Alpha Over Node

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Alpha Over Node.

The Alpha Over node is used to layer images on top of one another. Alpha Over does not work on the colors of an image.

Inputs

Factor
Controls the amount of influence the node exerts on the output image.
Image
The background image.
Image
The foreground image. Where the image pixels has an alpha greater than 0, the background image will be overlaid.

Properties

Convert Premultiplied
Strange Halos or Outlines.
Premultiply
Mix Factor. See Alpha Channel.

Outputs

Image
Standard image output.

Strange Halos or Outlines

This section clarifies the functionality of the premultiplied alpha button. An alpha channel has a value of between 0 and 1. To make an image transparent (to composite it over another one), the RGB pixel values are multiplied by the alpha values (making the image transparent (0) where the alpha is black (0), and opaque (1) where it is white (1)).

To composite image A over image B, the alpha of image A gets multiplied by image A, thus making the image part of A opaque and the rest transparent. Then the alpha channel of A is inverted and multiplied by image B, thus making image B transparent, where A is opaque and vice versa. To get the final composite the resultant images are added.

A premultiplied alpha is, when the image (RGB) pixels are already multiplied by the alpha channel, therefore, the above compositing operation does not work too well, and Convert Premultiplied has to be enabled. This is only an issue in semitransparent area and edges usually. The issue normally occurs in a node setup, in which two images previously combined with alpha, then are combined again with yet another image. The previously combined image was already multiplied (premultiplied) and needs to be converted as such (hence, Convert PreMul).

If multiplied twice artifacts like a white or clear halo occur around where the image meet, since the alpha value is being squared or cubed. It also depends on whether or not the image has been rendered as a premultiplied, or as a straight RGBA image.

Examples

In this example, an image of a Cube is superimposed on a cliff background. Use the PreMultiply button, when the foreground image and background images have a combined Alpha that is greater than 1.00; otherwise, you will see an unwanted halo effect. The resulting image is a composite of the two source images.

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Assembling a composite image using Alpha Over.

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Animated see-through/sheer SFX using Alpha Over on frame 11.

In this example, we use the Factor control to make a sheer cloth or onion-skin effect. This effect can be animated, allowing the observer to “see-through” walls (or any foreground object) by hooking up a Time node to feed the Factor socket as shown below. In this example, over the course of 30 frames, the Time node makes the Alpha Over node produce a picture that starts with the background cliff image, and slowly bleeds through the cube. This example shows frame 11 just as the cube starts to be revealed.