Blend modes  Blend modes (or mixing modes) in digital image editing and computer graphics are 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). However, as each pixel has a numerical representation, there exist a large number of ways to blend two layers. Most graphics editing programs like Adobe Photoshop and GIMP allow the user to modify the basic blend modes, e.g. by 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" can also be referred to as the "blend layer" or "active layer". In the formulas shown on this page, values go from 0.0 (black) to 1.0 (white). 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 (making the actual formula formula_2), 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. The dissolve mode takes random pixels from both layers. With top layer opacity greater than that of the bottom layer, most pixels are taken from the top layer, while with low opacity most pixels are taken from the bottom layer. No anti-aliasing is used with this blend mode, so the pictures may look grainy and harsh. Adobe Photoshop generates a pseudo-random noise dither pattern on startup, with each pixel location in a 2D raster array assigned a gray value (R=G=B) and an alpha value of 1 (“on”). As the opacity of the top layer is reduced, the alpha value of some of the gray pixels is switched from 1 to 0 ("off"), with the result that image pixels corresponding to a gray-valued pixel in the raster array are either on (visible, opaque) or off (invisible, transparent), with no opacity gradation. Multiply and Screen blend modes are basic blend modes for darkening and lightening images, respectively.