Image Processing Toolbox User's Guide

Contrast-Limited Adaptive Histogram Equalization

As an alternative to using histeq, you can perform contrast-limited adaptive histogram equalization (CLAHE) using the adapthisteq function. While histeq works on the entire image, adapthisteq operates on small regions in the image, called tiles. Each tile's contrast is enhanced, so that the histogram of the output region approximately matches a specified histogram. After performing the equalization, adapthisteq combines neighboring tiles using bilinear interpolation to eliminate artificially induced boundaries.

To avoid amplifying any noise that might be present in the image, you can use adapthisteq optional parameters to limit the contrast, especially in homogeneous areas.

To illustrate, this example uses adapthisteq to adjust the contrast in a grayscale image. The original image has low contrast, with most values in the middle of the intensity range. adapthisteq produces an output image having values evenly distributed throughout the range.

• I = imread('pout.tif');
  J = adapthisteq(I);
  imshow(I)
  figure, imshow(J)
  
  
  

Image After CLAHE Equalization with Its Histogram

Histogram Equalization

Decorrelation Stretching

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