Introduction to Digital Image · PDF fileQuantization: The process of ... Spatial...
Transcript of Introduction to Digital Image · PDF fileQuantization: The process of ... Spatial...
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Introduction to Digital
Image Processing
Part I
Chester F. Carlson Center for Imaging Science,
RIT
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HumanSubject Process DisplayCapture
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What is a Digital Image?
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Digital Image:
“ Rendering of a continuously varying scene
with a finite array of picture elements,
where each one has a discrete intensity or
color
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Charge Coupled Device (CCD)
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Magnified View of a CCD Array
Individual pixel element
Close-up of a CCD Imaging Array
CCD
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Spatial Sampling:
Measuring a continuously varying image
at uniformly separated points in space and
assigning a value that corresponds to an
average light intensity within a small box
surrounding each point.
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=
==
=
=
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30 pixels
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63 pixels
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108 pixels
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180 pixels
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350 pixels
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980 pixels
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2215 pixels
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8700 pixels
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221,000 pixels
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Spatial Aliasing:
An effect seen in digital images which
have been inadequately sampled (too few
pixels) which gives rise to a misleading
appearance when compared to the
original.
Image distortions caused by spatial
aliasing are also known as sampling
artifacts.
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Online Demo: Spatial Sampling
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0 0 0 0 0 0
0 0 0 0 0 0
100 100 10010050
10
10
20
20
20 20
20 20
40
55
55
5
5
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Quantization:
The process of taking a set of continuous
values and mapping them into a finite
number of discrete steps represented by
integers.
39
56
45
75
62
99
64
101
228 178 106 193
183 143 84 162
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Number of Steps = Power of 2
A bit is a value, a position, and
an amount of information
Each added bit allows us to
double the number of values
we can represent with a binary
number.
The number of values that can be
represented is given by 2Nbits
e.g., 4 bits provides 24
= 16
different values
0
1
10
11
100
101
110
111
1000
1001
1010
1011
1100
1101
1110
1111
10000...
binary
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16...
decimal
1 bit
2 bits
3 bits
4 bits
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How Many Steps?
The simplest kind of digital image is known
as a ‚binary image‛ because the image
contains only two ‘colors’ - white and black
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Binary Images
Because binary images contain only two colors,
we can encode the image using just two
numbers, for example:
0 = black
1 = white
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
1 1 1 1 1 1 1 0 0 0 0 0 0 0 0
1 1 1 1 1 1 1 1 1 0 0 0 0 0 0
1 1 1 1 1 1 1 1 1 1 1 0 0 0 0
1 1 1 1 1 1 1 1 1 1 1 0 0 0 0
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2 shades of gray
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4 shades of gray
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8 shades of gray
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16 shades of gray
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32 shades of gray
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256 shades of gray
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Bit depth: bits per pixel
The number of possible gray levels is controlled
by the number of bits/pixel, or the ‘bit depth’ of the
image
gray levels
2
4
8
16
32
64
128
256
Bit depth;
bits/pixel
1
2
3
4
5
6
7
8
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Summary
“ Digital image:
“ Rendering of a continuously varying scene with a
finite array of picture elements, each one
containing a discrete numerical value which
represents an intensity
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Summary
“ Digital images are created by:
“ Spatial sampling
“ Quantizing
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Summary
“ Spatial sampling:
“ Measuring the scene at uniformly separated
points and assigning an average intensity to
the pixel at that location
“ The more samples taken, the better resolution
of the image
“ Insufficient sampling results in aliasing
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Summary
“ Quantization:
“ Converting the average intensity of a pixel to
a binary integer
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Summary
“ Number of available intensity levels or
colors is determined by the bit depth
“ Most gray scale images use 8 bits (256 levels)
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Color Images
In most cases, we also want to capture
color information
The way that we capture, store, view,
and print color digital images is based
on the way that humans perceive color
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Color Images
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Color Perception
The eyes have three different kinds of color receptors
(‘cones’); one type each for blue, green, and red light.
Color perception is based on how much light is
detected by each of the three ‘primary’ cone types
(red, green, and blue)
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Color Perception
Because we have three kinds of cones, every color
that we can see can be made up by combining red,
green, and blue light - the three ‚additive primaries‛
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RGB Color Images
Each one of the color images (‘planes’) is like a grayscale
image, but is displayed in R, G, or B
=
The most straightforward way to capture a color image is to
capture three images; one to record how much red is at each
point, another for the green, and a third for the blue.
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RGB Color Images
To capture a color image we record how much red,
green, and blue light there is at each pixel.
To view the image, we use a display (monitor or print)
to reproduce the color mixture we captured.
Q) How many different colors can a display produce?
A) It depends on how many bits per pixel we’ve got.
For a system with 8 bits/pixel in each of the red,
green, and blue (a ‘24-bit image’):
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RGB Color Images: 24-bit color
Every pixel in each of the
three 8-bit color planes can
have 256 different values (0-
255)
If we start with just the blue
image plane, we can make
256 different ‚colors of blue‛ 0
255
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RGB Color Images: 24-bit color
Every pixel in each of the three
8-bit color planes can have 256
different values (0-255)
If we start with just the blue
image plane, we can make 256
different ‚colors of blue‛
If we add red (which alone
gives us 256 different reds):
0 255
0
255
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RGB Color Images: 24-bit color
Every pixel in each of the three 8-
bit color planes can have 256
different values (0-255)
If we start with just the blue image
plane, we can make 256 different
‚colors of blue‛
If we add red (which alone gives us
256 different reds):
We can make 256 x 256 = 65,536
combination colors because for
every one of the 256 reds, we can
have 256 blues. 0 255
0
255
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RGB Color Images: 24-bit color
When we have all
three colors together,
there are 256 possible
values of green for
each one of the 65,536
combinations of red
and blue:
256 x 256 x 256 = 16,777,216 (‚> 16.7 million colors‛)
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Color Image = Red + Green + Blue
=
In a 24-bit image, each pixel has R, G, & B values
When viewed on a color display, the three images
are combined to make the color image.
212100139196
16375113149
37446372
95118155170
189
162
38
41608278
182
161
50
43576863
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Image quality factors
Two major factors which determine image quality are:
Bit depth -- controlled by number of colors or grey levels
allocated for each pixel
Spatial resolution -- controlled by spatial sampling.
Increasing either of these factors results in a larger
image file size, which requires more storage space
and more processing/display time.
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File Size Calculation
100 pixels
100 pixels
Bit depth = 8 bits per pixel (256 gray levels)
File size (in bits) = Height x Width x Bit Depth
100 x 100 x 8 bits/pixel = 80,000
bits/image
80,000 bits or 10,000 bytes
How much memory is
necessary to store an
image that is 100 x 100
pixels with 8 bits/pixel?
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File Size Calculation
1280 pixels
960 pixels
Bit depth = 24 bits per pixel (RGB color)
File size (in bits) = Height x Width x Bit Depth
960 x 1280 x 24 bits/pixel = 29,491,200 bits/image
29,491,200 bits = 3,686,400 bytes = 3.5 MB
How much memory is
necessary to store an
image that is 1280 x 960
pixels with 24 bits/pixel?
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File Size = 217 KFile Size = 1442 K
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Summary
“ Digital image:
“ Rendering of a continuously varying scene with a
finite array of picture elements, each one containing a
discrete numerical value which represents an
intensity or color
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Summary
“ Digital images are created by:
“ Spatial sampling
“ Quantizing
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Summary
“ Spatial sampling:
“ Measuring the scene at uniformly separated
points and assigning an average intensity to
the pixel at that location
“ The more samples taken, the better resolution
of the image
“ Insufficient sampling results in aliasing
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Summary
“ Quantization:
“ Converting the average intensity of a pixel to
a binary integer
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Summary
“ Number of available intensity levels or
colors is determined by the bit depth
“ Most gray scale images use 8 bits (256 levels)
“ Most color images use 24 bits (16.7 million
colors)
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Summary
Resolution and bit depth are major factors
in determining the quality of a digital
image AND the size of the image file
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Introduction to Digital
Image Processing
Part II
Chester F. Carlson Center for Imaging Science,
RIT
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A simple black & white image is a volume of information!
Two dimensions are spatial dimensions (cm for example)
The other dimension is lightness/darkness
Image Characterization
x
y
y
x
gray level
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y
x
Contour Plot:
Another way to represent 3 dimensional information
Lines are regions
of constant gray level
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The Histogram
A statistical way of looking at an image
Step #1: Sample the image
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Step #2: Quantize each pixel
Gray Value: R = 167
Gray Scale
R = 255 is white
R = 0 is black
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Step #3: Take the gray pixels out of the image.
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Step #4: Sort the pixels from darkest to lightest.
There are many pixels at this gray level.
Only a few at these gray levels.
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R = 0 R = 255
Each pixel in the image has its
place in the stacks of rearranged pixels.
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Step #5: Form a graph to
represent the sorted pixels.
0 255
Num
ber
Gray Level
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If you use very many levels,
the graph becomes sharper.
0 255
Num
ber
Gray Level
This is called the image Histogram.
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The Image Histogram Has Many Uses
Use #1: A way to describe Image "Brightness"
R
N
R
N
R
N
Average R = 50 Average R = 120 Average R = 200
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R
N
R
N
Same Average
R=120
Different Ranges=
Different contrasts
Range = 255
Range = 50
Range = 180
R
N
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Characterizing Color Images
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Summary – Characterizing Digital Images
1. Number of pixels - resolution
2. Bit depth - number of gray levels or colors
3. Histogram - brightness and contrast.
When you start changing the values in the image array, that’s
digital image processing.
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What is Digital Image Processing?
Mathematical operations on the numbers
that make up a digital picture
Results in a new digital picture which is
different from the original
REMEMBER :
Start with an image -- end with an image
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Low Contrast Normal Contrast
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High Noise Low Noise
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Unnatural Color Realistic Color
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Adjusted Perspective
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Sharpen
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Initial Image
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Final Image
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Dark Light
Algorithm: for each pixel in the original image, add 55 to its value
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The Transfer Function
Initial image Final image
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The Transfer Function
0
255
0 255Pixel value in initial image
Pix
el v
alue
in f
inal
im
age
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The Transfer Function
0
255
0 255Pixel value in initial image
Pix
el v
alue
in f
inal
im
age
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The Transfer Function
0
255
0 255Pixel value in initial image
Pix
el v
alu
e in
fin
al i
mag
e
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0
255
0 255initial image
final
im
age
Initial image Final image
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Location of the Transfer Function
Pix
el v
alu
e in
fin
al i
mag
e
0
255
0 255Pixel value in initial image
1
2
3
Line 1 will result in a brighter final image
Line 2 will result in equal brightness
Line 3 will result in a darker final image
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Initial Image
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Final Image
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Low Contrast Normal Contrast
Algorithm: ‚stretch‛ the histogram by defining max and min values
In initial image as black and white (0 and 255), and evenly distribute
all intermediate values between them
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0
255
0 255initial image
final
im
age
Initial image Final image
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Slope of the Transfer Function
0
255
0 255
1 2
3
Curve 1 will result in a more contrast in the final image
Curve 2 will result in equal contrast in the final image
Curve 3 will result in a less contrast in the final image
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Mapping
First class of mathematical operations that
can be performed on images
Definition:
Creating a pixel value for the final image by
employing a computational rule that uses only
the value of the corresponding pixel in the
initial image
Same rule is applied to all pixels, no matter
where they are located
This rule can be described in terms of a
transfer function
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Online Demos
Contrast Manipulation
Histogram Stretching
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Question…
What does the transfer function look like?
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Question…
What will the final image look like?
0
255
0 255initial image
final
im
age
200
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Introduction to Digital
Image Processing
Part III
Chester F. Carlson Center for Imaging Science,
RIT
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Question…
What will the final image look like?
0
255
0 255initial image
final
im
age
200
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threshold=200
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threshold = 100
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Question…
What does the transfer function look like?
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0
255
0 255initial image
fin
al
image
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Review
Digital images can be characterized by:
Resolution
Bit depth
Histogram
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Review
Histogram
Number of pixels at each allowable gray level
in a digital image
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Review
Digital image processing:
Performing mathematical operations on the
numbers that make up a digital image which
results in a new image different from the
original
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Review
One class of mathematical operations that
can be performed on digital images is
called mapping
Creating a pixel value for the final image by
employing a computational rule that uses only
the value of the corresponding pixel in the
initial image
This rule can be described in terms of a
transfer function
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Review
Shape and location of the transfer function
gives an indication of how the initial image
is being changed
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Review
Examples of mapping operations include:
Changes to brightness
Changes to contrast
Thresholding
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Arithmetic Combination of
Images
Next class of operations
Apply an arithmetic function to
corresponding pixel values of two different
images, resulting in a new, third image
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0
180 200
100
30 230
130
+
100
180
125 255
255
=
Image Addition
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Image Subtraction
0
180 200
100
30 170
130
-180
125 5
=
0
0
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Image Multiplication
0
180 200
100
30
130
X180
125 255
=
0
255 255
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Image Division
0
180 200
100
30
130
/180
125 1
=
0
1 6
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Noise Reduction
Noise: variations in an image not related
to the content of the scene
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average =
noise
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Average
Noisy Images
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Online Demo:
Image Averaging and Noise Removal
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Convolutions
Third class of mathematical operations
that can be performed on images
Also known as ‚neighborhood operations‛
Pixel values in new image are computed
from groups of pixels located near those
of the output pixel
The relative weighting of the neighboring
pixels is given by a matrix called a kernel
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average
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http://www.microscopy.fsu.edu/primer/java/digitalimaging/processing/kernelmaskoperation/
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http://www.microscopy.fsu.edu/primer/java/digitalimaging/processing/kernelmaskoperation/
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http://www.microscopy.fsu.edu/primer/java/digitalimaging/processing/kernelmaskoperation/
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http://www.microscopy.fsu.edu/primer/java/digitalimaging/processing/kernelmaskoperation/
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http://www.microscopy.fsu.edu/primer/java/digitalimaging/processing/kernelmaskoperation/
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http://www.microscopy.fsu.edu/primer/java/digitalimaging/processing/kernelmaskoperation/
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http://www.microscopy.fsu.edu/primer/java/digitalimaging/processing/kernelmaskoperation/
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http://www.microscopy.fsu.edu/primer/java/digitalimaging/processing/kernelmaskoperation/
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http://www.microscopy.fsu.edu/primer/java/digitalimaging/processing/kernelmaskoperation/
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http://www.microscopy.fsu.edu/primer/java/digitalimaging/processing/kernelmaskoperation/
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Online Demo: Convolution Kernel Mask Operations
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Sharpening Filter
Original Filtered
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Edge Finding Filter
Original Filtered
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Embossing Filter
Original Filtered
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Summary: Classes of
Operations
Mappings
Adjust brightness and contrast
Arithmetic combinations of images
Averaging for noise reduction
Convolutions (filters)
Noise reduction
Sharpening and softening
Edge detection
Etc.
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Introduction to Digital
Image Processing
Part IV
Chester F. Carlson Center for Imaging Science,
RIT
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What is Digital Image Processing?
Mathematical operations on the numbers
that make up a digital picture
Results in a new digital picture which is
different from the original
REMEMBER :
Start with an image -- end with an image
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What is Digital Image
Analysis?
“ Using the numbers in digital pictures to
get useful information or solve a
problem
REMEMBER:
Start with an image -- end with an answer
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Answers
Width of Genesee River = 193 feet
Length of ‚Quarter Mile‛ = 0.41 mile
Length of Perimeter Road = 3.1 miles
Area inside Perimeter Road = 346 acres
Number of cars in ‚E‛ Lot = 226