Apple Shake 4 - Manual (Page 434)

434 Chapter 15 Image Processing Basics
In the next example, an unpremultiplied image is accidentally filtered, to show you
what artifacts to look for.
Adding a filter to an unpremultiplied image—the wrong way:
1 In a preexisting node tree, add a Blur node to the munch_unpremult node, in an
attempt to blur it against the background.
The mask clips any soft gradation.
2 Copy and clone the Blur1 node. (Copy the node, then press Control-Shift-V.)
Note: Press Command-E or Control-E to activate the enhanced Node View and see that
the cloned Blur node is linked to the Blur1 node.
3 Connect the Blur1_Clone1 node to the munch_mask node.
You should notice that an unwanted glowing highlight has appeared around the edge
of the image.
To eliminate this glow, you should apply the blur to an image that has nothing added
to the rim (against a black background)—since black has a value of 0 and therefore
does not add to the filtering.