CHAPTER 7
546
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
C
0
and
α
0
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
–
α
g
) ×
C
0
+
α
g
×
C
g
where
C
0
is the selected backdrop color.