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                                              549
      SECTION 7.5                                          Specifying Transparency in PDF



7.5.3 Specifying Shape and Opacity

      As discussed under “Source Shape and Opacity” on page 526, the shape (f) and
      opacity (q) values used in the compositing computation can come from a variety
      of sources:

      • The intrinsic shape (fj ) and opacity (qj ) of the object being composited
      • A separate shape (fm ) or opacity (qm ) mask independent of the object itself
      • A scalar shape (fk ) or opacity (qk ) constant to be added at every point
      The following sections describe how each of these shape and opacity sources are
      specified in PDF.


      Object Shape and Opacity

      The shape value fj of an object painted with PDF painting operators is defined as
      follows:

      • For objects defined by a path or a glyph and painted in a uniform color with a
        path-painting or text-showing operator (Sections 4.4.2, “Path-Painting Opera-
        tors,” and 5.3.2, “Text-Showing Operators”), the shape is always 1.0 inside and
        0.0 outside the path.
      • For images (Section 4.8, “Images”), the shape is nominally 1.0 inside the image
        rectangle and 0.0 outside it. This can be further modified by an explicit or color
        key mask (“Explicit Masking” on page 351 and “Color Key Masking” on page
        351).
      • For image masks (“Stencil Masking” on page 350), the shape is 1.0 for painted
        areas and 0.0 for masked areas.
      • For objects painted with a tiling pattern (Section 4.6.2, “Tiling Patterns”) or a
        shading pattern (Section 4.6.3, “Shading Patterns), the shape is further con-
        strained by the objects that define the pattern (see Section 7.5.6, “Patterns and
        Transparency”).
      • For objects painted with the sh operator (“Shading Operator” on page 303), the
        shape is 1.0 inside and 0.0 outside the bounds of the shading’s painting
        geometry, disregarding the Background entry in the shading dictionary (see
        “Shading Dictionaries” on page 304).

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