User Manual

Table Of Contents
Operator
This menu is used to select the Operation mode used when the trailing objects overlap.
Changing the Operation mode changes how the overlapping objects are combined to produce
a result. This drop-down menu is visible only when the Apply mode is set to Normal.
The formula used to combine pixels in the trails node is always (fg object * x) + (bg object * y).
The different operations determine what x and y are, as shown in the description for
each mode.
The Operator Modes are as follows:
Over: The Over mode adds the foreground object to the background object by
replacing the pixels in the background with the pixels from the Z wherever the
foreground object’s Alpha channel is greater than 1.
x = 1, y = 1 - [foreground object Alpha]
In: The In mode multiplies the Alpha channel of the background object against the
pixels in the foreground object. The color channels of the foreground object are
ignored. Only pixels from the foreground object are seen in the final output. This
essentially clips the foreground object using the mask from the background object.
x = [background Alpha], y = 0
Held Out: Held Out is essentially the opposite of the In operation. The pixels in the
foreground object are multiplied against the inverted Alpha channel of the background
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 trailing 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, below is a quick
explanation.
NOTE: 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.
Chapter – 88 Effect Nodes 1965