2. Overview
Introduction
Image enhancement
Contrast enhancement
Linear contrast stretching
Non Linear contrast stretching
3. Image Enhancement
Definition: It is the process of improving the visual quality
of the image by manipulating the gray levels of the pixels
for a specific application.
Processing of image to bring out specific features of an
image.
Highlight certain characteristics of image.
4. Contrast Enhancement
Contrast is the difference in colour that makes an object (or its representation
in an image or display) distinguishable.
Contrast enhancement or stretching : Stretching the gray levels in an image to
increase the contrast between the features.
Used to increase the dynamic range of gray levels in the image
Example:
Number of gray levels possible in an 8-bit system = 256 (0 to 255).
The image pixel values are stretched to use the entire gray level range i.e. from
0 to 255.
6. Linear Contrast Stretching …
Min-max stretching:-
Corresponds to the maximum
number of display levels
0-255 in an 8-bit system
0-127 in a 7-bit system
in
The basic purpose of contrast enhancement is to expand the original
brightness values to make use of the full range of the radiometric scale of
the sensor.
7. Landsat TM Band-5 image and Histogram
Displayed in an 8-bit system
Image is vague
DN values range from 60-158
0-59 and 159-255 are not utilized
DN values are stretched to 0-255
Contrast is improved
Light tones appear lighter
Dark tones appear darker
Linearly stretched Landsat TM Band-5 image and Histogram
Linear Contrast Stretching…
8. Non-linear Contrast Stretching
DN values are not stretched linearly to uniformly
occupy the entire display range.
Histogram-equalized stretch
Piece-wise linear stretch
Logarithmic, Power law or Gaussian stretch
9. Histogram-Equalized Stretch
Image Histogram a graphical representation of the brightness value that
comprise an image. The brightness value are displayed along x-axis and
frequency of occurrence of each these values on y-axis of the graph.
DN values are enhanced based on their frequency in the original image
DN values corresponding to the peaks of the histogram are assigned to a
wider range
More display levels to the higher frequency region gives better
information enhancement
Original histogram
Adjusted histogram
Transformation
10. Histogram-Equalized Stretch…
DN values are enhanced based on their frequency in the original image
Example: Enhancement in an 8-bit display system
k
j
j
st
N
n
DN
0
255
DNst = Enhanced DN value
nj = Number of pixels having DN values in the jth range, in the
input image
k = Maximum number of DN ranges in the input image
N = Total number of pixels in the input image
12. Most of the image enhancement procedures are subjective, due to
which a procedure well suited to one application or one user, may not be
suitable to another application for another user.
“ One man’s enhancement is another man’s noise.”
