Equalize

The equalize transformation computes and applies a brightness curve to the input image so that the histogram of the output image on average matches a shape you specify. The curve on the left defines the desired output histogram shape. The histogram on the left is the histogram of the input image. The curve on the right is the adjustment curve applied to the input image to produce the output image. The histogram on the right is the histogram of the output image. The curve on the right is for reference purposes only and cannot be adjusted.

Standard histogram equalization works by trying to adjust the image to a flat curve as illustrated below. This causes each tonal values in the output image to be roughly equally represented, often bringing out details that would otherwise be obscured. In addition to flat histogram standard equalization, you can adjust the curve that shapes the output image histogram by changing the curve on the left to the desired histogram shape. The closer you make the curve to the input histogram, the more closely the curve on the right gets to a diagonal from the lower left to the upper right corner. For example, lowering the curve in the center increases midtone contrast by forcing the image to have more lights and darks than midtones.

Amount

The amount control lets you control how much of the equalize transformation is applied to the input image. You can apply a percentage of the transformation to the entire image, or you can specify an amount mask to restrict the effects of the transformation to only part of the input image.

Color Space

This control is only displayed if the input image is a color image. It lets you select which brightness component of the input image is equalized: the HSV value or the HSL lightness.

Input Curve

The curve control lets you specify the desired shape of the histogram of the output image. The histogram of the input image is displayed in the background for reference. When you force the histogram to be constant as illustrated above, the effect is called histogram equalization. This means that all brightness levels are roughly equally represented in the image, and sometimes this works well to extract detail from an image that would otherwise be hard or impossible to discern. By adjusting the curve, you can alter the distribution of different brightness levels in the image to add or remove detail in different parts of the tonal range.

Output Curve

The output curve displays the curve that Equalize applies to the input image to create the output image. The histogram in the background is the histogram of the output image.

In both the input and output curves, heavy histogram smoothing is selected since especially the output histogram tends to be noisy.