Spark: Knoll UnMult.

Knoll UnMult is used to generate mattes for light effects or other bright elements generated on a black background. It is also the perfect solution to cleanly composite elements with pre-multiplied alpha channels.

This flying saucer project is a good demonstration of light tracking. [play]

How UnMult works.

UnMult creates an alpha channel based on the maximum luminance of the RGB channels. It also makes the RGB channels "un-premultiplied", giving you full access to the RGB values of an element without any alpha-based attenuation.

By spreading the RGB pixels outward and keeping the edge detail in the alpha, UnMult produces perfectly clean composites without fringing. It has proven to be invaluable for cleanly extracting elements from black backgrounds and removing matte fringing.