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7.4. Soft Masks

As stated in earlier sections, the shape and opacity values used in compositing an object can include components called the mask shape (fm) and mask opacity (qm), which originate from a source independent of the object. Such an independent source, called a soft mask, defines values that can vary across different points on the page. The word soft emphasizes that the mask value at a given point is not limited to just 0.0 or 1.0 but can take on intermediate fractional values as well. Such a mask is typically the only means of providing position-dependent opacity values, since elementary objects do not have intrinsic opacity of their own.

A mask used as a source of shape values is also called a soft clip, by analogy with the “hard” clipping path of the opaque imaging model (see Section 4.4.3, “Clipping Path Operators”). The soft clip is a generalization of the hard clip: a hard clip can be represented as a soft clip having shape values of 1.0 inside and 0.0 outside the clipping path. Everywhere inside a hard clipping path, the source object's color replaces the backdrop; everywhere outside, the backdrop shows through unchanged. With a soft clip, by contrast, a gradual transition can be created between an object and its backdrop, as in a vignette.


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