CHAPTER 7
514
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.)