User Manual

object.
x = 1 - [background Alpha], y = 0
 Atop: Atop places the foreground object over the background object only where the
background object has a matte.
x = [background Alpha], y = 1 - [foreground Alpha]
 XOr: XOr combines the foreground object with the background object wherever either
the foreground or the background have a matte, but never where both have a matte.
x = 1 - [background Alpha], y = 1-[foreground Alpha]
Subtractive/Additive
This slider controls whether Fusion performs an Additive composite, a Subtractive composite,
or a blend of both when the duplicate objects overlap. This slider defaults to Additive assuming
the input image’s Alpha channel is premultiplied (which is usually the case). If you don’t
understand the difference between Additive and Subtractive compositing, here’s a quick
explanation.
An Additive blend operation is necessary when the foreground image is premultiplied, meaning
that the pixels in the color channels have been multiplied by the pixels in the Alpha channel.
The result is that transparent pixels are always black since any number multiplied by 0 always
equals 0. This obscures the background (by multiplying with the inverse of the foreground
Alpha), and then adds the pixels from the foreground.
A Subtractive blend operation is necessary if the foreground image is not premultiplied. The
compositing method is similar to an additive composite, but the foreground image is first
multiplied by its Alpha, to eliminate any background pixels outside the Alpha area.
While the Additive/Subtractive option is often an either/or mode in most other applications, the
Duplicate node lets you blend between the Additive and Subtractive versions of the
compositing operation. This can be useful for dealing with problem composites with bright or
dark edges.
For example, using Subtractive merging on a premultiplied image may result in darker edges,
whereas using Additive merging with a non-premultiplied image causes any non-black area
outside the foreground’s Alpha to be added to the result, thereby lightening the edges. By
blending between Additive and Subtractive, you can tweak the edge brightness to be just right
for your situation.
Gain
The Gain RGB controls multiply the values of the image channel linearly. All pixels are multiplied
by the same factor, but the effect is larger on bright pixels and smaller on dark pixels. Black
pixels are not changed since multiplying any number times 0 always equals 0.
Alpha Gain linearly scales the Alpha channel values of objects in front. This effectively reduces
the amount that the objects in the background are obscured, thus brightening the overall result.
When the Subtractive/Additive slider is set to Additive with Alpha Gain set to 0.0, the
foreground pixels are simply added to the background.
When Subtractive/Additive slider is set to Subtractive, this controls the density of the composite,
similarly to Blend.
Chapter – 88 Effect Nodes 1944