Previous Next

                                        515
SECTION 7.1                                               Overview of Transparency



Both shape and opacity vary from 0.0 (no contribution) to 1.0 (maximum contri-
bution). At any point where either the shape or the opacity of an object is 0.0, its
color is undefined. At points where the shape is 0.0, the opacity is also undefined.
The shape and opacity are subject to compositing rules; therefore, the stack as a
whole also has a shape and opacity at each point.

An object’s opacity, in combination with the backdrop’s opacity, determines the
relative contributions of the backdrop color, the object’s color, and the blended
color to the resulting composite color. The object’s shape then determines the de-
gree to which the composite color replaces the backdrop color. Shape values of
0.0 and 1.0 identify points that lie outside and inside a conventional sharp-edged
object; intermediate values are useful in defining soft-edged objects.

Shape and opacity are conceptually very similar. In fact, they can usually be com-
bined into a single value, called alpha, which controls both the color compositing
computation and the fading between an object and its backdrop. However, there
are a few situations in which they must be treated separately; see Section 7.3.5,
“Knockout Groups.” Moreover, raster-based implementations must maintain a
separate shape parameter to do anti-aliasing properly; it is therefore convenient
to have it be an explicit part of the model.

One or more consecutive objects in a stack can be collected together into a trans-
parency group (often referred to hereafter simply as a group). The group as a
whole can have various properties that modify the compositing behavior of ob-
jects within the group and their interactions with its backdrop. An additional
blend mode, shape, and opacity can also be associated with the group as a whole
and used when compositing it with its backdrop. Groups can be nested within
other groups, forming a tree-structured hierarchy.

Note: The concept of a transparency group is independent of existing notions of
group or layer in applications such as Adobe Illustrator®. Those groupings reflect
logical relationships among objects that are meaningful when editing those objects,
but they are not part of the imaging model.

Plate 16 illustrates the effects of transparency grouping. In the upper two figures,
three colored circles are painted as independent objects with no grouping. At the
upper left, the three objects are painted opaquely (opacity = 1.0); each object
completely replaces its backdrop (including previously painted objects) with its
own color. At the upper right, the same three independent objects are painted
with an opacity of 0.5, causing them to composite with each other and with the

Previous Next