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Left: histogram of original image, Right: histogram of enhanced image
Histogram stretching can be seen as a Window and Level contrast enhancement technique where the window ranges from the minimum to the maximum pixel values of the image.
The image shown below has been enhanced via histogram stretching. Also, its new histogram is plotted. Note that the histogram now ranges from 0 to 255. This operation can be performed by normalization, which is a procedure that maps the minimum value of the image to 0 and its maximum value to 255.
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Left: histogram of original image, Right: histogram of enhanced image
This normalization or histogram stretching operation is automatically performed in many display operators. To fully illustrate the difference between the two images above (original and stretched) a grey level scale was superimposed in both images to guarantee that the display operator will use the same greylevel scale.