User Manual

The Operator modes are as follows:
–
Over: The Over mode adds the foreground layer to the background layer by
replacing the pixels in the background with the pixels from the Z wherever the
foreground’s Alpha channel is greater than 1.
x = 1, y = 1-[foreground Alpha]
–
In: The In mode multiplies the Alpha channel of the background input against the
pixels in the foreground. The color channels of the foreground input are ignored.
Only pixels from the foreground are seen in the final output. This essentially clips the
foreground using the mask from the background.
x = [background Alpha], y = 0
–
Held Out: Held Out is essentially the opposite of the In operation. The pixels in
the foreground image are multiplied against the inverted Alpha channel of the
background image. Accomplish exactly the same result using the In operation and a
Matte Control node to invert the matte channel of the background image.
x = 1-[background Alpha], y = 0
–
Atop: Atop places the foreground over the background only where the background
has a matte.
x = [background Alpha], y = 1-[foreground Alpha]
–
XOr: XOr combines the foreground with the background wherever either the
foreground or the background has a matte, but never where both have a matte.
x = 1-[background Alpha], y = 1-[foreground Alpha]
 Subtractive/Additive slider: This slider controls whether Fusion performs an Additive
merge, a Subtractive merge, or a blend of both. This slider defaults to Additive merging
for most operations, assuming the input images are premultiplied (which is usually
the case). If you don’t understand the difference between Additive and Subtractive
merging, here’s a quick explanation.
–
An Additive merge 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 simply adds the
pixels from the foreground.
 A Subtractive merge is necessary if the foreground image is not pre-multiplied.
The compositing method is similar to an additive merge, but the foreground image
is first multiplied by its own Alpha to eliminate any background pixels outside the
Alpha area.
While the Additive/Subtractive option could easily have been a checkbox to select one
mode or another, the Merge node lets you blend between the Additive and Subtractive
versions of the merge operation—an operation that is occasionally useful for dealing
with problem composites with edges that are calling attention to themselves as too
bright or too dark.
For example, using Subtractive merging on a premultiplied image may lead to darker
edges, whereas using Additive merging with a non-premultiplied image will cause 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.
Chapter – 86 Composite Nodes 1913