About ink trapping

When an offset printed document uses more than one ink on the same page, each ink must be printed in register (perfectly aligned) with any other inks that it abuts, so that there is no gap where the different inks meet. However, it’s impossible to ensure exact registration for every object on every sheet of paper running through a printing press, so misregistration of inks can occur. Misregistration causes an unintended gap between inks.

You can compensate for misregistration by slightly expanding one object so that it overlaps an object of a different color—a process known as trapping. By default, placing one ink over another knocks out, or removes, any inks underneath to prevent unwanted color mixing; but trapping requires that inks overprint, or print on top of each other, so that at least a partial overlap is achieved.

Misregistration with no trap (left) and with trap (right)
Misregistration with no trap (left) and with trap (right)

Most traps employ spreading—expanding a light object into a dark object. Because the darker of two adjacent colors defines the visible edge of the object or text, expanding the lighter color slightly into the darker color maintains the visual edge.

Adobe In-RIP trapping

Acrobat can automatically trap color documents with the Adobe In-RIP Trapping engine, which is available on Adobe PostScript output devices that support Adobe In-RIP Trapping.

Adobe In-RIP Trapping can precisely calculate and apply any necessary adjustments to the edges of type and graphics throughout your document. It can apply effective trapping techniques to different parts of a single object, even if the object overlaps several different background colors. Trapping adjustments are made automatically, and you can define trap presets to address the trapping requirements of specific page ranges. The effects of trapping are apparent only on color separations generated by the trapping engine; you cannot see the results onscreen within the program.

The trapping engine decides where to trap by detecting contrasting color edges. It then creates traps based on the neutral densities (lightness or darkness) of abutting colors, in most cases by spreading lighter colors into adjacent darker colors. The trapping settings you specify in the Trap Presets palette modify the trapping engine’s results.

Requirements

Adobe In-RIP Trapping requires the following software and hardware:

  • A PPD (PostScript Printer Description) file for a printer that supports Adobe In-RIP Trapping. You must select this PPD by using the operating system driver.

  • An Adobe PostScript Level 2 or later output device that uses a RIP that supports Adobe In-RIP Trapping. To find out if a PostScript output device supports Adobe In-RIP Trapping, contact the manufacturer or your print service provider.

Trap a PDF

Trapping is a complex process that depends on the interaction of various color, ink, and printing factors; the correct settings vary, depending on specific press conditions. Do not change the default trap settings unless you’ve consulted with your print service provider.

  1. If necessary, choose Tools > Print Production > Trap Presets to create a trap preset with custom settings for your document and printing press conditions.
  2. Assign the trap preset to a page range.
  3. Choose File > Print to open the Print dialog box, and then click Advanced.
  4. Select Output from the list on the left.
  5. For Color, choose In-RIP Separations.
  6. For Trapping, choose Adobe In-RIP.

    Piezīme.

    This option works only when you target an output device that supports Adobe In-RIP Trapping.

  7. Click Ink Manager. As necessary, select an ink, specify the following options (only if your service provider recommends changing the settings), and then click OK:

    Type

    Choose an ink type that describes the selected ink.

    Neutral Density

    Type a value that differs from the default.

    Trapping Sequence

    Type a value to set the order in which inks are printed.

  8. Continue specifying other print options, and then click OK to print your document.

Specify settings using trap presets

A trap preset is a collection of trap settings you can apply to pages in a PDF. Use the Trap Presets dialog box for entering trap settings and saving a collection of settings as a trap preset. If you don’t apply a trap preset to a trapping page range, that page range will use the [Default] trap preset, a collection of typical trap settings that are applied to all pages of a new document.

Piezīme.

In Acrobat, trap presets and their assignments apply to the document only while it is open; trap settings are not saved in the PDF. This behavior is different from InDesign, where trap presets and their assignments are saved with the InDesign document.

Create or modify a trap preset

  1. Choose Tools > Print Production >Trap Presets.
  2. Select an existing preset, and then click Create.
  3. Specify the following options, and then click OK:

    Name

    Type a name for the preset. You cannot change the name of either of the two built-in presets: [No Trap Preset] and [Default].

    Trap Width

    Type values to specify the amount of overlap for inks.

    Trap Appearance

    Specify options for controlling the joins and ends of the traps.

    Images

    Specify settings that determine how to trap images.

    Trap Thresholds

    Type values to specify the conditions under which trapping occurs. Many variables affect the values you’ll need to enter here. For more information, consult with your print service provider, and see the other trapping topics.

Delete a trap preset

  1. In the Trap Presets dialog box, select the presets, and then click the Delete button.

    Piezīme.

    You cannot delete either of the two built-in presets: [No Trap Preset] and [Default].

Assign a trap preset to pages

You can assign a trap preset to a document or to a range of pages in a document. Pages with no abutting colors print faster if you disable trapping on those pages. Trapping doesn’t actually occur until you print the document.

Assign a trap preset
Trap assignments list presets you have applied to various pages; trap assignments are updated each time you click Assign.

  1. In the Trap Presets dialog box, click Assign.
  2. For Trap Preset, choose the preset you want to apply.
  3. Select the pages you want to apply the trap preset to.
  4. Click Assign.

    Piezīme.

    If you click OK without clicking Assign, the dialog box closes without changing the trap assignments. Trap assignments previously made using the Assign button are preserved.

  5. After you finish assigning trap presets, click OK.

Disable trapping on pages

  1. In the Trap Presets dialog box, click Assign.
  2. Select the pages you want to disable trapping on and choose [No Trap Preset] in the Trap Preset menu.
  3. Click Assign.
  4. After you finish updating the dialog box, click OK.

Trap preset options

You can change trap preset options whenever you create or edit a trap preset. The same trap preset options are available in Acrobat and InDesign. In Acrobat, you can view trap presets by choosing Tools > Print Production >Trap Presets. In InDesign, choose Window > Output >Trap Presets.

Trap widths

Trap width is the amount of overlap for each trap. Differences in paper characteristics, screen rulings, and printing press conditions require different trap widths. To determine the appropriate trap widths for each job, consult your commercial printer.

Default

Specifies the trap width in points for trapping all colors except those involving solid black. The default value is 0p0.25.

Black

Indicates the distance that inks spread into solid black, or the holdback amount—the distance between black edges and underlying inks for trapping rich blacks. The default value is 0p0.5. This value is often set to be 1.5 to 2 times the value of the default trap width.

In InDesign, the value you set for Black Color determines the value for a solid black or a rich black, a process black (K) ink mixed with color inks for increased opacity and richer color.

Piezīme.

(InDesign) If you choose Application Built‑In trapping, and you specify a Default trap width or Black trap width larger than 4 points, the resulting trap width is limited to 4 points. However, the value you specified will continue to be displayed, because if you switch to Adobe In‑RIP Trapping, traps larger than 4 points are applied as you specified.

Trap appearance

A join is where two trap edges meet at a common endpoint. You can control the shape of the outside join of two trap segments and the intersection of three traps.

Join Style

Controls the shape of the outside join of two trap segments. Choose from Miter, Round, and Bevel. The default is Miter, which matches earlier trapping results to retain compatibility with previous versions of the Adobe Trapping Engine.

Trap join examples
Trap join examples, left to right: miter join, round join, bevel join

End Style

Controls the intersection of three‑way traps. Miter (the default) shapes the end of the trap to keep it away from the intersecting object. Overlap affects the shape of the trap generated by the lightest neutral density object that intersects with two or more darker objects. The end of the lightest trap is wrapped around the point where the three objects intersect.

Close‑up of trap end examples
Close‑up of trap end examples: miter (left) and overlap (right)

Trap thresholds

Step

Specifies the color change threshold at which the trapping engine creates a trap. Some jobs need only the most extreme color changes trapped, while others require traps for more subtle color changes. The Step value indicates the degree to which components (such as CMYK values) of abutting colors must vary before trapping occurs.

To change how much the component inks in abutting colors can vary before causing those colors to trap, increase or decrease the value for Step in the New Trap Preset or Modify Trap Preset Options dialog box. The default is 10%. For best results, use a value from 8% to 20%. Lower percentages increase sensitivity to color differences and result in more traps.

Black Color

Indicates the minimum amount of black ink required before the Black trap width setting is applied. The default value is 100%. For best results, use a value no lower than 70%.

Black Density

Indicates the neutral density value at or above which InDesign considers an ink to be black. For example, if you want a dark spot ink to use the Black trap width setting, enter the neutral density value here. This value is typically set near the default of 1.6.

Sliding Trap

Determines when the trapping engine starts to straddle the centerline of the color boundary. The value refers to the proportion of the lighter color’s neutral density value to a darker, abutting color’s neutral density value. For example, setting the Sliding Trap value to 70% moves the point at which the trap begins to straddle the centerline to where the lighter color exceeds 70% of the darker color in neutral density (lighter color’s neutral density divided by darker color’s neutral density > 0.70). Colors of identical neutral density will always have their traps exactly straddle the centerline, unless the Sliding Trap is set to 100%.

Trap Color Reduction

Indicates the degree to which components from abutting colors are used to reduce the trap color. This setting is useful for preventing certain abutting colors (such as pastels) from making an unsightly trap that is darker than either color. Specifying a Trap Color Reduction lower than 100% begins to lighten the color of the trap; a Trap Color Reduction value of 0% makes a trap with a neutral density equal to the neutral density of the darker color.

Trapping imported graphics

You can create a trap preset to control traps within images, and to control traps between bitmap images (such as photographs and those saved in raster PDF files) and vector objects (such as those from a drawing program and vector PDF files). Each trapping engine handles imported graphics differently. It’s important to be aware of these differences when setting trapping options.

Trap Placement

Provides options for determining where the trap falls when you trap vector objects (including objects drawn in InDesign) to bitmap images. All options except Neutral Density create a visually consistent edge. Center creates a trap that straddles the edge between objects and images. Choke causes objects to overlap the abutting image. Neutral Density applies the same trapping rules as used elsewhere in the document. Trapping an object to a photograph with the Neutral Density setting can result in noticeably uneven edges as the trap moves from one side of the edge to another. Spread causes the bitmap image to overlap the abutting object.

Trap Objects To Images

Ensures that vector objects (such as frames used as keylines) trap to images, using the Trap Placement settings. If vector objects don’t overlap images in a trapping page range, consider turning this option off to speed trapping of that page range.

Trap Images To Images

Turns on trapping along the boundary of overlapping or abutting bitmap images. This feature is on by default.

Trap Images Internally

Turns on trapping among colors within each individual bitmap image (not just where they touch vector artwork and text). Use this option only for page ranges containing simple, high-contrast images, such as screen shots or cartoons. Leave it unselected for continuous-tone and other complicated images, as it will create bad traps. Trapping is faster when this option is unselected.

Trap 1-Bit Images

Ensures that 1‑bit images trap to abutting objects. This option doesn’t use the Image Trap Placement settings, because 1‑bit images use only one color. In most cases, leave this option selected. In some cases, such as with 1‑bit images where pixels are widely spaced, selecting this option may darken the image and slow the trapping.

About trapping black

When creating or editing presets, the value you type for Black Color determines what is considered solid black and rich black. A rich black is any black color that uses a support screen—additional percentages of one or more process inks to strengthen the black.

The Black Color setting is useful when you must compensate for extreme dot gain (as when using low-grade paper stock). These situations cause black percentages lower than 100% to print as solid areas. By screening back blacks or rich blacks (using tints of solid black) and decreasing the Black Color setting from its default of 100%, you can compensate for dot gain and ensure that the trapping engine will apply the proper trap width and placement to black objects.

When a color reaches the Black Color value, the Black trap width value is applied to all abutting colors, and keepaway traps are applied to rich black areas using the Black trap width value.

If support screens extend all the way to the edge of a black area, any misregistration causes the edges of support screens to become visible, creating an unwanted halo or distorting the edges of objects. The trapping engine uses a keepaway, or a holdback, for rich blacks to keep support screens a specified distance away from edges of reversed-out or light elements in the foreground, so that the light elements retain their sharpness. You control the distance of support screens from the edges of black areas by specifying the Black trap width value.

Piezīme.

If the element you’re trapping is a thin element, such as a black keyline around graphics, the trapping engine overrides the Black trap width setting and limits the trap to half the width of the thin element.

Adjusting ink neutral density values

By adjusting the ink neutral density (ND) values that the selected trapping engine uses, you can determine the precise placement of traps. The default ND values for process inks are based on the neutral density readings of process ink swatches that conform to industry standards in different parts of the world. The language version determines which standard it conforms to. For example, the ND values for the U.S. English and Canadian versions conform to the Specifications for Web Offset Publications (SWOP) solid ink density values published by the Graphic Arts Technical Foundation of North America. You can adjust process ink neutral densities to match printing industry standards in other parts of the world.

The trapping engine derives the ND values for a spot color from its CMYK equivalent. For most spot colors, the ND values of their CMYK equivalents are accurate enough for proper trap creation. Spot inks that aren’t easily simulated using process inks, such as metallic inks and varnishes, may need their ND values adjusted so that the trapping engine can trap them correctly. By typing new values, you can ensure that an ink that is observably darker or lighter is recognized that way by the trapping engine; the appropriate trap placement is then applied automatically.

You can get the appropriate neutral density value for a given ink by asking your commercial printer. The most accurate method of determining an ink’s ND value is by measuring a swatch of the ink with a commercial densitometer. Read the “V” or visual density of the ink (don’t use process filters). If the value differs from the default setting, type the new value in the ND text box.

Piezīme.

Changing the neutral density for a spot color affects only how that color will trap. It doesn’t change the appearance of that color in your document.

Follow these guidelines when adjusting ND values:

Metallic and opaque inks

Metallic inks are usually darker than their CMYK equivalents, while opaque inks obscure any ink beneath them. In general, you should set the ND values for both metallic and opaque spot colors much higher than their default values to ensure that these spot colors won’t spread.


Piezīme.

Setting an ink to Opaque or Opaque Ignore in the Type menu of the Ink Manager prevents an opaque ink from spreading into other colors, unless another opaque ink has a higher ND value.

Pastel inks

These inks are normally lighter than their process equivalents. You may want to set the ND value for these inks lower than their default values to ensure that they spread into adjacent darker colors.

Other spot inks

Some spot colors, such as turquoise or neon orange, are significantly darker or lighter than their CMYK equivalents. You can determine whether this is the case by comparing printed swatches of the actual spot inks to printed swatches of their CMYK equivalents. You can adjust the spot ink’s ND value higher or lower as necessary.

Customize trapping for specialty inks

Using certain inks involves special trapping considerations. For example, if you are using a varnish on your document, you don’t want the varnish to affect trapping. However, if you’re overprinting certain areas with a completely opaque ink, you don’t need to create traps for items underneath. Ink options are available for these situations. It’s usually best not to change the default settings, unless your prepress service provider recommends changing them.

Piezīme.

The speciality inks and varnishes used in the document may have been created by mixing two spot inks or by mixing a spot ink with one or more process inks.

  1. Open the Ink Manager and select an ink that requires special treatment.
  2. For Type, choose one of the following options, and then click OK:

    Normal

    Use for traditional process inks and most spot inks.

    Transparent

    Use for clear inks to ensure that underlying items trap. Use this option for varnishes and dieline inks.

    Opaque

    Use for heavy, nontransparent inks to prevent trapping of underlying colors but allow for trapping along the ink’s edges. Use this option for metallic inks.

    Opaque Ignore

    Use for heavy, nontransparent inks to prevent trapping of underlying colors and to prevent trapping along the ink’s edges. Use this option for those inks, such as metallic and varnishes, that have undesirable interactions with other inks.

Adjust the trapping sequence

The trapping sequence (also called the trapping order) matches the order in which inks are printed at the press, but it doesn’t match the order in which separations are produced at the output device.

The trapping sequence is particularly important when you’re printing with multiple opaque colors, such as metallic inks. Opaque inks with lower sequence numbers are spread under opaque inks with higher sequence numbers. This process prevents the last applied ink from being spread, and it still creates good traps.

Piezīme.

Don’t alter the default trapping sequence without first consulting with your prepress service provider.

  1. Open the Ink Manager. The current trapping sequence is displayed in the Sequence column of the inks list.
  2. Select an ink, type a new value for Trapping Sequence, and then press Tab. The sequence number of the selected ink changes, and the other sequence numbers change accordingly.
  3. Repeat the previous step for as many inks as necessary, and then click OK.

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