Blend modes

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 (x, y, z) 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.

Normal blend mode

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.

Dissolve

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

Multiply and Screen blend modes are basic blend modes for lightening or darkening images. There are several different combinations of them like Overlay or Soft Light (mentioned further down) and Vivid Light, Linear Light and Pin Light.

Multiply

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 can get values as high as 65025, 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.

creen

With Screen blend mode the values of the pixels in the two layers are inverted, multiplied, and then inverted 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

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.

oft Light

This is a softer version of Overlay. Applying pure black or white does not result in pure black or white.

Hard Light

Hard Light combines Multiply and Screen blend modes. As opposed to Overlay, the contrast is also increased.

Dodge and Burn

Dodge and burn change the lightness of the pictures. Dodge makes them brighter and burn makes them darker. Color dodge and color burn change the contrast to change the lightness. Linear dodge and linear burn change the brightness to change the lightness.

Dodge

This blend inverts both layers, divides them, and then the result is inverted again. This lightens the upper layer. (i.e color = layer1 / 1 - layer2 )

Color Dodge

This decreases the contrast to make the bottom layer reflect the top layer: the brighter the top layer, the more its colour affects the bottom layer. Blending with white gives white. Blending with black does not change the image. This effect is similar to changing the white point.

Linear Dodge

This increases the brightness (but does not change the contrast) to make the bottom layer reflect the top layer: the brighter the top layer, the more its colour affects the bottom layer. Blending with white gives white.

Burn

While dodge inverts, divides and inverts, "burn" inverts, multiplies and inverts. This darkens the top layer.

Color Burn

This darkens the top layer increasing the contrast to reflect the colour of the bottom layer. The darker the bottom layer, the more its colour is used. Blending with white produces no difference. This is similar to changing the black point.

Linear Burn

This darkens the top layer decreasing the brightness (not touching the contrast) to reflect the colour of the bottom layer. The darker the bottom layer, the more its colour is used. Blending with white produces no difference. This is similar to changing the black point.

Hard Mix

This blend mode combines Color Dodge and Color Burn. Dodge applies to values greater than 128 dodge and burn to values less than 128.

imple arithmetic blend modes

Divide

This blend mode simply divides pixel values of one layer with the other.

Addition

This blend mode simply adds pixel values of one layer with the other. In case of values above 256 (in the case of RGB), white is displayed.

ubtract

This blend mode simply subtracts pixel values of one layer with the other. In case of negative values, black is displayed.

Difference

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 Only

Darken takes the darkest value for each pixel from each layer.

Lighten Only

Lighten takes the lightest pixel from each layer.

Hue, Saturation and Brightness

The following blend modes look at the values of hues, saturation and brightness.

Hue

Hue takes the hue from the top layer, but takes the brightness and saturation from the bottom layer.
* Top: H
* Bottom: BS

aturation

Saturation takes its saturation from the top layer but brightness and hue from the bottom layer.
* Top: S
* Bottom: BH

Color

Color takes its brightness from the bottom layer, but hue and saturation from the top layer.
* Top: HS
* Bottom: B

Luminosity

Luminosity takes its brightness from the top layer, but hue and saturation from the bottom layer. It can be used to eliminate color halos caused by sharpening.
* Top: B
* Bottom: HS

ee also

*Alpha compositing
*Comparison of raster graphics editors
*Digital image editing
*Raster graphics
*Image processing

References

* [http://www.nathanm.com/photoshop-blending-math/ Blend modes math]
* [http://www.ronbigelow.com/articles/blend1/blend1.htm Ron Bigelow's tutorials]
* [http://docs.gimp.org/en/gimp-tool-curves.html The gimp manual]
* [http://livedocs.adobe.com/flash/9.0/UsingFlash/help.html?content=WSd60f23110762d6b883b18f10cb1fe1af6-7d5a.html Adobe's website on blend modes]
* [http://www.adobe.com/designcenter/indesign/articles/idsn3kbtrans/idsn3kbtrans.pdf Adobe "Master transparency and blends" pdf file]