18
Overprinting
While transparency and overprint are two distinct things, transparency will often affect
overprint. In the process of flattening, overprint instructions are preprocessed when
overprinted objects interact with transparent objects. This ensures that overprint settings
are reliably processed when a file is flattened. The visual appearance of the objects involved
does not change during flattening, nor does the amount of ink per plate.
Overprinting instructions are preprocessed by the flattener when:
•
Overprinting objects are transparent (not 100% opaque).
•
Overprinting objects are part of or within 1/72” of transparent objects or groups.
•
Simulate Overprints is selected on the Advanced tab of the Print dialog (for use when
printing proofs to low-end composite devices that cannot otherwise simulate overprints).
Note:
Changing overprinting
instructions at the
RIP can affect not
only objects set
to overprint, but
flattened transparency
as well.
In these cases, the overprinting instructions for the objects are executed and the results have
the same appearance as the original, but in a flattened form. When Preserve Overprints is
selected for separated output, flattening does not occur for objects that are not involved in
transparency, even when they are overprinted.
Overprint preview:
It is always a good idea to check the results of settings before progressing
with print output. To see an “ink preview” that approximates how blending, transparency,
and overprinting will appear in color-separated output, choose View > Overprint Preview,
which displays the results of overprinted colors. You can also use a desktop proofer to
simulate overprinted colors by choosing “Simulate” overprints in the Advanced tab of the
Print dialog when doing composite output. When creating separations, you should not select
“Simulate.” Rather, using the “Preserve” option means Illustrator CS will correctly separate
spot colors and overprinting.
Flattening transparency
When printing a file or converting it into a format that does not support native transparency
instructions, all transparent objects in a file as well as objects that interact with transpar-
ency must be flattened. The process of flattening removes the live transparency effects
and creates individual objects that take on the same appearance as the original transpar-
ent objects, but in a format that can be understood by the printing device or file format.
Overlapping transparent art is divided into atomic regions
when flattened, and live transparency effects are removed.
(On the right, regions have been shifted to show the different
atomic regions.)
