High contrast effect. Converts all colours to either black or white depending on their brightness values. Use the slider bar in the option window to set the medium brightness value. More powerful than the Threshold option, this option allows you to retain as many colours as you want. The levels box refers to the value of brightness.
Lower values result in more dramatic effects.
The higher the value the darker the image will become. This feature is a more powerful version of the levels option. The option allows you to distribute brightness values by drawing curves on a graph. An excellent starting tool. Allows you to apply the changes to individual colour channels and affect all colours equally across the spectrum.