CHAPTER 7
566
Transparency
Blend Modes and Overprinting
As stated in Section 7.6.2, “Spot Colors and Transparency,” each graphics object
that is painted affects all existing color components: all process colorants in the
transparency group’s color space as well as any available spot colorants. For color
components whose value has not been specified, a source color value of 1.0 is
assumed; when objects are fully opaque and the
Normal
blend mode is used, this
has the effect of erasing those components. This treatment is consistent with the
behavior of the opaque imaging model with the overprint parameter set to
false
.
The transparent imaging model defines some blend modes, such as
Darken
, that
can be used to achieve effects similar to overprinting. The blend function for
Darken
is
B
(
c
b
,
c
s
) =
min
(
c
b
,
c
s
)
In this blend mode, the result of compositing is always the same as the backdrop
color when the source color is 1.0, as it is for all unspecified color components.
When the backdrop is fully opaque, this leaves the result color unchanged from
that of the backdrop. This is consistent with the behavior of the opaque imaging
model with the overprint parameter set to
true
.
If the object or backdrop is not fully opaque, the actions described above are al-
tered accordingly. That is, the erasing effect is reduced, and overprinting an ob-
ject with a color value of 1.0 may affect the result color. While these results may
or may not be useful, they lie outside the realm of the overprinting and erasing
behavior defined in the opaque imaging model.
When process colors are overprinted or erased (because a spot color is being paint-
ed), the blending computations described above are done independently for each
component in the group’s color space. If that space is different from the native col-
or space of the output device, its components are not the device’s actual process
colorants; the blending computations affect the process colorants only after the
group’s results are converted to the device color space. Thus the effect is different
from that of overprinting or erasing the device’s process colorants directly. On the
other hand, this is a fully general operation that works uniformly, regardless of the
type of object or of the computations that produced the source color.
The discussion so far has focused on those color components whose values are
not
specified and that are to be either erased or left unchanged. However, the