Apple Shake 4 - Manual (Page 422)

422 Chapter 15 Image Processing Basics
Shake takes a third approach, giving you explicit control over premultiplication for
every image in your composition. Although this can be inconvenient, it helps you get
around the problems that are typical in other software packages.
Problems Caused When Premultiplication Is Ignored
There are two typical problems that occur when the premultiplied state of an image is
ignored:
• Unwanted fringing around a masked subject
• Unwanted side effects that occur when a node affects parts of the image that ought
not to be affected
The following exercise demonstrates these problems.
To experience the heartbreak of premultiplication errors:
1 Open the Image tab, and click the FileIn node.
The File Browser opens.
2 In the File Browser, navigate to the directory where you copied the tutorial media.
The default path is $HOME/nreal/Tutorial_Media/Tutorial_Misc/premult.
3 Select the bg.jpg and munch_pre.sgi images, then click OK.
The nodes appear in the Node View.
The image munch_pre.sgi, rendered in Maya, is premultiplied (the blacks in the alpha
match the blacks in the RGB), and has an embedded alpha channel.
In a Nutshell—the Rules of Premultiplication
If you don’t read the full explanation of the mathematics of premultiplication, here
are the two rules you must always follow when creating a composition in Shake:
• Rule Number 1: Always color correct unpremultiplied images. To unpremultiply an
image, use a Color–MDiv node.
• Rule Number 2: Always filter and transform premultiplied images. To premultiply an
image, use a Color–MMult node.
munch_pre.sgi munch_pre.sgi, alpha channel bg.jpg
Images from Munch’s Oddysee courtesy of Oddworld Inhabitants.