Working with Color Traps

Color trapping controls how colors overlap and abut when printed. InDesign offers moderate controls over trapping ‚ enough to set the basics document-wide without getting into the expertise level of a commercial printer. It's also a feature that novice users can abuse terribly, which is one reason InDesign hides these options. If you don't know much about trapping, leave the features of the program at the default settings. Before you use InDesign trapping tools, study some books on color publishing, talk to your printer, and experiment with test files that you don't want to publish. If you're experienced with color trapping ‚ or after you become experienced ‚ you'll find InDesign trapping tools easy to use.

You'll still want to use the trapping tools within the illustration product with which you create your EPS and PDF graphics, because these tools will help you to finely control the settings for each image's specific needs. Also, if you're using a service bureau that does high-resolution scanning for you and strips these files into your layout before output, check to make sure that the bureau is not also handling trapping for you with a Scitex or other high-end system. If it is, make sure you ask whether and when you should be doing trapping yourself.

Note ‚

If you're printing to a color laser, dye-sublimation, ink-jet, or thermal wax printer, don't worry about trapping. You're not getting the kind of output resolution at which this level of image fine-tuning is relevant. But if you're outputting to an imagesetter (particularly if you're outputting to negatives ) for eventual printing on a standard web offset press (SWOP) or other printing press, read on.

Using choking versus spreading

So what is trapping, anyway? Trapping adjusts the boundaries of colored objects to prevent gaps between abutting colors. Gaps can occur because of misalignment of the negatives, plates, or printing press ‚ all of which are impossible to avoid.

Colors are trapped by processes known as choking and spreading. Both make an object slightly larger ‚ usually a fraction of a point ‚ so that it overprints the abutting object slightly. The process is called choking when one object surrounds a second object, and the first object is enlarged to overlap the second. The process is known as spreading when you enlarge the surrounded object so that it leaks (bleeds) into the surrounding object.

The difference between choking and spreading is the relative position of the two objects. Think of choking as making the hole for the inside object smaller (which in effect makes the object on the outside larger), and think of spreading as making the object in the hole larger.

Tip ‚

The object made larger depends on the image, but you generally bleed the color of a lighter object into a darker one. If you did the opposite , you'd make objects seem ungainly larger. Thus, choke a dark object inside a light one, and spread a light object inside a dark one. If the objects are adjacent, spread the light object.

Figure 29-5 shows the three types of trapping techniques. Spreading (at upper-right) makes the interior object's color bleed out; choking (at lower-left) makes the outside color bleed in, in effect making the area of the choked element smaller. The dashed lines show the size of the interior object; as you can see in the image at right, when you choke a darker object into a lighter one, the effect is to change its size (here, the interior object gets smaller). At upper-left is an untrapped image whose negatives shifted slightly during printing, causing a gap.

Figure 29-5: Three kinds of traps: spreading (upper-right), choking (lower-left), and centering (lower-right), with an untrapped image in the upper-left that was misregistered during printing.

There's a third type of trapping technique supported by InDesign: Called centering, it both chokes and spreads , splitting the difference between the two objects. This makes traps look nicer, especially between light and dark colors where regular choking and spreading can encroach on the light object, as shown in Figure 29-5 at lower-right.

Finally, InDesign offers a fourth trapping method ‚ neutral density ‚ that adjusts the trap as color hues and tints shift to minimize the possibility of darker lines where colors trap. But this can be dangerous ‚ for a bitmap image, it will most likely create an uneven edge as the trapping changes from pixel to pixel.

In practice, trapping also involves controlling whether colors knock out or overprint. The default is to knock out ‚ cut out ‚ any overlap when one element is placed on top of another. If, for example, you place two rectangles on top of each other, they print like the two rectangles on the right side of Figure 29-6. If you set the darker rectangle in this figure to overprint, the rectangles print as shown on the left side of the figure. Setting colors to overprint results in mixed colors, as on the left, while setting colors to knock out results in discrete colors, as on the right.

Figure 29-6: The two kinds of untrapped options: overprint (left) and knockout.

In InDesign, you use the Attributes pane (Window ‚ Attributes) for individual objects ‚ text, frames , shapes , and lines ‚ to pick from the pane's four trapping options: Overprint Fill, Overprint Stroke, Overprint Gap, and Nonprinting. Most objects won't have all four settings available, since not all objects have fills, gaps, and strokes, but all objects have Nonprinting available, which prevents the object from printing. All four options are normally unchecked, since you usually want objects' fills, gaps, and strokes to knock out (and you want the object to print). Figure 29-7 shows the pane.

Figure 29-7: The Attributes pane lets you set whether objects overprint or knock out (the default, or unchecked, status).

Specifying trapping presets

In InDesign, document-wide trapping settings are handled as part of the printing process (see Chapter 31). But you specify them like you do colors or other document attributes, via a pane. In this case, you use the Trap Presets pane (Window ‚ Trap Presets), shown in Figure 29-8. Choose New Preset from the pane's palette menu to create a new preset, or select an existing preset and choose Preset Options to modify it. (Figure 29-9 shows the New Preset dialog box.) You can also delete and duplicate trap presets from the palette menu, as well as import them from other documents (via the Load Trap Presets option).

Figure 29-8: The Trap Presets pane.

Figure 29-9: The New Preset dialog box.

Note ‚

Consult your service bureau or commercial printer before changing the default settings. They'll know what setting you should used based on the paper, inks, printing press, and image types you're using.

New Feature ‚

Trap Presets is the new name for the Trap Styles pane, but they're functionally the same otherwise . InDesign is adopting the name presets for what used to be called styles, to distinguish saved groups of attributes from text styles such as boldface and italics, which are often called just styles.

Here are the settings you can change when creating or modifying a trap preset:

Name: Give the trap style a name in the Name field ‚ make sure it's a meaningful name, like Choke, rather than New Preset 1, so you'll know what it's for.
Trap Width: Set your trap width (the amount of overlap you want between adjacent colors). The default is 0.25 point, a common setting. The Default field is for all colors except black and white. You set the trap width separately for black, using the Black Width field; this value is usually one and a half to two times as much as the regular trap settings because it controls how colors spread into black objects. The value is higher because you have more leeway with black ‚ spreading a color into it won't change it from being black, while, for example, spreading yellow into blue makes a green, so you want to minimize that spreading.
Trap Appearance: You can choose the type of end for traps, just as you do for strokes, with options of Miter, Round, and Bevel for both Join Style and End Style. In almost every case, you want to select Miter, since this keeps the trap confined to the abutting images. If you choose another option, the choke or spread will extend past the trapped objects a tad. If you think of the boundary of the trap as a line, then choosing Bevel or Round would extend that line slightly past the objects. If the objects have white or a light color on either side, readers might notice that slight extension.
Images: InDesign lets you control how trapping is applied with several options: Trap Placement plus four picture-specific settings, Trap Objects to Images, Trap Images to Images, Trap Inside Images, and Trap 1-Bit Images.
- Image Trap Placement determines how to handle trapping between an image and an abutting solid color. Your choices are Center, which has the trap straddle the edge between the image and the abutting color object; Choke, which has the abutting color object overprint the image by the Trap Width amount; Spread, which has the image overprint the abutting color object by the Trap Width amount; Neutral Density, defined earlier; and Normal, which traps each pixel in the image individually and thus can result in an uneven edge between the image and the abutting object.
- Trap Objects to Images turns on trapping for images and any abutting objects such as text or graphics created in InDesign.
- Trap Images to Images turns on trapping for images and any abutting images.
- Internal Image actually will trap colors within the bitmap image. Use this only for high-contrast bitmaps, such as cartoons and computer screenshots, where the color has fewer gradations and more broad, consistent swaths.
- Trap 1-Bit Images will trap black-and-white bitmaps to any abutting objects (including those underneath). This prevents the black portions from having a white ghost area around them if there is any misregistration when printing.
Trap Thresholds: These guide InDesign in how to apply your trapping settings:
- Step gives InDesign the color-difference threshold before trapping is implemented. The default is 10%, a value that will trap most objects. A higher value will trap fewer objects. The way this works is that the value represents the difference in color variance between adjacent objects, and you're telling InDesign not to worry about colors that are within the percentage difference. This is usually a low setting because that means you don't care if there are traps between similar colors, since the human eye will notice misregistration less between them because they are, in fact, similar. In most cases, keep this value between 8% and 20%.
- Black Color defines at what point InDesign should treat a dark gray as black for the trap width in Black Width. For coarse paper, which usually absorbs more ink, dark tints and grays often end up looking like a solid color ‚ 85% black will appear as 100% black. Use Black Color Limit in such cases so that an 85% black object traps as if were a 100% black object.
- Black Density is similar to Black Color Limit, except that it treats dark colors as black (like navy blues), based on their ink density. You can enter a value of 0 to 10, with 1.6 being the default (0 is full black, while 10 is white).
- Sliding Trap adjusts the way a choke or spread works. The normal value is 70%, which tells InDesign ‚ when the difference in ink density (a good measure of color saturation) is 70% or more ‚ not to move the darker color so much into the lighter color. The greater the contrast between two colors, the more the lighter object is distorted as the darker color encroaches on it. At 0%, all traps are adjusted to the centerline between the two objects, while at 100%, the choke or spread is done at the full trap width.
- Trap Color Reduction controls over-inking that some traps can create. The default is 100%, which means that the overlapping colors in a trap are produced at 100%, which in some cases can cause the trap to be darker than the two colors being trapped, due to the colors mixing. Choosing a lower value in Trap Color Reduction will lighten the overlapping colors to reduce this darkening. A value of 0% will keep the overlap no darker than the darker of the two colors being trapped.

Specifying trapping to pages

Once you've created trapping presets, you apply them. (The [Default] preset, which you can modify, is applied to all pages by default.) To do so, choose the Assign Trap Presets option in the Trap Presets pane's palette menu. Figure 29-10 shows the dialog box.

Figure 29-10: The Assign Trap Presets dialog box.

The process is simple: Choose the desired trap preset from the Trap Preset pop-up menu, then select the pages to apply it to in the Pages section (click All, or Range; if you click Range, enter the page numbers in the Range field). Click the Assign button to complete the application of the trap preset to the selected pages. You can set multiple presets in a document, to different page ranges, so if you want, select another trap preset and assign it to the desired pages. InDesign will show you what pages have what trap presets applied at the bottom of the dialog box. When done, click Done.

Note ‚

If your document uses sections, you can enter page numbers in the Range field using the section numbering style (such as A-2, D-3, F-5-F-10) or by using absolute page numbers (+1, +5, +10-+15).