8.7. Equalize

The Equalize command automatically adjusts the brightness of colors across the active layer so that the histogram for the Value channel is as nearly flat as possible, that is, so that each possible brightness value appears at about the same number of pixels as every other value. You can see this in the histograms in the example below, in that pixel colors which occur frequently in the image are stretched further apart than pixel colors which occur only rarely. The results of this command can vary quite a bit. Sometimes Equalize works very well to enhance the contrast in an image, bringing out details which were hard to see before. Other times, the results look very bad. It is a very powerful operation and it is worth trying to see if it will improve your image. It works on layers from RGB and Grayscale images. If the image is Indexed, the menu entry is insensitive and grayed out.

8.7.1. Activate the Command

  • You can access this command from the image menubar through ColorsAutoEqualize

  • or by using the keyboard shortcut Shift+Page_Down.

8.7.2. Equalize example

Figure 15.131. Original image

Original image

The active layer and its Red, Green, Blue histograms before Equalize.

Figure 15.132. Image after the command

Image after the command

The active layer and its Red, Green, Blue histograms after treatment.

Histogram stretching creates gaps between pixel columns giving it a striped look.