Blend 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.
The currently most common numerical representation of colors is the one used in RGB (red, green, blue) images, where three numbers (r, g, b) can take values between 0 and 255, each of them indicating how much red, green and blue the pixel contains. This means for example that (255, 0, 0) is intense red and (0, 0, 255) is pure blue. There are other color models, which have other number representations. For the purpose of blend modes, the principles are very similar for different color models, even though not all blend modes can be applied with all color models.
Most applications, like Adobe Photoshop and the GIMP, allow the user to modify the basic blend modes - for example by applying different levels of opacity to the top picture.
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This is the standard blend mode which simply takes each pixel from the top layer if present. Otherwise the bottom layer is used. The article about layers has several examples of this.
The dissolve mode takes random pixels from both layers. With high opacity, most pixels are taken from the top layer. 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.
Multiply and Screen blend modes are basic blend modes for darkening and lightening images respectively. There are several different combinations of them like Overlay or Soft Light (mentioned further down) and Vivid Light, Linear Light and Pin Light.
Multiply blend mode multiplies the numbers for each pixel of the top layer with the corresponding pixel for the bottom layer. As a simple multiplication for 8-bit/channel can get values as high as 65025 (255*255), which is far higher than the maximum allowed value, 255, the result is divided by 255.
The result is a darker picture.
Formula: Result Color = (Top Color) * (Bottom Color) /255
If the two layers contain the same picture, multiply blend mode is equivalent to an exponential curve.
If one layer contains a homogeneous colour, for example the gray color (200, 200, 200), multiply blend mode is equivalent to a curve which simply is a straight line.
For image editing it is sometimes more convenient to simply go to the curves dialog of the software, as it gives more flexibility in the shape of the curves.
With Screen blend mode the values of the pixels in the two layers are negated, multiplied, and then negated again. This is in some way the opposite of multiply.
The result is a brighter picture.
Formula: Result Color = 255 - [((255 - Top Color)*(255 - Bottom Color))/255]
Just like for multiply blend mode, using the curves dialog in the software sometimes is easier to use, as it increases the flexibility and control for the user.
Overlay combines Multiply and Screen blend modes. Light parts of the picture become lighter and dark parts become darker. An overlay with the same picture looks like an S-curve.
This is a softer version of Overlay. Applying pure black or white does not result in pure black or white.
Hard Light combines Multiply and Screen blend modes. As opposed to Overlay, the contrast is also increased.
Dodge and burn change the lightness of the pictures, inspired by the dodging and burning performed in a darkroom. Dodging lightens an image, while burning darkens it.
This blend mode simply divides pixel values of one layer with the other.
This blend mode simply adds pixel values of one layer with the other. In case of values above 255 (in the case of RGB), white is displayed.
This blend mode simply subtracts pixel values of one layer with the other. In case of negative values, black is displayed.
Difference subtracts the top layer from the bottom layer or the other way round, to always get a positive value. Blending with black produces no change, as values for all colours are 0. (The RGB value for black is 0,0,0). Blending with white inverts the picture.
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. Exclusion is a very similar blend mode with lower contrast.
Darken takes the darkest value for each pixel from each layer.
Lighten takes the lightest pixel from each layer.
Rarely, applications such as Aviary's Peacock will supply boolean arithmetic blend modes. These combine the binary expansion of the hexadecimal color at each pixel of two layers using boolean logic gates. The top layer's alpha controls interpolation between the lower layer's image and the combined image.
Photoshop’s hue, saturation, color, and luminosity blend modes are based on a color space with dimensions that the article HSL and HSV calls hue, chroma, and luma. Note that this space is different from both HSL and HSV, and only the hue dimension is shared between the three; see that article for details.
Unlike all of the previous blend modes described, which operate on each image channel independently, in each of these modes, some dimensions are taken from the bottom layer, while the remainder are taken from the top layer. Colors which end up out of gamut are brought inside by mapping along lines of constant hue and luma. This makes the operations uninvertible – after a top layer has been applied in one of these blend modes, it is in some cases impossible to restore the appearance of the original (bottom) layer, even by applying a copy of the bottom layer in the same blend mode above both.
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. The Luminosity mode is commonly used for image sharpening, because human vision is much more sensitive to fine-scale lightness contrast than color contrast. See Contrast (vision).
Few editors other than Photoshop implement this same color space for their analogs of these blend modes. 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.