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Chapter 15 Image Processing Basics 433
The Over node has a premultiplication parameter built into it. In the following tree, the
preMultiply flag in the Over parameters is turned on, which allows you to omit the
MMult node entirely.
You are not required to apply an MDiv for each color correction. You only need to add a
single MDiv to the beginning of the branch of your node tree where you need to
perform color correction. After the first MDiv, you can stack as many correctors up as
you want.
After you’ve added all the color-correction nodes you require, add an MMult node to
the end, and the image is ready for further compositing.
Filters and Premultiplication
Now to the second rule of premultiplication—using filters (such as Blur).
Identical tree with
premultiplication handled by Over1
Over1 parameters
Multiple corrections on one MDiv
Remember This
Rule Number 2: Only apply Filter and Transform nodes to premultiplied images.