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                                                     525
      SECTION 7.2                                                   Basic Compositing Computations



      The above formulas apply to RGB spaces. Blending in CMYK spaces (including
      both DeviceCMYK and ICCBased calibrated CMYK spaces) is handled in the fol-
      lowing way:

      • The C, M, and Y components are converted to their complementary R, G, and
        B components in the usual way. The formulas above are applied to the RGB col-
        or values. The results are converted back to C, M, and Y.
      • For the K component, the result is the K component of Cb for the Hue, Satura-
        tion, and Color blend modes; it is the K component of Cs for the Luminosity
        blend mode.

      Note: An additional standard blend mode, Compatible, is a vestige of an earlier de-
      sign and is no longer needed but is still recognized for the sake of compatibility. Its
      effect is equivalent to that of the Normal blend mode. See “Compatibility with
      Opaque Overprinting” on page 567 for further discussion.


7.2.5 Interpretation of Alpha

      The color compositing formula

            ⎛ αs ⎞               αs
      C r = ⎜1 – ----- ⎟ × C b + ----- × [ ( 1 – α b ) × C s + α b × B ( C b , C s ) ]
            ⎝ αr ⎠               αr

      produces a result color that is a weighted average of the backdrop color, the
      source color, and the blended B (Cb , Cs ) term, with the weighting determined by
      the backdrop and source alphas αb and αs . For the simplest blend mode, Normal,
      defined by
           ·
      B ( cb , cs ) = cs

      the compositing formula collapses to a simple weighted average of the backdrop
      and source colors, controlled by the backdrop and source alpha values. For more
      interesting blend functions, the backdrop and source alphas control whether the
      effect of the blend mode is fully realized or is toned down by mixing the result
      with the backdrop and source colors.

      The result alpha, αr, is actually a computed result, described below in Section
      7.2.6, “Shape and Opacity Computations.” The result color is normalized by the
      result alpha, ensuring that when this color and alpha are subsequently used to-

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