# Adjust Image Contrast Using Histogram Equalization

This example shows how to adjust the contrast of a grayscale image using histogram equalization.

Histogram equalization involves transforming the intensity values so that the histogram of the output image approximately matches a specified histogram. By default, the histogram equalization function, histeq, tries to match a flat histogram with 64 bins such that the output image has pixel values evenly distributed throughout the range. You can also specify a different target histogram to match a custom contrast.

### Original Image Histogram

Read a grayscale image into the workspace.

Display the image and its histogram. The original image has low contrast, with most pixel values in the middle of the intensity range.

figure
subplot(1,3,1)
imshow(I)
subplot(1,3,2:3)
imhist(I)

### Adjust Contrast Using Default Equalization

Adjust the contrast using histogram equalization. Use the default behavior of the histogram equalization function, histeq. The default target histogram is a flat histogram with 64 bins.

J = histeq(I);

Display the contrast-adjusted image and its new histogram.

figure
subplot(1,3,1)
imshow(J)
subplot(1,3,2:3)
imhist(J)

### Adjust Contrast, Specifying Number of Bins

Adjust the contrast, specifying a different number of bins. With a small number of bins, there are noticeably fewer gray levels in the contrast-adjusted image.

nbins = 10;
K = histeq(I,nbins);

Display the contrast-adjusted image and its new histogram.

figure
subplot(1,3,1)
imshow(K)
subplot(1,3,2:3)
imhist(K)

### Adjust Contrast, Specifying Target Distribution

Adjust the contrast, specifying a nonflat target distribution. This example demonstrates a linearly decreasing target histogram, which emphasizes small pixel values and causes shadows to appear darker. Display the target histogram.

target = 256:-4:4;
figure
bar(4:4:256,target)

Adjust the histogram of the image to approximately match the target histogram.

L = histeq(I,target);

Display the contrast-adjusted image and its new histogram.

figure
subplot(1,3,1)
imshow(L)
subplot(1,3,2:3)
imhist(L)