Biomedical Image Processing – V

This post is a continuation from Biomedical Image Processing – IV.

Contrast Stretching

Contrast stretching (a.k.a. normalization) is very similar to histogram equalization, but it maps only a certain portion of the histogram to the full range. An example of this image enhancement technique is illustrated in Figure 17. If a and b are the lowest and highest pixel values available (e.g., 0 and 255, respectively, in an 8-bit gray scale image), and c and d are the lowest and highest current pixel values, then the mapping function works in the following manner, where Pout is the output pixel and Pin is the input pixel:


8-bit gray scale images

In an 8-bit gray scale image, any values of Pout below 0 or above 255 are set to 0 and 255, respectively.


Figure 17. Example of contrast stretching [].

Logarithmic and Exponential Operators

Logarithmic/exponential (a.k.a. “raise to power”) operators are used to enhance low/high pixel values, respectively, where a relative loss of information occurs in the high/low pixel values.


Leave a Reply

Your email address will not be published. Required fields are marked *