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To add a Mask Object you can:
You are offered three alternatives
Masks in Version 9 also have another property which enables complex masks to be generated from a mixture of black and white objects: the ability to switch the “Alpha Channel” on and off. Here is an example using two black rectangles with a white centre rectangle. Remember “white reveals” and “black conceals”..
These two rectangles can be used to generate a composite mask. The two rectangles are now used as mask stencils in a user-defined mask. Rectangle 2 is above Rectangle 1. For Rectangle 2, if “Convert to Alpha Image” is ON, then the black area where Rectangle 2 overlaps Rectangle 1 becomes transparent.
Convert to Alpha image is ON For Rectangle 2
In the next example, “Convert to Alpha Image” is OFF for Rectangle 2 and the black area blocks out the area where it overlaps Rectangle 1
Convert to Alpha image is OFF for Rectangle 2
More complex masks are possible using this technique and an example is shown below.