Apple Shake 4 - Manual (Page 528)

528 Chapter 19 Using Masks
Using Side Input Masks to Limit Effects
You can attach a mask to the side input of a node, thereby limiting that node’s effect
on the input image. In the following screenshots, a mask image (actually an RGrad
image node modified by a Move2D node) is used to limit the effect of a Brightness node
that’s connected to the source image of the car.
By connecting the mask image to the side input of the Brightness node, parts of the
source image remain unaffected by the Brightness operation.
Important: The side input is meant to be used with effects nodes only. Do not use side
input nodes to mask images.
You can set up a node to use a side input mask in one of two ways. You can connect an
existing mask image to the side input node of any effect node, or you can open an
effect node’s parameters and create an automatically connected side input mask using
the Mask controls.
Source image Mask image
Masked Brightness node Corresponding node tree