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                                              516
      CHAPTER 7                                                              Transparency



      gray and white backdrop. In the lower two figures, the three objects are combined
      as a transparency group. At the lower left, the individual objects have an opacity
      of 1.0 within the group, but the group as a whole is painted in the Normal blend
      mode with an opacity of 0.5. The objects thus completely overwrite each other
      within the group, but the resulting group then composites transparently with the
      gray and white backdrop. At the lower right, the objects have an opacity of 0.5
      within the group and thus composite with each other. The group as a whole is
      painted against the backdrop with an opacity of 1.0 but in a different blend mode
      (HardLight), producing a different visual effect.

      The color result of compositing a group can be converted to a single-component
      luminosity value and treated as a soft mask. Such a mask can then be used as an
      additional source of shape or opacity values for subsequent compositing opera-
      tions. When the mask is used as a shape, this technique is known as soft clipping;
      it is a generalization of the current clipping path in the opaque imaging model
      (see Section 4.4.3, “Clipping Path Operators”).

      The notion of current page is generalized to refer to a transparency group consist-
      ing of the entire stack of objects placed on the page, composited with a backdrop
      that is pure white and fully opaque. Logically, this entire stack is then rasterized
      to determine the actual pixel values to be transmitted to the output device.

      Note: In contexts where a PDF page is treated as a piece of artwork to be placed on
      some other page—such as an Illustrator artboard or an Encapsulated PostScript
      (EPS) file—it is treated not as a page but as a group, whose backdrop may be de-
      fined differently from that of a page.


 7.2 Basic Compositing Computations

      This section describes the basic computations for compositing a single object
      with its backdrop. These computations are extended in Section 7.3, “Transparen-
      cy Groups,” to cover groups consisting of multiple objects.

7.2.1 Basic Notation for Compositing Computations

      In general, variable names in this chapter consisting of a lowercase letter denote a
      scalar quantity, such as an opacity. Uppercase letters denote a value with multiple
      scalar components, such as a color. In the descriptions of the basic color compos-
      iting computations, color values are generally denoted by the letter C, with a

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