LAYER MODES

Layer modes are essentially an offsetting or rendering rule of the RGB values of the pixels in two neighbouring visible layers. The layer mode of the upper layer determines how the pixel values are offset against the layer below. GIMP then displays the result of this calculation. The outcome often looks rather strange, but it can be extremely useful. For example, the positions of layers can be checked to make sure they are pixel perfect with the Grain Extract mode, where any deviations clearly stand out. The subject is covered in detail in the GIMP manual at https://docs.gimp.org/2.10/en_GB/gimpconcepts-layer-modes.html, including the formulas used for offsetting.

In practice, it is often helpful to place your mouse cursor over the Mode button and then use the mouse wheel to try out all the different modes in turn. However, a lot of modes have very strong effects, so they tend to only be used on layers that have reduced opacity.

A typical application of layer modes is to darken or lighten highlights and shadows. To lighten dark images, for example, the Screen mode is a good choice. It has the effect that dark areas are lightened much more than light areas, which corresponds to how the human eye perceives things. You can see GIMP’s Screen layer mode in action in the screenshot (above-right). In some situations, the Addition and Dodge modes can also be helpful. Several copies of a layer can be superimposed to intensify the effect.

For images that are too bright, the Overlay and Multiplication layer modes, along with the different Light modes (Soft, Hard, Vivid), should be tried.