A

addendum
Trapping

This chapter is provided as additional addendum material from Real

World InDesign CS4, by Olav Martin Kvern and David Blatner, from
Peachpit Press. Find more information at www.blatner.com/rwid and
www.indesignsecrets.com.

A “trap” is a method of overlapping abutting colored objects to com-

pensate for the imperfect registration of printing presses. Because
registration, even on good presses with good operators, can be off by
a quarter point or more, abutting elements in your publication may
not end up abutting perfectly when the publication is printed by your
commercial printer. What happens then? The paper shows through
where you don’t want it to (see Figures 2 and 3 on the Color Pages).
Do we need to tell you what happens when you take your work to
a press that’s badly out of register or run by turkeys? Disaster. Before
this happens to you, talk with your commercial printer regarding the
tolerances of their presses and/or operators. Don’t ask them if they’re
turkeys—it’s considered rude.
In the “good ol’ days” printers handled all your trapping for you.
Then came the “bad ol’ days” when designers had to do most of it
themselves. The basic method of creating manual traps involves
adding additional strokes to objects and then setting those strokes
to overprint using the Attributes panel (see Figures 1, 2, and 3 in the
Color Pages—at the end of this chapter—for examples).

637
638  r    cs4

But in the past few years, output providers have taken to doing it
all “in-RIP”—this is far, far better than you trying to do it yourself.
And, in many cases (especially with digital printing) no trapping
may be required anyway. So check with your printer first.

Manual Trapping If you can’t (or don’t want to) use InDesign’s automatic trapping
methods (In-RIP or built-in), you can still trap your publication—
you’ll just have to do it yourself. We describe this process first
because we believe that you should know how to add and subtract,
multiply and divide before you ever use a calculator.
However, before we start describing manual trapping techniques,
we need to state that InDesign’s automatic trapping methods can
almost always trap your publications better than you can (assuming
that you have both deadlines to meet and a finite amount of patience),
and if you use them, you usually won’t even have to think about trap-
ping. Also, most output providers prefer to use post-process trapping
software; in this case, they’ll want you to leave trapping turned off
(it’s off by default). After all, if they trap your already-trapped docu-
ment, it ain’t gonna’ be pretty. As always, check with your service
provider before doing anything rash.

Object-Level Overprinting. The key to trapping, in InDesign and

any other software, is in controlling which objects—or which parts
of objects—print on top of other objects as the printing press prints
your publication. The only way to make manual trapping work is
to control the overprinting characteristics of individual objects (see
Figures 1, 2, and 3 on the Color Pages).
Luckily, you can. Any InDesign path can be specified as an over-
printing object (that is, it won’t knock a hole in any objects behind
it when you print), regardless of the object’s color. The controls for
object-level overprinting are the Overprint Fill and Overprint Stroke
options found in the Attributes panel (see Figure A-1). These controls,
used in combination with InDesign’s Paste Into command, can be
used to create virtually any trap.
We have to stress the importance of the Weight Changes Bound-
ing Box option on the Stroke panel menu. You cannot create traps
when this option is turned off, so you’ll have to make sure it’s turned
on as you follow any of the procedures in this section. (We don’t usu-
ally have this turned on, which is one reason we prefer to use auto
trapping rather than manual trapping.)
When you’re working with trapping, you’ll be creating spreads
(outlines of objects, in the same color as the object, that are slightly
larger than the object itself) and chokes (outlines of the object that
 chapter A. trapping  639
Figure A-1
Overprint off Overprint on
Attributes Panel

Color 1
Separations: Color 1 Color 2 Color 1 Color 2
Color 2

are the same color as the underlying object’s color). Spreads make the
object larger so that the edges of the object print over the underlying
object; chokes make the area knocked out of the underlying object
smaller than the outline of the foreground object.
Use chokes when the foreground object is a darker color than the
background object; use spreads when the foreground object is lighter.
In other words, trap from light colors into darker colors. Sound sub-
jective? It is. We use chokes when we’re trapping type—text charac-
ters often look distorted when you use spreads (the eye is very critical
when it comes to text). Some folks ask why you don’t just always over-
print every color on top of every other color. The answer is that inks
are almost never fully opaque, so if you print solid cyan on top of
solid magenta, you’ll get purple—probably not the effect you were
looking for. With trapping, you’ll still get purple, but only in a tiny
stripe along the edge between the cyan and magenta objects.

Overprint Preview. InDesign has a powerful feature called Over-

print Preview (in the View menu), which gives you an onscreen pre-
view of the strokes and fills that you set to overprint. You may not
want to work with Overprint Preview turned on all the time because
there is a slight performance hit, but it’s certainly worth leaving it
on when trapping, or when zooming in to 2000% or more to see if
something is trapping correctly.

Overprinting Black. By default, all objects on an InDesign page

knock out of all objects behind them—unless they’re black. InDesign,
by default, overprints black ink. Most of the time, this is a good thing.
To turn off black overprinting, display the General panel of the Pref-
erences dialog box and turn off the Overprint Black option.
Remember, however, that black ink isn’t really fully opaque, even
if it looks like it is on screen. Overprinting solid areas of black on
an image or other page objects can look mottled, and unfortunately
even Overprint Preview won’t display this problem unless you have
color management enabled (which we discuss later in this chapter).
If you need larger areas of solid black in a full-color document, con-
sider making a rich black, a CMYK black that is made of 100 percent
640  r    cs4

black plus perhaps 25 percent each of cyan, magenta, and yellow (see
Figure 4 in the Color Pages for an example).

Solid Color Trapping. It’s more important to trap abutting color

fields in publications when you’re printing using spot colors or areas
of single process colors (like where 100 percent cyan text sits on
top of 50-percent magenta background) than it is in most process
color publications. Usually when you’re working with process colors,
you’ll almost always see some ink between abutting objects, so you’re
less likely to see paper-colored gaps showing a poor trap.
The easiest way to demonstrate how spot-color trapping works
is to show you some examples. As you work through these exam-
ples, you’ll trap an ellipse into a rectangle by manipulating the color,
width, and overprinting specifications of the path that surrounds the
ellipse. First, draw the colored objects.
1. Create a rectangle. Fill the rectangle with a spot color (“Color 1”).
Set the rectangle’s stroke color to None.
2. Draw an ellipse on top of the rectangle. Make sure that the
ellipse is entirely inside the rectangle. Fill the ellipse with a dif-
ferent color from that of the rectangle (“Color 2”). Set the stroke
of the ellipse to None.
3. Save the file.
The ellipse needs to be trapped, or you’ll run the risk of having
paper-colored lines appear up around the ellipse when you print the
publication. You can either spread or choke the ellipse, or both.
To spread the ellipse, use these steps (see Color Pages Figure 3).
1. Select the ellipse.
2. Press F10 to display the Stroke panel.
3. Turn on the Weight Changes Bounding Box option from the
Stroke panel menu, then enter a new line width in the Weight
field. The line width you enter in the Weight field should be
equal to twice the trap amount—if you enter “2”, you’ll get a
stroke of one point on either side of the path. If your commercial
printer has asked for a trap of .5 points, enter “1” in the Weight
field.
4. While the object is still selected, turn on Overprint Stroke in the
Attributes panel.
 chapter A. trapping  641

When you print, the fill of the ellipse knocks out the background,
but the stroke overprints. That means the ellipse is larger than the
hole that’s been knocked out of the background rectangle. You’ve
just created a spread.
To choke the ellipse, change the stroke color of the ellipse to the
same as the background color. Now when you print, the hole that’s
knocked out of the background rectangle is slightly smaller than the
original ellipse.
Choose Revert from the File menu to get the file ready for the next
procedure.

Trapping Across Color Boundaries. The techniques described

above work well as long as objects don’t cross color boundaries. If
the objects do cross color boundaries (especially going from a color
background to a white background), it’s too obvious that you’ve
changed the shapes of the objects. What do you do?
1. Drag the ellipse so that it’s partially outside of the rectangle, and
then press Command-Shift-[/Ctrl-Shift-[ to send it to the back
(behind the rectangle).
2. Press Command-C/Ctrl-C to copy the ellipse.
3. Now select the rectangle and select Paste Into from the Edit
menu (or press Command-Option-V/Ctrl-Alt-V). This duplicate
should appear in exactly the same place as the original, but
nested inside the rectangle.
If for some reason it doesn’t show up in the same place, then
select the original ellipse, and note the values in the X and Y
fields of the Transform panel. Then use the Direct Selection tool
to select the nested ellipse, turn off Show Content Offset in the
Transform panel menu, and enter the X and Y values into the
Transform panel fields. InDesign moves the copy of the ellipse
into the same position as the original ellipse.
4. Without deselecting the duplicate ellipse, enter a stroke weight
for the trap in the Weight field of the Stroke panel. Make sure
that the fill and stroke colors are the same in the Swatches panel.
5. Turn on the Overprint Stroke option in the Attributes panel.
At this point, the ellipse you pasted inside the rectangle spreads
slightly, while the part of the ellipse outside the rectangle remains
the same size and shape. If you want the rectangle’s color to choke
into the ellipse, then change the color of the nested ellipse’s stroke to
match the rectangle’s fill color.
642  r    cs4

What happens when the object you need to trap overlaps more
than one other, differently colored object? In this case, you can run
into trouble. The trap you use for one background color might not be
the trap you want to use for the other. You might want to spread one
and choke the other, depending on the colors you’re using. In these
cases, you can use the same basic techniques described above for the
overlapping and/or abutting objects. But, at this point, we have to
urge you to save yourself some trouble and use either of InDesign’s
automatic trapping methods.

Trapping Lines. The trapping techniques above work well for filled
paths, but what of open paths? After all, you can’t apply two different
stroke properties to a single path. Instead, you clone the path and
make the width of the clone larger or smaller for the spread or choke
you want. One of the strokes overprints; the other line knocks out.
Follow these steps to spread an open path.
1. Draw a rectangle and fill it with a spot color.
2. Draw a path inside the rectangle. Create another spot color and
apply it to the path. Do not set this path to overprint.
3. Select the path and clone it with Step and Repeat.
4. Increase the stroke weight of this duplicate path by twice the
amount of spread you need (remember, PostScript strokes grow
out from their centers) and turn on the Overprint Stroke option
in the Attributes panel to make the stroke overprint.

That’s all there is to it. The original path knocks a hole in the back-
ground rectangle, and the clone of the path spreads to just a little bit
beyond the edges of the knockout. Of course, this doesn’t create a
trap around the ends of the line, just the sides.
To choke the path, follow these steps.
1. Draw a rectangle. Create a spot color and fill the rectangle
with it.
2. Draw a path inside the rectangle. Create another spot color
and apply it to the line. Set this path to overprint.
3. Select the path and clone it.
 chapter A. trapping  643

4. Decrease the weight of the path by twice the amount of choke

you need in the Stroke panel. Turn off the Overprint Stroke
option in the Attributes panel.

This time, the cloned path is narrower than the original and
knocks out an area that’s slightly smaller than the original path, cre-
ating a choke.

Trapping Text. Text is usually the element in a publication that

needs trapping the most. For whatever reason, it’s easier to notice
poor trapping around text than around other elements. At the same
time, traps that are too large distort the shapes of the characters
you’re trapping. It tends to be a problem with small type, especially
serif type.
Here’s how to create a spread for text (see Figure 3 on the Color
Pages).
1. Draw a rectangle, create a spot color (“Color 1”), and apply it to
the rectangle.
2. Enter text in a text frame. Position the text frame on top of the
rectangle so that the text is entirely within the area occupied by
the rectangle.
3. Create a second spot color (“Color 2”) and apply it to the text in
the text frame.
4. While the text is still selected, display the Stroke panel. Enter the
stroke weight you want (remember, it’s two times the amount of
trap you want) in the Weight field. Turn on the Overprint Stroke
option in the Attributes panel.

The next example shows how you can choke text by making the
shape the characters knock out of the background a bit smaller than
the characters themselves.
1. Draw a rectangle, create a spot color (“Color 1”), and apply it to
the rectangle.
2. Create a text frame. Position the text frame on top of the rect-
angle so that it’s entirely within the rectangle.
3. Create a second spot color (“Color 2”). Select all the text in the
text frame and apply “Color 2” to the fill of the text.
4. Clone the text frame using Step and Repeat with zero offsets.
644  r    cs4

5. Select the text in the duplicate frame, set its fill color to None
and give it a stroke color the same as the background rectangle
(“Color 1”).
6. Enter the stroke weight you want for the trap in the Weight field
of the Stroke panel. Turn on the Overprint Stroke option in the
Attributes panel.
If text crosses color boundaries, use the techniques described ear-
lier for trapping overlapping paths.

Process-Color Trapping. Process-color trapping is a bit simpler than

spot-color trapping, because it’s usually less critical that process-
colored elements have traps (because many of these colors are built
of multiple process colors, so there are shared colors—if one color
shifts on press, then the other colors are still there to cover the white
paper). However, it can be far harder to figure out exactly what color
to make the stroke for a process-colored object. When you’re talking
about trapping two process-colored graduated fills, watch out!
The main thing to keep in mind, however, is that for each of the
process inks the ink percentage used in the topmost object in any
stack of objects always wins—they knock out all percentages of that
ink behind them, regardless of any overprinting settings.
Unless, that is, the ink percentage is zero. If, for example, the per-
centage of cyan used in the fill color of the topmost object in a stack
of objects is zero, turning Overprint off makes the path knock out
any other cyan in the area covered by the path. Overprinting the
fill, in this case, means that the area taken up by the fill disappears
from the cyan plates—the percentage of cyan in the next object in
the stack shows through the area where the objects overlap. If your
head is spinning at this point, just go see Figures 1 and 2 on the Color
Pages for a visual explanation.
Another way to think of this is to think of each ink in a process
color as behaving like a separate spot ink.

Simple Process-Color Trapping. In process-color trapping, you’ve

got to make your overprinting strokes different colors from either
the background or foreground objects. Why? Because process colors
have a way of creating new colors when you print them over each
other. It’s what they do best.
As in the spot-color trapping section earlier, we’ll demonstrate
process-color trapping techniques by example.
 chapter A. trapping  645

1. Create a rectangle that’s filled with “Color 1,” which is specified

as 20C 100M 0Y 10K.
2. On top of this rectangle, draw an ellipse and fill it with “Color 2,”
which is specified as 0C 100M 50Y 0K.
3. Select both objects and set their stroke to None.
4. Save the file.
The ellipse needs to be trapped, or you run the risk of having cyan-
colored lines appearing around the ellipse when the publication is
printed—which could happen if the cyan and yellow plates aren’t
in good register, or if your paper stretches. Whether you spread or
choke the ellipse depends on its color. If the ellipse is darker than
the background rectangle, choke the ellipse. If the ellipse is a lighter
color than the background rectangle, spread the ellipse. In this case,
the ellipse is a lighter color, so you’ll use a spread. To spread the
ellipse, follow these steps.
1. Create a new process color in the Swatches panel containing
only those colors in “Color 2” having higher values than “Color
1.” Quick quiz: what component colors in “Color 2” have higher
values than their counterparts in “Color 1”? If you said 50Y,
you’re the lucky winner. Specify a new color: 0C 0M 50Y 0K
(we’ll call this “Color 3”).
2. Select the ellipse.
3. Press F10 to display the Stroke panel, if it’s not already visible.
Enter the stroke weight you want for your stroke in the Weight
field. It should be twice the width of your desired trap.
4. Apply the color swatch “Color 3” to the stroke of the ellipse and
set it to overprint.

When you print, all the areas around the ellipse have some dot
value inside them, and the new colors created where the objects abut
won’t be too obvious. Choose Revert from the File menu to get ready
for the next example.
What if the ellipse is the darker color? If it were, we’d have to
choke it. To choke the ellipse, follow these steps.
1. Select the ellipse and fill it with “Color 1.” Select the rectangle
and fill it with “Color 2.”
646  r    cs4

2. Create a new color (“Color 3”) that contains only the largest
color component in “Color 1.” That’s 100M, so “Color 3” should
be specified as 0C 100M 0Y 0K.
3. Use the Weight field in the Stroke panel to specify the weight of
the trap you want.
4. Set the stroke color to “Color 3.”
5. Turn on the Overprint Stroke option in the Attributes panel.
When you print, the stroke you applied to the ellipse guarantees
that there’s no gap around the ellipse, even if you run into registra-
tion problems when you print the publication.

Automatic Trapping At this point, you’ve read your way through the manual trapping
techniques and are seriously considering hiring 20 house elves
to take care of your trapping needs. But wait—InDesign includes
two powerful automatic trapping methods: In-RIP trapping and
InDesign built-in trapping.
What are the differences between the two automatic trapping
methods? InDesign’s built-in trapping cannot do the following
things (all of which can be accomplished by in-RIP trapping).
▶ Trap gradients created in InDesign.
▶ Use trap widths greater than 4 points.
▶ Be used with in-RIP separations.
▶ Be used with most OPI or DCS workflows.
▶ Create traps inside color images, or trap InDesign objects to
images contained in DCS, EPS, or PDF files.
▶ Trap imported vector graphics or type within PDF or EPS files.
See “Built-in Trapping and Imported Graphics,” later in this
chapter.

Don’t let the length of the above list discourage you—InDesign’s

built-in trapping can take care of the trapping needs of most publi-
cations and printing processes. If, however, you see an item in the
above list that is crucial to your publication, then you’d probably
better use in-RIP trapping.
Of course, in-RIP trapping only works with PostScript printers
that have trapping built in. All PostScript 3 devices should be able
to handle this, but if you have a PostScript Level 2 printer, check
with the manufacturer to see if it has this feature. In-RIP trapping
 chapter A. trapping  647

also only works when you’re also using in-RIP color separations.
Most people don’t use in-RIP separations (they separate on a host
machine), so they can’t use in-RIP trapping either. In that case, we
recommend third-party post-process trapping software that creates
traps for you.
Note that both forms of automatic trapping work best with
PostScript and OpenType fonts; this is another instance where
TrueType fonts may cause problems.

Built-in Trapping and Imported Graphics. When InDesign ele-

ments overlap an imported EPS or PDF graphic, InDesign won’t be
able to trap the InDesign elements properly and you can get odd
trapping results. If the elements don’t touch each other, or don’t need
to be trapped, this isn’t a problem. If the InDesign object does touch
the graphic, however, you can sometimes work around the problem
by adjusting the frame that contains the graphic. If the InDesign ele-
ments overlap an empty area in the graphic, edit the shape of the
frame containing the graphic so that it doesn’t touch the elements.
Alternatively, you can copy and paste paths from Illustrator or
FreeHand (or any other application capable of putting data on the
system Clipboard in Illustrator format), thereby converting the
paths into InDesign objects—but this will only work for very simple
graphics.

Specifying Trapping Settings. While InDesign’s default trap set-

tings are reasonably good for most sheetfed print jobs, you will likely
have to change the trap settings for different jobs or for different
pages within a job (InDesign does not let you adjust the trap preset
for individual objects on your page). You can do all of this with the
Trap Presets panel (see Figure A-2). Note that this is different from
InDesign 1.x, which hid trapping settings in the Print dialog box.
All documents start with two trap presets: Default and No Trap
Preset. We’ll see how you can use No Trap Preset later when we talk
about assigning trap presets to pages. If you just want to change
the trapping values for the entire document, edit Default by double-
clicking on it in the Trap Presets panel. Or, to create a new trap preset,
Option/Alt-click on the New Trap Preset button at the bottom of the
panel (or select New Trap Preset from the panel menu).
The New Trap Preset and the Modify Trap Preset dialog boxes
are functionally equivalent. Here’s a quick rundown of what all these
cryptic features are for.
648  r    cs4

Trap Width. The value you enter in the Default field sets the trap
width for all inks except solid black—you set that trap width using
the nearby Black field. If you’re using InDesign’s built-in trapping,
the trapping width is never greater than four points (regardless of the
value you enter in these two fields).
If you’re working with a rich black (that is, a color containing
other process inks in addition to black), the value you enter in the
Black Width field also sets a kind of “margin” of black ink at the
edges of a rich black object. This margin is known as the “holdback”
or “holdback area.”
The holdback area comes in handy at the edges of a rich black
object. If, for example, you’ve placed white text over a rich black
area, the holdback area prevents the non-black inks from appearing
around the edges of the white characters.

Trap Appearance. Those of you who really care about how your
traps appear can adjust the look and feel of trap lines with the Join
Style and End Style pop-up menus. Join Style determines how corner
points in trap segments will appear (mitered, rounded, or beveled).
End Style determines how the trap lines act at their end points (the
default, Mitered, keeps the trap lines slightly away from each other,
while Overlap lets the trap lines… well, overlap). We typically just
leave both of these set to their default values.

Trap Thresholds. Imagine that you have two process colors in your
publication. Color 1 is defined as 60C 20M 0Y 0K, and Color 2 is 65C
15M 0Y 5K. Do these two colors need to be trapped if they happen
to end up next to each other on your InDesign pages? That depends
on you, your publication, and your commercial printer. The fields in
the Trapping Thresholds section of the New Trap Preset dialog box
control when and how InDesign traps the boundaries between colors
in a publication.
The Step field sets the percentage of difference between each color
component to trigger automatic trapping. In the above example of
Color 1 and Color 2, using the default value of 10 percent in the Step
field means that InDesign would not trap the two colors—there’s not
enough difference between the inks that make up the two colors. If
you lowered the value in the Step field to 5 percent, InDesign would
trap the objects (because the C, M, and K ink components vary by
that percentage).

Black Color. How much black ink has to be used in a color before
InDesign applies the holdback defined by the value you entered in
 chapter A. trapping  649

the Black field? That depends on what you enter in the Black Color
Limit field. Enter 100 percent when you want to apply a holdback to
colors containing 100 percent black ink, or lower the percentage to
apply a holdback to colors containing less black ink.

Black Density. InDesign traps colors in a publication based on their

ink neutral densities (see “Editing Ink Neutral Densities,” later in this
chapter). Lighter colors typically spread into areas of darker colors,
which usually produces a less obvious trap. You can manipulate the
way that InDesign traps objects by changing the neutral densities in
the Ink Manager dialog box. Or you can use the Black Density field
to redefine the density InDesign thinks of as black. By default, black
ink is set to an ink neutral density of 1.6.
The value you enter in the Black Density Limit field also affects
InDesign’s application of the Black holdback area. By reducing the
value in the Black Density Limit field, you instruct InDesign to apply
the holdback to inks other than black.

Sliding Trap. When gradients abut, the colors at the edges of the
gradients vary along the border between the two gradients. You can’t
use a simple spread or choke for the entire length of the boundary
Figure A-2
To create a
Trapping Settings trapping preset,
select New Preset
from the menu or
Option/Alt-click
the New Preset
button.

InDesign displays the New Trap Preset dialog box.

650  r    cs4

between gradients—at some point, the trap will just become too
obvious.
One way to solve this problem is to use a centerline trap—a trap
that extends equally on either side of the boundary between the gra-
dients. Another method is to use a sliding trap—which changes from
a spread to a centerline trap, and then to a choke, depending on the
ink neutral densities of the colors used in the gradients.
The value you enter in the Sliding Trap Limit field defines the
point (or points) at which the trap switches from a spread to a cen-
terline trap, and from a centerline trap to a choke. This value is a
percentage of the difference between ink neutral densities (note that
this is unlike the percentage in the Step field, which is the differ-
ence between the color components making up a color). Enter 0%
(zero percent) to force InDesign to use a centerline trap for the entire
length of the trap, or enter 100% to make InDesign apply a spread
along the length of the boundary between the two gradients. Other
values (such as the default 70 percent) apply sliding traps.

Trap Color Reduction. The value you enter in this field defines the
colors InDesign creates as it builds traps. When the value in this
field is 100 percent, some color combinations can result in a trapping
color (or colors) that is darker than either of the original colors. To
avoid this, enter a smaller value in this field. Enter 0 (zero) to set the
neutral density of the objects created by the trapping system to the
neutral density of the darkest color (note that this doesn’t necessarily
mean it’s the same color).

Image Settings. The controls in the Images section of the New Trap
Preset dialog box define the way that InDesign traps InDesign page
items to imported graphics. Note that “image,” in the context of this
dialog box, means any imported graphic—not just bitmaps.

Trap Placement. Choose Center to apply a centerline trap (see “Slid-

ing Trap,” earlier) to the boundary between the InDesign object and
the imported graphic. Choose Choke to extend the InDesign objects
into the area inside the imported graphic. Choose Neutral Density
to apply the trap based on the ink neutral density of the abutting
colors. Choose Spread to spread the colors from the image into the
InDesign object.

Trap Objects to Images. Turn this option on to apply automatic

trapping to areas where InDesign objects abut imported images.
 chapter A. trapping  651

InDesign uses the trapping method you chose from the Image Trap
Placement pop-up menu to trap the objects (see above).

Trap Images to Images. This feature lets InDesign build traps where
two bitmapped images abut each other.

Trap Images Internally. Turn this option on to apply in-RIP trap-

ping to areas of color inside imported bitmap images. Most scanned
images (photographs) don’t need trapping—this option is for syn-
thetic images (such as screen shots) containing abutting areas of flat
color. Turn this option off for faster trapping.

Trap 1-bit Images. Turn this option on to trap bilevel (black and
white) images to InDesign objects. This affects images to which
you’ve applied colors in InDesign.

Trapping Ranges If the pages in your publication have differing trapping needs, you
can use trapping ranges to vary the trapping presets used to trap the
publication. For instance, if one or more pages don’t require trapping
at all (perhaps they include only black text or objects don’t overlap at
all), your document will print faster if you turn off trapping for those
pages. To change one or more page’s trap preset, select Assign Trap
Preset from the Trap Presets panel menu.
In the Assign Trap Preset dialog box, select a trapping preset from
the Trap Preset pop-up menu, then enter the page range you want
to trap using the style (separate individual pages with commas and
page ranges with hyphens). Make sure you click the Assign button
before clicking Done, or else InDesign ignores your change. To turn
off trapping for a page or range of pages, select [No Trap Preset] from
the Trap Preset pop-up menu.

Editing Ink When you use either automatic trapping method, the trapping
Neutral Densities system (whether in InDesign or in a PostScript RIP) bases its trap-
ping decisions on the inks used in abutting objects on an InDesign
page. In general, the trapping system tries to spread lighter inks into
darker inks. How can the system tell which inks are lighter or darker?
By comparing the ink neutral density values between the objects. A
color’s neutral density is sort of like “how dark would the color be if
this were in grayscale.”
In general, InDesign assigns ink neutral density values based on
the CMYK values of the colors you’ve used in a publication. If you’ve
652  r    cs4

used spot colors (inks), or have created colors using the RGB or LAB
color model, InDesign uses the process color equivalent of the color.
Most of the time, this approach works pretty well. But we can
think of three cases in which you might want to edit the ink neutral
density values.
▶ Varnishes. A varnish should almost always be the lightest ink on
the page, so set its ink neutral density to 0 (zero). This way, the
trapping system will spread the varnish into abutting objects.
▶ Metallic Pantone inks. Metallic inks are more opaque than
other inks, and they’re also somewhat reflective. Spreading a
metallic ink into an abutting area of some other color is almost
certain to create an obvious trap. Set the ink neutral density of
metallic inks to 1.7 (the value of black ink) or higher—that way,
they will be considered the darkest ink on the page by the trap-
ping system (and all other inks will spread into them).
▶ Pantone fluorescent inks. Like metallic inks, fluorescent inks
are more opaque than other inks, and they’re also very bright.
In general, you want every other ink to spread into an area of
fluorescent ink, so you set the ink neutral density to a high value.
We think that 1.6 makes a good setting for fluorescent inks—
that way, they’ll spread into black areas, but most other inks will
spread into the fluorescent ink.
▶ Pantone pastel inks. These are very light colors and should be
treated in much the same way as you’d treat a varnish. Set the
ink neutral density value to .15—approximately the same value
as that of process yellow ink.

To edit the neutral density value for an ink, follow these steps.
1. Open the Ink Manager dialog box from the Swatches panel
menu or the Print dialog box.
2. Select the ink you want to edit.
3. To set the ink type, choose an option from the Type pop-up
menu. You use these options to declare to the trapping system
that a specific ink doesn’t follow the usual trapping rules. This
can come in handy when you’re working with certain types of
spot inks: varnishes, metallic colors, fluorescent colors, and
pastel colors. You can also control the inks trapping behavior by
manipulating its ink neutral density, as in Step 5.
▶ Choose Normal for all process inks and for most spot colors.
 chapter A. trapping  653

▶ Choose Transparent for varnishes and very light spot inks

(Pantone pastels, for example).
▶ Choose Opaque for very opaque inks, such as Pantone
metallic or fluorescent colors.
▶ Choose OpaqueIgnore for inks you want to have the
trapping system ignore entirely—for instance,
nontransparent metallic inks or varnishes.
4. Enter a new value in the Neutral Density field. Do this only if
you’ve set the Type pop-up menu to Normal. Use this approach
if your publication contains “specialty inks” (the ink types listed
in Step 4) and you need to define the ways these inks trap to
each other.
5. The Trapping Sequence field offers a way for you to tell InDesign
the order in which the inks will be printed (this has no effect at
all on the order in which the color separations will print). While
it’s rare that you’d need this, the Trapping Sequence feature can
be useful if you’re printing multiple opaque inks. For instance,
InDesign can then know to spread other inks underneath the
last-printed opaque ink.
6. Click the OK button to close the Ink Manager dialog box and
apply your changes.
Note that changing the neutral density values for an ink has no
effect on how the color appears on screen, even when Overprint Pre-
view is turned on. This only affects InDesign’s trapping behavior.
654  r    cs4
color figure 1
Spot color 1 plate Spot color 2 plate
Overprint and Knockout

Objects colored with spot

color 1 set to knock out
(Overprint Fill option off)

Spot color 1 knocks out spot color 2

Objects colored with spot Spot color 1 plate Spot color 2 plate
color 1 set to overprint
(Overprint Fill option on)

Spot color 1 overprints spot color 2

color figure 2
Overprint off Where the overprinting object’s percentage
Overprinting and Overprint on of a process color is zero, the background
Process Colors color will show through.

We’ve always wondered

what it really means to say
that you’re overprinting
process colors, so we created
this figure to explain it to
ourselves. We hope it works
for you, too.

Rectangle:
80C 20M 80Y 10K These objects contain percentages of
each process color, so overprinting
0C 80M 0Y 40K and non-overprinting objects print
identically on each plate.
20C 80M 20Y 40K
This lavender circle causes an unexpected
Circle:
effect when overprinted. The magenta,
20C 20M
yellow, and black plates print as expected,
but the 20-percent cyan in the circle
overrides the cyan in the background.
 chapter A. trapping  655
color figure 3
Trapping Closed
Paths and Text

Unless we’ve been lucky, Select the path you want to trap and press F10
you’ll see the paper showing to display the Stroke palette. Add a stroke to the
through around the cyan object that’s twice the width of the spread you
circle in this example. To want, and turn on the Overprint option.
prevent the paper from
showing, you need to trap
the object.

If you could separate the fill

and the stroke, you would see
something like this.

To create a choke, apply The fill is set to …the stroke is set to

an overprinting stroke the knock out… overprint.
color of the background
rectangle to the ellipse.

When you print, the the stroke of the circle

overprints the background square, while
the fill knocks out. This creates a spread.

This example has not been

IOLANTHE
trapped, so you’ll probably
see paper showing through
around the text characters.
or the peer and the peri

The cyan stroke overprints

IOLANTHE
the background objects,
creating a spread. In general,
you want to spread the
lighter color (cyan, in this or the peer and the peri
example) into the darker
color (magenta). Trapped using a spread.

Because InDesign prints the When you choke

IOLANTHE
fill of text over the stroke, we lighter characters,
had to create a duplicate text the apparent
frame containing characters shape of the char-
with a magenta stroke or the peer and the peri acters changes
and a fill of “None” (not a good thing).
Trapped using a choke.