| Image
Processing Articles
Inde |
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| Overview |
| Contrast
stretching (often
called normalization)
is a simple image
enhancement technique
that attempts to improve
the contrast in an
image by `stretching'
the range of intensity
values it contains
to span a desired
range of values, e.g.
the the full range
of pixel values that
the image type concerned
allows. Low contrast
images can be due
to the poor illumination,
lack of dynamic range
in the imaging sensor,
or due to the wrong
setting of the lens.
The idea behind the
contrast stretching
is to increase the
dynamic range of intensity
level in the processed
image. |
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| General
Working |
| The
general working of
the contrast stretching
operation on grayscale
image is to apply
the following equation
on each of the pixels
in the input image
to form the corresponding
output image pixel: |
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| The
erosion can also be
applied on the graylevel
images also in a single
pass. During the passing
through the image,
the structuring element
is applied on each
pixel of the image,
such that the origin
of the structuring
element is applied
on that particular
pixel. In this case
the corresponding
pixel of the output
image contains the
minimum of the pixels
surrounding it. In
this case, only those
pixels are compared
with each other, where
the structuring element
contains. |
|
 |
|
| where,
I( x,y ) represents
the images, on the
left side it represents
the output image,
while on the right
side it represents
the xth pixel in the
yth column in the
input image. In this
equation the "min"
and "max"
are the minimum intensity
value and the minimum
intensity value in
the current image.
Here "no. of
intensity levels"
shows the total number
of intensity values
that can be assigned
to a pixel. For example,
normally in the gray-level
images, the lowest
possible intensity
is 0, and the highest
intensity value is
255. Thus "no.
of intensity levels"
is equal to 255. |
|
| The
contrast stretching
operation is applied
on the grayscale images
in two passes. In
the first pass the
algorithm calculates
the minimum and the
maximum intensity
values in the image,
and in the second
pass through the image,
the above formula
is applied on the
pixels. |
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| Guidelines
for Use |
|
| The
visual effects can
be improved by using
contrast stretching.
For example take the
following image: |
|
 |
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| After
applying the contrast
stretching operation
on the image the following
image is obtained: |
|
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|
| Sample
Project: |
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| The
algorithm is coded
in C# using unsafe
so the quality and
speed of the program
may not be affected.
The class BitmapData
is used to read and
process the pixels
in the image. This
is the speicality
of C# to provide such
a speed even on image
processing applications.
There is a set of
modules that are designed
to implement the algorithm.
The interface designed
for the sample project
as: |
|
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| If
you have the low contrast
image, by choosing
in the browser opened
when you click File>Open,
then click Operation>Contrast
Stretch, the stretched
image will be displayed
in the imagebox. |
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Project Files
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