Contrast Stretching or Contrast Normalization an Image in MATLAB
Contrast stretching is a simple image
enhancement technique that improves the contrast in an
image by expanding the dynamic range of intensity values it contains. It can only apply a linear scaling function
to the image pixel values.
Some Mathematical Modelling: (source: ed.ac.uk)
It is necessary to specify the
upper and lower pixel value limits over which the image is to be
normalized. Often these limits will just be the minimum and maximum
pixel values that the image type concerned allows. For example for 8-bit
graylevel images the lower & upper limits might be 0 and 255. Call
the lower and the upper limits a and b respectively.
The simplest of stretching scans the image to find the
lowest & highest pixel values in the image. Call
these c and d. Then each pixel P is scaled using the
following function:
Values below 0 are set to 0 and values about 255 are set to 255.
MATLAB implementation of the above concept of Contrast Stretching or Contrast Normalization on an image is:
X = imread('image.jpg'); %reading a grayscale image
figure(1);
imshow(X);
title('Original Image')
a = min(X(:)); %minimum pixel of image X
b = max(X(:)); %maximum pixel of image X
X= (X-a).*(255/(b-a)); %just using the formula above
figure(2);
imshow(X);
title('Contrast Streached Image')
figure(1);
imshow(X);
title('Original Image')
a = min(X(:)); %minimum pixel of image X
b = max(X(:)); %maximum pixel of image X
X= (X-a).*(255/(b-a)); %just using the formula above
figure(2);
imshow(X);
title('Contrast Streached Image')
Results of Contrast Stretching in MATLAB:
 |
| The original Image: Not Contrast Stretched |
 |
| The Resultant Image: Contrast Stretched |
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