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                                                       546
      CHAPTER 7                                                              Transparency



7.4.1 Deriving a Soft Mask from Group Alpha

      In the first method of defining a soft mask, the color, shape, and opacity of a
      transparency group G are first computed by the usual formula
      〈 C, f, α 〉 = Composite ( C 0 , α 0 , G )

      where C0 and α0 represent an arbitrary backdrop whose value does not contrib-
      ute to the eventual result. The C, f, and α results are the group’s color, shape, and
      alpha, respectively, with the backdrop factored out.

      The mask value at each point is then derived from the alpha of the group. Since
      the group’s color is not used in this case, there is no need to compute it. The alpha
      value is passed through a separately specified transfer function, allowing the
      masking effect to be customized.


7.4.2 Deriving a Soft Mask from Group Luminosity

      The second method of deriving a soft mask from a transparency group begins by
      compositing the group with a fully opaque backdrop of some selected color. The
      mask value at any given point is then defined to be the luminosity of the resulting
      color. This allows the mask to be derived from the shape and color of an arbitrary
      piece of artwork drawn with ordinary painting operators.

      The color C used to create the mask from a group G is defined by
                                         ·
      〈 C g , f g , α g 〉 = Composite ( C 0 , 1, G )
                  C = ( 1 – αg ) × C0 + αg × Cg

      where C0 is the selected backdrop color.

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