User Manual

Premultiplied alpha image (Top), and straight alpha image (Bottom).
The Rules of Premultiplication
In general, when you’re compositing multiple images together, and one or more has a built-in
alpha channel, you want to make sure you follow these general rules:
 Images with alpha connected to a Merge node must be premultiplied.
 Always color-correct images that are not premultiplied.
 Always filter and transform images that are premultiplied.
How Do You Know You’ve Made a
Premultiplication Mistake?
Improper handling of premultiplication manifests itself in a few obvious ways:
 You see thin fringing around a subject composited with a Merge node.
 You notice a node adjustment affecting parts of the image that shouldn’t be affected by
that operation.
 You’ve combined RGB and alpha channels from different sources, and the
checkerboard background pattern in the viewer (if enabled) is only semi-transparent
when it should be fully transparent.
If you spot these sorts of issues, the good news is they’re easy to fix using either the internal
settings of the nodes causing the problem or with dedicated nodes to force the premultiplied
state of the image at specific points in your node tree.
Chapter – 69 Understanding Image Channels 1396