User Manual

Table Of Contents
A bit of fringing at the edge of a
foreground element surrounded
by transparency.
Click to select the Merge node for that particular composite, and look for the Subtractive/
Additive slider.
The Subtractive/Additive slider, which can be used
to fix or improve fringing in composites.
Drag the slider all the way to the left, to the Subtractive position, and the fringing disappears.
A clip with alpha exhibits fringing (left),
and after fixing fringing by dragging the
Subtractive/Additive slider to the left (right).
The Subtractive/Additive slider, which is only available when the Apply mode is set to Normal,
controls whether the Normal mode performs an Additive merge, a Subtractive merge, or a
blend of both. This slider defaults to Additive merging, which assumes that all input images with
alpha transparency are premultiplied (which is usually the case). If you dont understand the
difference between Additive and Subtractive merging, here’s a quick explanation:
An Additive merge, with the slider all the way to the right, 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 is always 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, with the slider all the way to the left, is necessary if the
foreground image is not premultiplied. 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.
The Additive/Subtractive slider lets you blend between two versions of the merge operation,
one Additive and the other Subtractive, to find the best combination for the needs of your
particular composite. Blending between the two is occasionally useful for dealing with problem
composites that have edges that are calling attention to themselves as either too bright
or too dark.
Chapter – 70 Compositing Layers in Fusion 1415