After you add an image mask to a layer, the following parameters become available in the Image Mask Inspector:
Mask Source: An image well to assign a layer (a shape, text, an image, a movie, particle emitter, numbered image sequence, and so on) to use to mask a layer.
Frame: A slider (available when you use a movie or image sequence as the assigned image) that sets the start frame for playback.
Hold Frame: A checkbox (available when you use a movie or an image sequence as the assigned image) that, when selected, freezes the frame specified in the Frame parameter. That frame is then used as the mask for the masked layer’s total duration.
Offset: Value sliders that offset the X and Y values of the mask’s position, relative to the layer being masked, allowing you to change which part of the layer is masked.
Wrap Mode: A pop-up menu that increases the area an image mask affects (if the image mask is smaller than the layer it’s assigned to). There are three options:
None: The object is used as is.
Repeat: The object is duplicated and tiled to fill up the full width and height of the image.
Mirror: The layer is duplicated, tiled, and reversed to fill the full width and height of the image. This may create a more seamless repetition for some images.
Source Channel: A pop-up menu that sets which channel of the assigned image is used to create the mask. There are five options:
Red: Uses the red channel as a grayscale mask.
Green: Uses the green channel as a grayscale mask.
Blue: Uses the blue channel as a grayscale mask.
Alpha: Uses the alpha channel from the mask layer.
Luminance: Uses the aggregate luminance from the red, green, and blue channels as a grayscale mask.
Mask Blend Mode: A pop-up menu that determines how an image mask interacts with the alpha channel of the layer to which it’s 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. There are four menu options:
Add: Adds the mask to the alpha channel (the default setting). 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: Replaces 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, with the current mask. You can add masks above, set to whatever blending mode you like.
Intersect: Masks out all regions of the layer that do not overlap the mask itself (but does not replace a layer’s original alpha channel). This includes other masks applied to the same layer that appear nested underneath the intersecting mask in the Layers list.
Invert Mask: A checkbox that, when selected, reverses the mask—swapping its solid and transparent areas. This is useful if you need to switch the solid and transparent areas of an existing mask.
Stencil: A checkbox that preserves the layer’s geometric transforms. 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 (available when Stencil is deselected) that stretches the mask to fit the boundaries of the image and centers the mask on the layer’s center point.