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    CHAPTER 7                                                                Transparency



7.1 Overview of Transparency

    The original Adobe imaging model paints objects (fills, strokes, text, and images),
    possibly clipped by a path, opaquely onto a page. The color of the page at any
    point is that of the topmost enclosing object, disregarding any previous objects it
    may overlap. This effect can be—and often is—realized simply by rendering ob-
    jects directly to the page in the order in which they are specified, with each object
    completely overwriting any others that it overlaps.

    Under the transparent imaging model, all of the objects on a page can potentially
    contribute to the result. Objects at a given point can be thought of as forming a
    transparency stack (or stack for short). The objects are arranged from bottom to
    top in the order in which they are specified. The color of the page at each point is
    determined by combining the colors of all enclosing objects in the stack accord-
    ing to compositing rules defined by the transparency model.

    Note: The order in which objects are specified determines the stacking order but not
    necessarily the order in which the objects are actually painted onto the page. In
    particular, the transparency model does not require a consumer application to ras-
    terize objects immediately or to commit to a raster representation at any time before
    rendering the entire stack onto the page. This is important, since rasterization often
    causes significant loss of information and precision that is best avoided during inter-
    mediate stages of the transparency computation.

    A given object is composited with a backdrop. Ordinarily, the backdrop consists
    of the stack of all objects that have been specified previously. The result of com-
    positing is then treated as the backdrop for the next object. However, within cer-
    tain kinds of transparency groups (see below), a different backdrop is chosen.

    When an object is composited with its backdrop, the color at each point is com-
    puted using a specified blend mode, which is a function of both the object’s color
    and the backdrop color. The blend mode determines how colors interact; differ-
    ent blend modes can be used to achieve a variety of useful effects. A single blend
    mode is in effect for compositing all of a given object, but different blend modes
    can be applied to different objects.

    Compositing of an object with its backdrop is mediated by two scalar quantities
    called shape and opacity. Conceptually, for each object, these quantities are de-
    fined at every point in the plane, just as if they were additional color components.
    (In actual practice, they are often obtained from auxiliary sources rather than be-
    ing intrinsic to the object.)

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