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Chapter 8
Image Enhancement
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Introduction and Overview
Image enhancement:
emphasize, sharpen and/or smooth image
features for display and analysis
to facilitate the development of a solution
Application-specific developed empirically
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We define the enhanced image as E (r,c), and
application feedback is an important part of the process
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Enhancement methods operate in both the spatial and
frequency/spectral domains
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Enhancement techniques are as follows:
• Point operations : Each pixel is modified
by an equation that is not dependent on
other pixel values
• Mask operations : Each pixel is modified
according to the values in a small
neighborhood (subimage)
• Global operations : All pixel values in the
image are taken into consideration
Spatial domain processing methods
include all three types, but frequency
domain operations, are global operations
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Enhancement techniques are as follows:
• Point operations
• Mask operations
• Global operations :
• Spatial domain processing methods
include all three types, but frequency
domain operations, are global operations
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Frequency domain : mask operations
Enhancement techniques:
 preprocessing, and in post processing
End result
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Gray Scale Modification
 Gray level scaling or gray level transformation
 improve image contrast
 measure of the distribution and range of gray levels
 the difference between the brightest and darkest pixel values and
how the intermediate values are arranged
 Brightness
 Mapping Equations
 Histogram Modification
 Adaptive Contrast Enhancement
 Color Enhancement
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Mapping Equations
Based on a mathematical function
linear, non-linear: piecewise linear (Point
Operation)
Applications: contrast enhancement and
feature enhancement
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Primary operations: to compress or stretch
slope of the line is between zero and one,
slope is greater than one
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Example:
For the ranges 0 to 28 and 75 to 255 the input
equals the output. For the range 28 to 75, we
want to stretch the range from 28 to 255. To do
this we need a linear equation. If we use the
standard form y = mx + b, where m is the slope,
and b is the y-intercept, we can find the equation
as follows (note that in this case y corresponds
to the mapping equation M[ ], and x is the input
image gray level (brightness) values I(r,c) ):
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1) We know two points on the line, (28,28) and
(75,255), so:
2)
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3) So the equation of the line for the range
between 28 and 75 is:
4) Therefore:
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Gray-level Stretching with Clipping at Both Ends
Original image
Modified image with stretched gray levels
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A digital negative
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Digital Negative
Mapping Equation
Original image
Modified by the inverse mapping equation
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