Blend modes

Blend modes (or Mixing modes) in digital image editing are used to determine how two layers are blended into each other. The default blend mode in most applications is simply to hide the lower layer with whatever is present in the top layer. However, as each pixel has a numerical representation, a large number of ways to blend two layers is possible. Note that the top layer is not necessarily called a "layer" in the application. It may be applied with a painting or editing tool.

Most graphics editing programs, like Adobe Photoshop and GIMP, allow the user to modify the basic blend modes - for example by applying different levels of opacity to the top picture.

Normal blend mode

This is the standard blend mode which uses the top layer alone, without mixing its colors with the layer beneath it.

Where a is the value of a color channel in the underlying layer, and b is that of the corresponding channel of the upper layer. The result is most typically merged into the bottom layer using "simple" (b over a) alpha compositing, but other Porter-Duff operations are possible. The compositing step results in the top layer's shape, as defined by its alpha channel, appearing over the bottom layer.