Apple Shake 4 - Manual (Page 435)

Chapter 15 Image Processing Basics 435
Adding a filter to an unpremultiplied image—the right way:
m To fundamentally change the compositing order, you should premultiply the
foreground with the mask with a SwitchMatte node, then apply an Over node to create
the composite. (KeyMix is only used for unpremultiplied images.)
The following image shows the same tree with a single premultiplied image. The
preMultiply parameter is disabled in the Over1 node (otherwise a black edge appears
around the image).
Because transforms do a bit of filtering, technically speaking, you should perform
transforms on a premultiplied image as well, although the tolerances are a little more
forgiving.
To Sum Up
Make sure your image is unpremultiplied before you do any color correction, by
applying an MDiv to premultiplied images. Then, apply an MMult prior to applying
transforms and filters. Finally, use any of the layering nodes to composite the images
together.
Blur on a
premultiplied image Over1 detail