Hang around a print shop long enough, and you’ll hear the term overprint.
In the world of prepress, overprinting is a way to control how color
separated plates interact with each other. A printing press imprints
each color on a piece of paper, one after the other, as it runs through
the press. Because of this process, you need to consider certain issues
when making color separations.
For example, say you design
some blue text over a yellow background. When those colors are
separated and printed on press, the blue and yellow mix, resulting
in green text on a yellow background. Therefore, under normal
conditions, when pages are separated, color that appears underneath
other objects is removed so that the color on top is unaffected. In this
example, the blue text removes, or knocks out, the yellow background underneath it, allowing the blue to appear correctly when printed.
Overprinting,
however, is a method of overriding a knockout and forcing overlapping
colors to mix on press. In our example, setting the blue text to
overprint means that the yellow background still appears behind it, and
the result on press is green text on a yellow background (Figure 1).
Why Overprint?
You’d want to apply an
overprint when you specifically want to mix colors on press. Some
designers who work with low-budget jobs that print in two or three spot
colors can simulate other colors by mixing those spot colors. Before
transparency rolled around, designers would also specify overprints to
simulate objects being transparent; you could also simulate shadows or
shading by overprinting with black over other elements.
Overprinting is also essential
when you’re creating plates for custom dyes and varnishes. For example,
if you want to create a spot varnish for a particular photo, you need
to create a spot color called Varnish and set it to overprint, because
this allows the photo that appears beneath it to print (otherwise, the
varnish knocks out the photo).
You can easily specify
overprinting from the Attributes panel (Window > Attributes). With
an object selected, you can force the fill, the stroke, or both to
overprint. Remember that Illustrator also allows you to specify whether a
stroke is painted in the centerline, inside, or outside a path, and you
should be aware that if you overprint a stroke that’s on the inside or
the centerline of a path, the stroke also overprints the fill of that
object.
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Those who work in packaging use overprints all the time to create traps—colors
that share borders with other colors that overlap slightly. This is
because commonly used package materials and the printing processes used
(called flexographic printing, or flexo)
don’t always result in perfect printing. Remember that the requirements
for printing a couple hundred brochures and printing several million
containers of milk can be quite different. The next time you see a bag
of potato chips or a bottle of soda, take a close look at the label;
you’ll be able to see the overprint traps. These are usually created in
Illustrator by setting just the stroke to overprint.
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Handling the Limitations of Overprints
Let’s get technical for a
moment. You’ll encounter some limitations when it comes to using
overprints. First, whereas one color plate can overprint another, an
overprint cannot overprint its own plate. For example, if you have a
color that contains cyan and you set it to overprint over a background
that contains cyan, you won’t get an overprint on the cyan plate.
Second, sometimes users
specify overprinting for objects colored white. Usually, white is always
a knockout (because it lets the white paper show through), and setting a
white object to overprint would kind of defeat the purpose. However,
these things do happen accidentally. You might have a logo that you
created that’s colored black and that you’ve set to overprint. Then you
might come upon a situation where you need a reverse (white) version of
the logo, so you might just open the file, color it white, and save it
with a different name, forgetting that you set the fill to overprint.
This would most likely result in the file not printing properly, because
either the white overprints (making it entirely transparent) or the RIP
doesn’t process the file correctly.
Previewing Overprints
Because overprints are really
PostScript commands that you use when you’re printing color separations,
you’ll always have a problem with displaying overprints onscreen or
when you’re printing composite proofs to show a client. In the past, the
only real way to proof overprints was by printing separations and
creating a matchprint proof or by investing in expensive prepress
plug-ins. More often than not, a designer would show a proof to a client and say, “It won’t look like this when it’s actually printed.” If only there were a better way...
Illustrator offers that
better way. By choosing View > Overprint Preview, you can actually
see on your monitor what the effects of overprint commands are.
Additionally, in the Output panel of the Print dialog box, the Simulate
Overprint option, when activated, prints composites as they will look
with overprints applied. This is perfect for showing clients exactly
what they are going to get. The Simulate Overprint option is also
available in the Advanced panel of the PDF dialog box, so you can even
show your client an accurate proof via PDF. You disable Simulate
Overprint when you choose to print separations—it’s available only when
you’re printing composites.
Although overprints are
useful (and essential in some workflows), our advice is to talk to your
printer before you use them, because some printers prefer to specify
overprints themselves.
Handling Transparency Effects That Disappear or Print as White Boxes
Has the following scenario ever happened to you?
You create some artwork
that contains two spot colors (let’s say Pantone Blue 072 and Red 032).
The logo has a drop shadow behind it, and you’ve correctly set the
Illustrator Drop Shadow effect to use the Blue 072 spot color, not
black. On the Illustrator artboard, the logo appears correctly against
the spot color background (Figure 2).
Then
you save the art as a PDF/X-1a file because it will be used in an ad
and you want to make sure it will print correctly. Or you save your
document using Acrobat 4 (PDF 1.3) compatibility. Alternatively, you
save your file as an EPS file because maybe you’re required to place
this logo into a QuarkXPress document. The point here to focus on is
that you’re saving your file to a flattened format.
The “problem” is that when you
open the PDF in Acrobat or Reader, or when you place the file into
QuarkXPress or InDesign and print the file to your laser or ink-jet
printer, it comes out looking incorrect—either the drop shadow
disappears completely (Figure 3) or a white box appears where the transparent effect should blend into the background (Figure 4).
The key items to focus on here
are that you have used a transparent effect and you’ve used a spot
color. Now, you’ll know what’s happening and what the solution is.
Note
Some
RIPs have built-in settings to ignore overprints in files and instead
use their own settings for overprints. This often results in output that
isn’t desirable. You can easily fix these issues by instructing the RIP
to honor the overprints in your files. For example, Rampage RIPs have a
setting called Preserve Application Overprint that, when activated,
results in perfect output. |
When you have a transparent
effect, the result is a mixture of the inks. In this case, the shadow,
which is Pantone Blue 072, blends right into the Red 032 background. By
default, when one color sits on top of another color, a knockout occurs. In other words, the area
beneath the top shape is removed from the lower object. Otherwise, the
top color will print on top of the bottom color when the paper is run
through the printing press, causing the two inks to mix. In the case of
the red and blue colors, the result would be purple in appearance.
However, in this case, where you want the drop shadow to blend into the background on press, you have to override that knockout by specifying an overprint.
The thing is, Illustrator
already knows this, so no action is required on your part. When you
print your file from Illustrator, all these settings are done
automatically, so your file looks great when you print it—either as a
composite or as separations. The same applies when you save your file
from Illustrator as a native Illustrator file and place it into InDesign
or when you create a PDF with Acrobat 5 compatibility (PDF 1.4) or
newer.
But when you save your file
to a format that doesn’t support transparency, Illustrator has to
flatten the transparency. And in that process, Illustrator realizes that
in order to preserve the spot colors so that they print in separations
correctly, the drop shadow must be set to overprint the background color
(in Illustrator CS4 and CS5, the spot color is set to overprint
instead).
The problem is that overprint
commands are honored only when you print your file as separations. When
you are previewing your document onscreen or when you are printing a
composite proof of your file, the overprint commands aren’t used, and
either the result will be white where overprinting should occur or the
transparency effect will simply disappear. The file will print correctly
when you print as separations, because, at that time, the overprints
are honored (as they should be).
This issue is easy to solve in InDesign, Acrobat, or Reader:
Tip
If
you’re using QuarkXPress (at least up through version 8.0) you really
don’t have an option, because that program doesn’t allow you to simulate
overprint commands when printing composite proofs. One workaround is to
create two versions of your file: one that uses spot colors that will
separate correctly when you print separations and another version where
you’ve converted your spot colors to process colors. When you convert to
process colors, you don’t need the overprints, and the file will print
with the correct appearance on a composite proof. |
In InDesign, choose
View > Overprint Preview. This will allow you to view overprints on
your screen. When printing composite proofs, select the Simulate
Overprints box in the Output panel of the Print dialog box to get the
correct appearance in your printouts.
In
Acrobat or Reader, choose Always in the Use Overprint Preview popup
menu (in the Page Display panel in Preferences) to view the file
correctly on your screen. When printing composite proofs, choose Print,
and then click the Advanced button. Then select the Simulate
Overprinting box in the Output section of the dialog box. The file will
then print with the correct appearance.
