Keywords: front-page

# Histogram Equalization

[Laboratory]

Histogram equalization is as a contrast enhancement technique with the objective to obtain a new enhanced image with an uniform histogram. This can be achieved by using the normalized cumulative histogram as the grey scale mapping function.

The intermediate steps of the histogram equalization process are:

1. Take the cumulative histogram of the image to be equalized
2. Normalize the cumulative histogram to 255
3. Use the normalized cumulative histogram as the mapping function of the original image

These intermediate steps are illustrated below.

a)original image; b)its histogram
a) | b)

a)normalized cumulative histogram of the original image; b)histogram equalized image
a) | b)

a)histogram of the equalized image; b)cumulative histogram of the equalized image
a) | b)

Due to the discrete nature of the problem, the resultant histogram is not uniform as desired, but you can see from the cumulative equalized histogram that it does approximate to a straight line.

You can get a much uniformer histogram if you artificially increase the quantization of the original image before applying the equalization. You can check this in the "Other Examples" section.

- Lab: Histogram Equalization
- Other Examples