EE 4780: Introduction to
Computer Vision
Introduction
EE 4780
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Instructor: Bahadir K. Gunturk
Office: EE 225
Email: bahadir@ece.lsu.edu
Tel: 8-5621
Office Hours: MW 10:00 – 12:00
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EE 4780
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We will learn the fundamentals of digital image processing and
computer vision.
Lecture slides, problems sets, solutions, study materials, etc. will be
posted on the class website. [www.ece.lsu.edu/gunturk/EE4780]
Textbook is not required.
References:
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Gonzalez/Woods, Digital Image Processing, Prentice-Hall, 2/e.
Forsyth/Ponce, Computer Vision: A Modern Approach, Prentice-Hall.
Duda, Hart, and Stork, “Pattern Classification,” John Wiley&Sons, 2001.
Shapiro/Stockman, Computer Vision, Prentice-Hall.
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Horn, “Robot Vision,” MIT Press, 1986.
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Grading Policy
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Your grade will be based on
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Problem Sets: 30%
Midterm: 30%
Final: 40%
Problem Sets
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Mini projects: Theoretical problems and MATLAB assignments
4-5 Problem Sets
Individually or in two-person teams
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Digital Image Acquisition
Sensor array
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When photons strike, electron-hole pairs are generated
on sensor sites.
Electrons generated are collected over a certain period
of time.
The number of electrons are converted to pixel values.
(Pixel is short for picture element.)
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Digital Image Acquisition
Two types of quantization:
1. There are finite number of
pixels. (Spatial resolution)
2. The amplitude of pixel is
represented by a finite
number of bits. (Gray-scale
resolution)
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Digital Image Acquisition
Take a look at
this cross
section
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Digital Image Acquisition
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256x256 - Found on very cheap
cameras, this resolution is so low that
the picture quality is almost always
unacceptable. This is 65,000 total pixels.
640x480 - This is the low end on most
"real" cameras. This resolution is ideal
for e-mailing pictures or posting pictures
on a Web site.
1216x912 - This is a "megapixel" image
size -- 1,109,000 total pixels -- good for
printing pictures.
1600x1200 - With almost 2 million total
pixels, this is "high resolution." You can
print a 4x5 inch print taken at this
resolution with the same quality that you
would get from a photo lab.
2240x1680 - Found on 4 megapixel
cameras -- the current standard -- this
allows even larger printed photos, with
good quality for prints up to 16x20
inches.
4064x2704 - A top-of-the-line digital
camera with 11.1 megapixels takes
pictures at this resolution. At this setting,
you can create 13.5x9 inch prints with
no loss of picture quality.
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Image Resolution
Don’t confuse image size and resolution.
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Bit Depth – Grayscale Resolution
8 bits
7 bits
6 bits
5 bits
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Bit Depth – Grayscale Resolution
4 bits
2 bits
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3 bits
1 bit
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Matrix Representation of Images
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A digital image can be written as a matrix
x[0,1]
 x[0, 0]
 x[1, 0]
x[1,1]
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x[n1 , n2 ] 
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 x[ M  1, 0]
x[0, N  1] 
x[1, N  1] 
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x[ M  1, N  1] MxN
35 45 20
 43 64 52 
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10 29 39 
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Digital Color Images
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xR [n1 , n2 ]
xG [n1 , n2 ]
xB [n1 , n2 ]
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Color Displays
CRT
LCD
Polarize to control the amount of light passed.
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Video
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x[n1 , n2 , n3 ]
n1 = vertical position
n2 = horizontal position
n3 = frame number
~24 frames per second.
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Why do we process images?
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To facilitate their storage and transmission
To prepare them for display or printing
To enhance or restore them
To extract information from them
To hide information in them
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Image Processing Example
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Image Restoration
Original image
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Blurred
Restored by Wiener
filter
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Image Processing Example
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Noise Removal
Noisy image
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Denoised by Median filter
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Image Processing Example
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Image Enhancement
Histogram
equalization
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Image Processing Example
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Artifact Reduction in Digital Cameras
Original scene
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Captured by a digital camera
Processed to reduce artifacts
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Image Processing Example
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Image Compression
Original image
64 KB
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JPEG compressed
15 KB
JPEG compressed
9 KB
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Image Processing Example
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Object Segmentation
“Rice” image
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Edges detected using Canny
filter
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Image Processing Example
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Resolution Enhancement
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Image Processing Example
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Watermarking
Original image
Watermarked
image
Generate
watermark
Hidden message
Secret key
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Image Processing Example
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Face Recognition
Search in the
database
Surveillance video
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Image Processing Example
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Fingerprint Matching
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Image Processing Example
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Segmentation
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Image Processing Example
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Texture Analysis and Synthesis
Photo
Pattern repeated
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Computer generated
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Image Processing Example
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Face detection and tracking
http://www-2.cs.cmu.edu/~har/faces.html
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Image Processing Example
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Face Tracking
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Image Processing Example
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Object Tracking
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Image Processing Example
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Virtual Controls
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Image Processing Example
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Visually Guided Surgery
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Cameras
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First camera was invented in 16th century.
It used a pinhole to focus light rays onto a wall or
translucent plate.
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Take a box, prick a small hole in one
of its sides with a pin, and then
replace the opposite side with a
translucent plate.
Place a candle on the pinhole side,
you will see an inverted image of the
candle on the translucent plate.
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Perspective Projection
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Perspective projection equations
x' y' z'
 
x
y
z
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Pinhole Camera Model
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If the pinhole were really reduced to a point, exactly one light ray would
pass through each point in the image plane.
In reality, each point in the image place collects light from a cone of rays.
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Pinhole Cameras
Pinhole too big many directions are
averaged, blurring the
image
Pinhole too small diffraction effects blur
the image
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Cameras With Lenses
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Most cameras are equipped with lenses.
There are two main reasons for this:
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To gather light. For an ideal pinhole, a single light ray would
reach each point the image plane. Real pinholes have a finite
size, so each point in the image plane is illuminated by a cone
of light rays. The larger the hole, the wider the cone and the
brighter the image => blurry pictures. Shrinking the pinhole
produces sharper images, but reduces the amount of light
and may introduce diffraction effects.
To keep the picture in sharp focus while gathering light from a
large area.
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Compound Lens Systems
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Real Lenses
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Rays may not focus at a single point.
Spherical aberration
Spherical aberration can be eliminated completely by designing aspherical lenses.
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Real Lenses
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The index of refraction is a function of wavelength.
Light at different wavelengths follow different paths.
Chromatic aberration
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Real Lenses
Chromatic Aberration
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Real Lenses
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Special lens systems using two or more pieces of glass with different
refractive indeces can reduce or eliminate this problem. However, not
even these lens systems are completely perfect and still can lead to
visible chromatic aberrations.
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Real Lenses
Causes of distortion
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Barrel Distortion & Pincushion Distortion
Stop (Aperture)
Chief ray
(normal)
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Real Lenses
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Barrel Distortion & Pincushion Distortion
Distorted
Corrected
http://www.vanwalree.com/optics/distortion.html
http://www.dpreview.com/learn/?/Image_Techniques/Barrel_Distortion_Correction_01.htm
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Real Lenses
Vignetting effect in a two-lens system. The shaded part of
the beam never reaches the second lens. The brightness
drop in the image perimeter.
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Real Lenses
Optical vignetting example. Left: f/1.4. Right: f/5.6.
f-number
focal length to diameter ratio
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Real Lenses
Long exposure time
Short exposure time
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Real Lenses
Flare
Hood may prevent flares
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Real Lenses
Flare
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Compound Lens Systems
http://www.dpreview.com/learn/?/glossary/
http://www.cartage.org.lb/en/themes/Sciences/Physics/Optics/Optical/Lens/Lens.htm
http://www.vanwalree.com/optics.html
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Digital Camera Pipeline
(from Katz and Gentile)
Auto-exposure algorithms measure brightness over discrete scene regions to
compensate for overexposed or underexposed areas by manipulating shutter
speed and/or aperture size. The net goals here are to maintain relative
contrast between different regions in the image and to achieve a good overall
quality.
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Digital Camera Pipeline
Auto-focus algorithms divide into two categories. Active methods use infrared
or ultrasonic emitters/receivers to estimate the distance between the camera
and the object being photographed. Passive methods, on the other hand,
make focusing decisions based on the received image in the camera.
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Digital Camera Pipeline
Lens distortion correction
This set of algorithms accounts for the physical properties of lenses that warp
the output image compared to the actual scene the user is viewing. Different
lenses can cause different distortions; for instance, wide-angle lenses create
a "barrel distortion", while telephoto lenses create a "pincushion distortion“.
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Digital Camera Pipeline
Vignetting (shading distortion) reduces image brightness in the area around
the lens. Chromatic aberration causes color fringes around an image. The
media processor needs to mathematically transform the image in order to
correct for these distortions.
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Digital Camera Pipeline
Sensor's output needs to be gamma-corrected to account for eventual display,
as well as to compensate for nonlinearities in the sensor's capture response.
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Digital Camera Pipeline
Image stability compensation, or hand-shaking correction is another area of
preprocessing. Here, the processor adjusts for the translational motion of the
received image, often with the help of external transducers that relate the
real-time motion profile of the sensor.
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Digital Camera Pipeline
White balance is another important stage of preprocessing. When we look at
a scene, regardless of lighting conditions, our eyes tend to normalize
everything to the same set of natural colors. For instance, an apple looks
deep red to us whether we're indoors under fluorescent lighting, or outside in
sunny weather. However, an image sensor's "perception" of color depends
largely on lighting conditions, so it needs to map its acquired image to appear
natural in its final output. This mapping can be done either manually or
automatically.
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Digital Camera Pipeline
Demosaicking (Bayer interpolation) estimates missing color samples in singlechip cameras.
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Digital Camera Pipeline
In this stage, the interpolated RGB image is transformed to the targeted
output color space (if not already in the right space). For compression or
display to a television, this will usually involve an RGBYCbCr matrix
transformation, often with another gamma correction stage to accommodate
the target display. The YCbCr outputs may also be chroma subsampled at this
stage to the standard 4:2:2 format for color bandwidth reduction with little
visual impact.
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Digital Camera Pipeline
Postprocessing
In this phase, the image is perfected via a variety of filtering operations before
being sent to the display and/or storage media. For instance, edge
enhancement, pixel thresholding for noise reduction, and color-artifact
removal are all common at this stage.
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Digital Camera Pipeline
Display / Compress / Store
Once the image itself is ready for viewing, the image pipe branches off in two
different directions. In the first, the postprocessed image is output to the target
display, usually an integrated LCD screen (but sometimes an NTSC/PAL
television monitor, in certain camera modes). In the second, the image is sent
to the media processor's compression algorithm, where industry-standard
compression techniques (JPEG, for instance) are applied before the picture is
stored locally in some storage medium (e.g., Flash memory card).
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