capturing
 A basic image capture system
contains a lens and a detector. Film
detects far more visual information
than is possible with a digital
system.
 With digital imaging, the detector is
a solid state image sensor called a
charge coupled device...CCD for
short
 On an area array CCD, a matrix of
hundreds of thousands of
microscopic photocells creates
pixels by sensing the light intensity
of small portions of the image
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Image Capture
 To capture images in color, red,
green and blue filters are placed
over the photocells.
 Film scanners often use three
linear array image sensors
covered with red, green and blue
filters.
 Each linear image sensor,
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containing thousands of
photocells, is moved across the
film to capture the image one-lineat-a-time.
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Image Definitions
 Pixel – A sample of the image intensity
quantized to an integer value
 Image – A two dimensional array of pixels
 Pixel
– Row and column indices [ i, j] are integer
values
– Pixels have intensity values
 0 to 255 grayscale images
 RGB value [vector value] color
images
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Pixel Array
Pixel [4,4]
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↓i →j
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Pixel Concept Map
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Pixel
 The
quality of a scanned
image is determined by pixel
size, or spatial resolution; and
by pixel depth, or brightness
resolution
 This relates to the two basic
steps in the digital capture
process:
 In step one, sampling
determines pixel size
and brightness value.
 In step two quantization
determines pixel depth
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Image File Formats
 Images are stored in a computer in one
of the following formats, depending on
the application of the images stored.
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– Tagged Image Format [.tif]
– Portable Network Graphics [.png]
– Joint Photographic Experts Group
[.jpeg, .jpg]
– Bitmap [.bmp]
– Graphics Interchange Format [.gif]
– Raster Images [.ras]
– Postscript [.ps]
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Machine Vision
End of Lecture 2