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1108 Chapter 12 Using Shapes and Masks
 Replace: This setting uses the current mask to completely replace the layer’s original
alpha channel, as well as any other masks that are applied to the same layer that
appear underneath the current mask in the Layers list. You can add additional masks
above, set to whatever blending mode you like.
 Intersect: Masks that are set to Intersect do not replace a layer’s original alpha
channel. Instead, they mask out all regions of the layer that do not overlap the mask
itself. This includes other masks applied to the same layer that appear nested
underneath the intersecting mask in the Layers tab.
Invert Mask: When this checkbox is selected, the mask is inverted—its solid and
transparent areas are swapped. This is useful if you need to switch the solid and
transparent areas of a mask you’ve already created.
Stencil: When this checkbox is selected, the layer’s geometric transforms are respected
by the mask. If you’re creating a mask with transformed Scale, Position, Rotation, or
other parameters, select this checkbox to use the modified layer.
Stretch: When this checkbox is selected, the mask is stretched to fit the boundaries of
the image and remains centered on the layer’s center point.
Note: Stencil and Stretch are mutually exclusive.
Applying Filters to Image Masks
Like shape masks, you can add filters to image masks to further manipulate the
transparency effect. In particular, you may be able to improve the mask that’s created
by using color correction filters such as Brightness, Contrast, and Gamma to manipulate
the contrast of the mask, pulling out or reducing detail to create the transparency
effect you need.
Filtering image masks works identically to filtering shape masks. For more information,
see “Applying Filters to Masks” on page 1100.
Applying Behaviors to Image Masks
You can also apply behaviors to image masks to create animated transparency effects.
For more information, see “Applying Behaviors to Masks” on page 1101.
Using Masks to Aid Keying Effects
Keying is rarely a one-step process. Although adding a keying filter is the first step,
there is usually additional work that needs to be done to create a good key that retains
detail around the edge of your subject. The mask tools and alpha channel filters
presented in this section can be used to turn a decent key into a great one.