Comp Alpha Tool

The Comp Alpha tool lets you composite two mattes together. It has front and back inputs, and works by extracting a matte from the front image and compositing it over the alpha channel of the back input using a choice of compositing operators. The back is the primary input and the output inherits the format of the back input.

This tool only affects alpha. If the back is an RGBA image, the color part is simply copied to the output.

Note The alpha output of this tool is always clamped to the [0,1] interval.

The Comp Alpha tool has the following parameters:

Front Channel: Selects which channel to use for the front (default is alpha).
Front Invert: Inverts the front before o using it (default is off).
Front Intensity: Specifies the intensity of the front layer. Default is 100% and range is [0,1].
Front Opacity: Controls the opacity of the front in the compositing. If the opacity is less then one, the front will get more transparent and you will start seeing the back through it. Default is 100%; range is [0,1].
Back Channel: Selects which channel to use for the back (default is alpha).
Back Invert: Inverts the back before using it (default is off).
Back Intensity: Specifies the intensity of the back layer. Default is 100%; range is [0,1].
Comp Mode: Determines which compositing mode will be used (default is Over)—see Compositing Operators.
Correlation: Specifies how the two input mattes are correlated. This can be used to improve the quality of the composite in special cases. For example, if you composite two mattes that share a good portion of their outline, you should indicate if they are Adjacent or Superposed. By default, the correlation mode is None, assuming that normally, the input mattes are not correlated.

Compositing Operators

The following table lists the compositing operators applicable between front and back input images:

Operator	Result:
Over	Composites the front over the back. The output will cover any area covered by either the front or the back. Where the front and back overlap, the output will show the front.
Replace	Completely replaces the back image with the front image.
Atop	Similar to the Over operator, but the output will cover the same area covered by the back.
Inside	Composites the front over the back, but the output will cover only the area covered by both front and back.
Outside	The output will be equal to the front except that the part of the front overlapping the back image will be missing. No part of the back will be visible in the output in any case.
Cutout	The output will be equal to the back except that the part of the back covered by the front image will be missing. No part of the front will be visible in the output in any case. Basically this operator is equal to the Outside operator with the front and back roles reversed.
Xor	Both front and back are copied to the output except for the part where they overlap.

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