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Mask Blend Mode: A pop-up menu that determines how an image mask interacts with
the alpha channel of the layer to which it is applied. When a layer has multiple image
masks, each mask can have a different Mask Blend mode. When this happens, each mask
adds to, or subtracts from, the layer’s alpha channel according to the selected mode. The
final alpha channel is the combined result of all image masks applied. These modes are
identical to the Mask Blend Mode options available for shape masks. The four options
are:
• Add: This is the default selection for new masks. The mask is added to the alpha channel.
This is useful for adding back regions of an image that other masks are cutting out.
• Subtract: Subtracts a mask from the alpha channel. This is useful for creating holes in
the middle of layers, or for masking out additional regions of an image that are
untouched by other masks.
• Replace: This setting uses the current mask to completely replace the layer’s original
alpha channel, as well as any other masks applied to the same layer that appear
underneath the current mask in the Layers list. You can add masks above, set to
whatever blending mode you like.
• Intersect: Masks 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 list.
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 an existing mask.
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: A checkbox that becomes available when Stencil is deselected. Selecting this
checkbox stretches the mask to fit the boundaries of the image and centers the mask on
the layer’s center point.
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 must be done to create a good key that retains detail
around the edge of your subject. You can use the mask tools and alpha channel filters
presented in this section to turn a decent key into a great one. For more information
about using Motion’s keying filters see Keying.
Important: When using masks with keying, apply any masks after applying the keyer.
1220 Chapter 20 Using Shapes, Masks, and Paint Strokes