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                                              548
      CHAPTER 7                                                              Transparency



7.5.1 Specifying Source and Backdrop Colors

      Single graphics objects, as defined in Section 4.1, “Graphics Objects,” are treated
      as elementary objects for transparency compositing purposes (subject to special
      treatment for text objects, as described in Section 5.2.7, “Text Knockout”). That
      is, all of a given object is considered to be one element of a transparency stack.
      Portions of an object are not composited with one another, even if they are de-
      scribed in a way that would seem to cause overlaps (such as a self-intersecting
      path, combined fill and stroke of a path, or a shading pattern containing an
      overlap or fold-over). An object’s source color Cs , used in the color compositing
      formula, is specified in the same way as in the opaque imaging model: by means
      of the current color in the graphics state or the source samples in an image. The
      backdrop color Cb is the result of previous painting operations.


7.5.2 Specifying Blending Color Space and Blend Mode

      The blending color space is an attribute of the transparency group within which
      an object is painted; its specification is described in Section 7.5.5, “Transparency
      Group XObjects.” The page as a whole is also treated as a group, the page group
      (see Section 7.3.6, “Page Group”), with a color space attribute of its own. If not
      otherwise specified, the page group’s color space is inherited from the native color
      space of the output device.

      The blend mode B (Cb , Cs ) is determined by the current blend mode parameter in
      the graphics state (see Section 4.3, “Graphics State”), which is specified by the BM
      entry in a graphics state parameter dictionary (Section 4.3.4, “Graphics State Pa-
      rameter Dictionaries”). Its value is either a name object, designating one of the
      standard blend modes listed in Tables 7.2 and 7.3 on pages 520 and 524, or an ar-
      ray of such names. In the latter case, the application should use the first blend
      mode in the array that it recognizes (or Normal if it recognizes none of them).
      Therefore, new blend modes can be introduced in the future, and applications
      that do not recognize them have reasonable fallback behavior. (See implementa-
      tion note 72 in Appendix H.)

      Note: The current blend mode always applies to process color components but only
      sometimes to spot colorants; see “Blend Modes and Overprinting” on page 566 for
      details.

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