Blend modes
Most graphics editing programs, such as Adobe Photoshop and GIMP, allow users to modify the basic blend modes, for example by applying different levels of opacity to the top "layer".
The result is most typically merged into the bottom layer using "simple" (b over a) alpha compositing (making the actual formula
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").
Multiply blend mode takes the RGB channel values from 0 to 1 of each pixel in the top layer and multiplies them with the values for the corresponding pixel from the bottom layer.
If the two layers contain the same picture, multiply blend mode is equivalent to a quadratic curve, or gamma correction with γ=2.
This is also equivalent to using this gray value as opacity when doing "normal mode" blend with a black bottom layer.
The result is the opposite of Multiply: wherever either layer was darker than white, the composite is brighter.
If one layer contains a homogeneous gray, Screen blend mode is equivalent to using this gray value as opacity when doing "normal mode" blend with white top layer.
The inverse relationship between Overlay and Hard Light makes them "commuted blend modes".
[example needed] There are a variety of different methods of applying a soft light blend.
Pegtop's formula[6] is smoother and corrects the discontinuity[better source needed] at b = 0.5:
This blend simply divides pixel values of one layer with the other, but it's useful for brightening photos if the colour is on grey or less.
This blend mode simply adds pixel values of one layer with the other.
This blend mode simply subtracts pixel values of one layer with the other.
One of the main utilities for this is during the editing process, when it can be used to verify alignment of pictures with similar content.
A few applications, such as Aviary's Peacock and KDE's Krita,[1] supply boolean arithmetic blend modes.
These combine the binary expansion of the hexadecimal color at each pixel of two layers using boolean logic gates.
Note: this space is different from both HSL and HSV, and only the hue dimension is shared between the three.
Colors which end up out of gamut are brought inside by mapping along lines of constant hue and luma.
Because these blend modes are based on a color space which is much closer than RGB to perceptually relevant dimensions, it can be used to correct the color of an image without altering perceived lightness, and to manipulate lightness contrast without changing the hue or chroma.
(See Contrast (vision)) Few editors other than Photoshop implement this same color space for their analogs of these blend modes.
[3] Instead, they typically base their blend modes on HSV (aka HSB) or HSL.
Blend modes based on HSV are typically labeled hue, saturation, and brightness.
Using HSL or HSV has the advantage that most operations become invertible (at least in theory), but the disadvantage that the dimensions of HSL and HSV are not as perceptually relevant as the dimensions of the space Photoshop uses.
The result of applying several of these modes depends linearly on the pixel level of the top layer.
Many image manipulation programs do not allow such masks; for them this equivalence holds only for grayscale top layers.)
When blending modes are used with these tools, the result is calculated based on pixels already existing on the target layer.
Subsequent strokes that overlap are then calculated based on the tool's blending mode, and the result is applied directly to the layer.
Put differently, painting tools alter the pixels on a layer; blend modes applied to two layers don't alter any pixels, but only affect the resulting visual image.
This distinction is useful to create various effects on a single layer, such as when applying dodge and burn techniques, where painting with a low opacity in screen or multiply modes allows the user to build up or reduce the results in a more organic way on a single layer.
