Apple Shake 4 - Manual (Page 431)

Chapter 15 Image Processing Basics 431
Since munch_unpremult is already an unpremultiplied image, you get the same clean
result.
Managing Premultiplication
The above steps are an elaborate illustration to explain Over, KeyMix, and
premultiplication. This explanation can be simplified a bit.
The basic difference between the KeyMix and Over nodes is:
• KeyMix is used for unpremultiplied foreground images. The alpha channel can be
anywhere, including in the foreground image.
• Over is used for premultiplied foreground images, but can also enable
premultiplication if necessary for unpremultiplied images. The alpha channel is in the
foreground element.
Shake does not require you to constantly separate your alpha channel and to perform
IMult and IDiv operations with the second image. Instead, there are nodes to do this for
you. The following examples duplicate the current complex tree with simplified
versions.
Remember This
Rule Number 1: Only color correct unpremultiplied images.
IDiv and IMult3 deleted,
ContrastLum moved up IAdd1 detail