Premultiplication and unpremultiplication are essential concepts for any compositing artist to master. Premultiply refers to performing a multiplication operation on the alpha and color channels of an image. Unpremultiplying then, refers to dividing the RGB image color by the alpha. Without the proper use of these techniques, edge artifacts and/or incorrect color correction can occur.
To help clarify how and when to use premultiply and unpremultiply, Laura Smith and Digital-Tutors have created a free tutorial for Nuke that covers this topic. The video can be accessed by going to Digital-Tutors’ blog via the link below: