Premultiply by an image?

[slepa]

How does one go about premultiplying an image by another image, rather than just a single color?

Reason for asking: I'm comping images with heavy contrast between the live-action background and the CG foreground. Motion blur looks smudgy as hell when the CG subject gets silhouetted by light. The CG can be rendered with the BG embedded, but I don't see why this just can't be done after the fact.

Any ideas?

[mayniak]

use the image you want to be the premultiplier in the mask input of an mmult node. If you want to use only one image and have the image multiply via one of its own channels other than the alpha do a reorder and tell the color you want to use to be the alpha and mmult that image