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Opacity masks

An opacity mask is an object whose shape and fill controls the opacity (transparency) of the objects it masks. The topmost object in a selection of objects works as the opacity mask. The masking is controlled by the value level (grayscale equivalent) of the top object's fill. In the mask, black or dark values will make the underlying masked object(s) totally transparent (seethrough); white or very light values will make the underlying masked object(s) opaque; and shades of gray (mid-range color values) will make the underlying masked object(s) partially transparent.

To create an opacity mask:

1.
Arrange the object to be used as the mask and the object or objects to be masked, then select them all . As the mask, you may use editable text, a placed (linked or embedded) image, an object containing a pattern or gradient, or a mesh.

Figure 1. Two objects are selected: a normal type object containing a radial fill and a placed bitmap image.


2.
Choose Make Opacity Mask from the Transparency palette menu . The Make Opacity Mask command links the object(s) and the mask. On the Transparency palette, a thumbnail of the object(s) being masked will appear on the left and a thumbnail for the mask will appear on the right, with the link icon between them . (If the thumbnails aren't visible, choose Show Thumbnails from the Transparency palette menu.)

Figure 2. After applying the Make Opacity Mask command


(See also figures and on the following page.)

Figure 1. A rectangle with a pattern fill on top of an irregular shape containing a solid gray fill: The pattern works as the opacity mask.


Figure 2. After choosing the Make Opacity Mask command, the value levels in the pattern fill mask out areas in the underlying object. We put a black rectangle behind the other objects to demonstrate how the whites are transparent.


If you used only two objects in step 1, those objects will be combined into one; if you used more than two objects, they will be nested within a <Group>. The new object or <Group> name will have a dashed underline, indicating the presence of an opacity mask.

  • The object-mask combination can be transformed, recolored, or assigned transparency attributes, effects, or styles— like any object.


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