In digital image editing and computer graphics, blend modes are algorithms used to determine how two layers are blended with each other. The default blend mode in most applications is simply to obscure the lower layer by covering it with whatever is present in the top layer (see alpha compositing); because each pixel has numerical values, there also are many other ways to blend two layers.
Most graphics editing programs, such as Adobe Photoshop and GIMP, allow users to modify the basic blend modes—by, for example, applying different levels of opacity to the top "layer". The top "layer" is not necessarily a layer in the application; it may be applied with a painting or editing tool. The top "layer" also is called the "blend layer" and the "active layer".
In the formulas shown on this page, values go from 0.0 (black) to 1.0 (white).