Quite often if an image is using an alpha channel, to save on additional multiplications during compositing, the color of the image is multiplied by the alpha value. This is what a premultiplied alpha is, and it needs to be treated differently than a regular alpha channel when compositing.

for you geeks out there that really want to understand WHY we need that pesky Premult node in NukeJoey Korenman - SchoolofMotion.com

Most compositing applications will have some methods for dealing with image Premultiplication. In Nuke there is a Premultiplication (Premult) Node that will allow you to premultiply the alpha channel with the color of the image.

Joey Korenman quickly shows what Premultiplication is in composting and why you would need to use it with a simple example explaining the process and the difference between Premult and no Premultiplication in images.

There is a strong bit on how to manage premultiplied alphas when color correcting with 32 bit EXR files in Nuke. Joey shows how to approach remove artifacts that can creep into the image and composite if premultiplied alphas are not handled correctly.

 

 

Going Further Understanding Premultiplication

Joey goes into great detail with his School of Motion post “Premultiplication Demystified” where he describes what Premultiplication is and how it is handled and why it is needed. Be sure to check out the “Premultiplication Demystified” here.