SECTION 7.2
525
Basic Compositing Computations
The above formulas apply to
RGB
spaces. Blending in
CMYK
spaces (including
both
DeviceCMYK
and
ICCBased
calibrated
CMYK
spaces) is handled in the fol-
lowing way:
•
The
C, M,
and
Y
components are converted to their complementary
R, G,
and
B
components in the usual way. The formulas above are applied to the
RGB
col-
or values. The results are converted back to
C, M,
and
Y.
•
For the
K
component, the result is the
K
component of
C
b
for the
Hue
,
Satura-
blend mode.
tion
, and
Color
blend modes; it is the
K
component of
C
s
for the
Luminosity
Note:
An additional standard blend mode,
Compatible
, is a vestige of an earlier de-
sign and is no longer needed but is still recognized for the sake of compatibility. Its
effect is equivalent to that of the
Normal
blend mode. See “Compatibility with
7.2.5 Interpretation of Alpha
The color compositing formula
α
s
⎛
α
s
⎞
C
r
=
⎜
1
– -----
⎟
×
C
b
+ -----
× [ (
1
–
α
b
) ×
C
s
+
α
b
×
B
(
C
b
,
C
s
)
]
α
r
⎝
α
r
⎠
produces a result color that is a weighted average of the backdrop color, the
source color, and the blended
B
(C
b
,
C
s
) term, with the weighting determined by
the backdrop and source alphas
α
b
and
α
s
. For the simplest blend mode,
Normal
,
defined by
·
B
(
c
b
,
c
s
) =
c
s
the compositing formula collapses to a simple weighted average of the backdrop
and source colors, controlled by the backdrop and source alpha values. For more
interesting blend functions, the backdrop and source alphas control whether the
effect of the blend mode is fully realized or is toned down by mixing the result
with the backdrop and source colors.
The result alpha,
α
r
, is actually a computed result, described below in Section
result alpha, ensuring that when this color and alpha are subsequently used to-