Chapter 9

Histogram processing

What will we learn?

- What is the histogram of an image?
- How can the histogram of an image be computed?
- How much information does the histogram provide about the image?
- What is histogram equalization and what happens to an image whose histogram is equalized?
- How can the histogram be modified through direct histogram specification and what happens to an image when we do it?
- What other histogram modification techniques can be applied to digital images and what is the result of applying such techniques?

9.1 Image histogram: definition and example

The histogram of a monochrome image is a graphical representation of the frequency of occurrence of each gray level in the image. The data structure that stores the frequency values is a 1D array of numerical values, , whose individual elements store the number (or percentage) of image pixels that correspond to each possible gray level.

Each individual histogram entry can be expressed mathematically as

Here, 0, 1, ..., , where is the number of gray levels of the digitized image, and denotes the cardinality ...