The plugin Equalizer 2 offers three methods of equalization, with options to make this process more exploitable. Roughly speaking, equalization is a process that generate for each image a theoretically "optimal" curve, which once applied to that image, improve global contrast in a relevant way.
New in version 2: The biggest change is that the three parameters (highlights, shadows, amount) has been replaced by a more useful one named "High Frequency Impact" (few explanations in first example).
Example 1: In the left picture below, the white pixels predominates. Because of this, a "standard" equalization (see HFI=100, at right) boost the contrast in the white area to the max. This contrast boost correspond to a "big jump" in the applied equalization curve (see red arrow). On the center pic below (HFI=20) , the HFI parameter, allows to reduce the impact of the predominant colors.
HFI = 20
HFI = 100
Curve for HFI = 20
Curve for HFI = 100
Example 2: Here we apllied the "Levels" equalization method (HFI=30, see center pic) . With this method, the option "Restrict Color Shifts" is available and can be occasionally helpful. Here, this option helped to remove the unwanted pink color at the fawn's muzzle (see right pic).
Restrict Color Shifts
Here's a brief description of the three equalization methods, and options:
Contrast: Usually the most reliable method. Apply a single curve. Notice that the equalization curve might differ , if you check the "Preserve hue" option.
Levels: This method can help sometimes to remove color cast. Because it applies a different equalization curve to each channel, it can also lead to the apparition of new unwanted colors. To limit the "damages", the option "Restrict Color Shifts" was added.
Luminosity: Only equalize the luminosity. Three different ways to calculate the lumosity are suggested (use the small slider). The value "Perceptual" is the most representative of our perception of luminosity.