CHAPTER 7
516
Transparency
gray and white backdrop. In the lower two figures, the three objects are combined
as a transparency group. At the lower left, the individual objects have an opacity
of 1.0 within the group, but the group as a whole is painted in the
Normal
blend
mode with an opacity of 0.5. The objects thus completely overwrite each other
within the group, but the resulting group then composites transparently with the
gray and white backdrop. At the lower right, the objects have an opacity of 0.5
within the group and thus composite with each other. The group as a whole is
painted against the backdrop with an opacity of 1.0 but in a different blend mode
(
HardLight
), producing a different visual effect.
The color result of compositing a group can be converted to a single-component
luminosity value and treated as a
soft mask.
Such a mask can then be used as an
additional source of shape or opacity values for subsequent compositing opera-
tions. When the mask is used as a shape, this technique is known as
soft clipping;
it is a generalization of the current clipping path in the opaque imaging model
(see Section 4.4.3, “Clipping Path Operators”).
The notion of
current page
is generalized to refer to a transparency group consist-
ing of the entire stack of objects placed on the page, composited with a backdrop
that is pure white and fully opaque. Logically, this entire stack is then rasterized
to determine the actual pixel values to be transmitted to the output device.
Note:
In contexts where a PDF page is treated as a piece of artwork to be placed on
some other page—such as an Illustrator artboard or an Encapsulated PostScript
(EPS) file—it is treated not as a page but as a group, whose backdrop may be de-
fined differently from that of a page.
7.2 Basic Compositing Computations
This section describes the basic computations for compositing a single object
with its backdrop. These computations are extended in Section 7.3, “Transparen-
7.2.1 Basic Notation for Compositing Computations
In general, variable names in this chapter consisting of a lowercase letter denote a
scalar quantity, such as an opacity. Uppercase letters denote a value with multiple
scalar components, such as a color. In the descriptions of the basic color compos-
iting computations, color values are generally denoted by the letter
C,
with a