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                                              551
      SECTION 7.5                                          Specifying Transparency in PDF



      group and apply the mask to the group as a whole. These considerations also apply
      to the current alpha constant (see the next section).


      Constant Shape and Opacity

      The current alpha constant parameter in the graphics state (see Section 4.3,
      “Graphics State”) specifies two scalar values—one for strokes and one for all
      other painting operations—to be used for the constant shape (f k) or constant
      opacity (qk) component in the color compositing formulas. This parameter can
      be thought of as analogous to the current color used when painting elementary
      objects. (Note, however, that the nonstroking alpha constant is also applied when
      painting a transparency group’s results onto its backdrop; see also implementa-
      tion note 72 in Appendix H.)

      The stroking and nonstroking alpha constants are set, respectively, by the CA and
      ca entries in a graphics state parameter dictionary (see Section 4.3.4, “Graphics
      State Parameter Dictionaries”). As described above for the soft mask, the alpha
      source flag in the graphics state determines whether the alpha constants are inter-
      preted as shape values (true) or opacity values (false).

      Note: The note at the end of “Mask Shape and Opacity,” above, applies to the cur-
      rent alpha constant parameter as well as the current soft mask.


7.5.4 Specifying Soft Masks

      As noted under “Mask Shape and Opacity” on page 550, soft masks for use in
      compositing computations can be specified in one of the following ways:

      • As a soft-mask dictionary in the current soft mask parameter of the graphics
        state; see “Soft-Mask Dictionaries,” below, for more details.
      • As a soft-mask image associated with a sampled image; see “Soft-Mask Images”
        on page 553 for more details.
      • (In PDF 1.5) as a mask channel embedded in JPEG2000 encoded data; see Sec-
        tion 3.3.8, “JPXDecode Filter,” and the SMaskInData entry of Table 4.39 for
        more details.

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