Trapping color (Acrobat Pro)

In offset printing, when using multiple inks on the same page, precise alignment is crucial to avoid gaps where the inks meet. However, it's challenging to achieve perfect alignment on every sheet, leading to misregistration and unintended gaps between inks.

To compensate for misregistration, you can employ trapping, which involves slightly expanding one object to overlap with a different colored object. By default, one ink knocks out any inks underneath to avoid color mixing. Trapping requires inks to overprint, allowing for at least a partial overlap and better alignment.

Trapping typically involves spreading—a light object is expanded into a dark one. This approach ensures that the visible edge of the object or text is defined by the darker color. By slightly extending the lighter color into the darker color, the visual edge remains intact.

Adobe In-RIP trapping

Acrobat offers automatic trapping for color documents using the Adobe In-RIP Trapping engine, available on Adobe PostScript output devices that support it.

The In-RIP Trapping engine precisely calculates and applies adjustments to type and graphics edges throughout your document. It can effectively trap different parts of an object even if it overlaps several background colors. Trapping adjustments happen automatically, and you can create trap presets for specific page ranges. Trapping effects are visible only on color separations generated by the trapping engine, not on the screen within the program.

The trapping engine detects contrasting color edges and applies traps based on neutral densities (lightness or darkness) of adjacent colors, often by expanding lighter colors into darker ones. You can modify the trapping engine's results through the Trap Presets palette settings.

To use Adobe In-RIP Trapping, you need:

• A PostScript printer with the appropriate PPD file (PostScript Printer Description) selected through the operating system driver.
• An Adobe PostScript Level 2 or later output device using a RIP that supports Adobe In-RIP Trapping. Check with the manufacturer or print service provider to confirm support for Adobe In-RIP Trapping on your PostScript output device.

Trap a PDF

Trapping is a complex process influenced by various color, ink, and printing factors. The correct settings depend on your specific press conditions. Unless you've consulted with your print service provider, it's best not to change the default trap settings.

If needed, follow these steps to create a trap preset with custom settings for your document and printing press conditions:

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.

Create or modify a trap preset

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.

Disable trapping on pages

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 going to the All tools menu > Use print production > Trap Presets.

In InDesign, select Window > Output > Trap Presets.

Trap widths

Trap width refers to the overlap amount for each trap, and it varies based on paper characteristics, screen rulings, and printing press conditions. Your commercial printer can help determine the appropriate trap widths for your job.

Here are the default settings:

• Default: Specifies the trap width in points for all colors except solid black, set to 0p0.25 by default.
• Black: Indicates how far inks spread into solid black or the holdback amount (distance between black edges and underlying inks) for trapping rich blacks. The default value is 0p0.5. Often, it's set to be 1.5 to 2 times the default trap width.

In InDesign, the value you set for Black Color determines the trap width for solid black or rich black. Rich black is a combination of process black (K) ink with color inks to achieve increased opacity and richer color.

Consult your commercial printer to ensure you use the correct trap widths for optimal printing results.

Trap appearance

In trapping, a join refers to the common endpoint where two trap edges meet. You have control over the shape of this outside join for two trap segments and the intersection of three traps.

• Join Style: This option lets you choose the shape of the outside join for two trap segments. There are three choices: Miter, Round, and Bevel. The default is Miter, maintaining compatibility with previous versions of the Adobe Trapping Engine.

• End Style: For the intersection of three-way traps, you can choose between Miter (default) and Overlap. Miter shapes the end of the trap to avoid intersecting objects. Overlap affects the shape of the trap created by the lightest neutral density object that intersects with two or more darker objects. The end of the lightest trap wraps around the point where the three objects intersect.

You can experiment with these settings to achieve the desired trapping results. For further clarity or assistance, consult your print service provider.

Trap thresholds

• Step: This setting determines the color change threshold that triggers trapping. Some jobs require traps for subtle color changes, while others need traps for more extreme changes. The Step value indicates how much components (like CMYK values) of adjacent colors must vary before trapping happens. To adjust this, modify the Step value in the New Trap Preset or Modify Trap Preset Options dialog box. The default is 10%, and a recommended range is 8% to 20%. Lower percentages increase sensitivity to color differences and create more traps.
• Black Color: This specifies the minimum amount of black ink needed before applying the Black trap width setting. The default value is 100%, and it's best to use a value no lower than 70% for optimal results.
• Black Density: Indicates the neutral density value at or above which an ink is considered black. Use this to set dark spot inks for the Black trap width. The typical default is 1.6.
• Sliding Trap: This determines when the trapping engine starts to straddle the centerline of the color boundary. The value is the proportion of the lighter color's neutral density to the darker color's neutral density. For example, a Sliding Trap value of 70% means trapping begins when the lighter color's neutral density exceeds 70% of the darker color's neutral density. If the colors have identical neutral density, their traps will always straddle the centerline, unless Sliding Trap is set to 100%.
• Trap Color Reduction: This controls how abutting color components contribute to reducing the trap color. It prevents certain abutting colors (e.g., pastels) from creating unsightly traps that are darker than either color. Setting a Trap Color Reduction below 100% lightens the trap color; 0% creates a trap with a neutral density matching that of the darker color.

Trap imported graphics

Creating a trap preset helps control traps within images and between bitmap images (e.g., photos and raster PDFs) and vector objects (e.g., drawings and vector PDFs). Each trapping engine treats imported graphics differently, so it's essential to understand these distinctions when setting trapping options.

Trap Placement

• Center: Creates a trap straddling the edge between objects and images, providing a visually consistent edge.
• Choke: Causes objects to overlap the adjacent image.
• Neutral Density: Applies standard trapping rules used throughout the document. Beware, trapping objects to photographs with Neutral Density may result in uneven edges.
• Spread: Makes the bitmap image overlap the adjacent object.

Trap Objects To Images

• Ensures vector objects (e.g., keylines) trap to images using the selected Trap Placement settings. Consider turning it off for pages where vector objects don't overlap images to speed up trapping.

Trap Images To Images

• Enables trapping along the boundaries of overlapping or abutting bitmap images. This feature is on by default.

Trap Images Internally

• Traps colors within each individual bitmap image, not just where they touch vector artwork and text. Use only for simple, high-contrast images (e.g., screen shots or cartoons). For continuous-tone and complex images, leave it unselected to avoid poor traps and speed up trapping.

Trap 1-Bit Images

• Ensures 1-bit images trap to adjacent objects. It doesn't use the Image Trap Placement settings since 1-bit images have only one color. Leave it selected in most cases. However, for widely spaced pixels in 1-bit images, selecting this option may darken the image and slow down trapping.

Trap black color

When creating or editing presets, the "Black Color" value determines what's considered solid black and rich black. Rich black is any black color using additional percentages of process inks to strengthen it—a support screen.

This setting is handy when compensating for extreme dot gain (common with low-grade paper). In such cases, black percentages lower than 100% may print as solid areas. To address this, you can use tints of solid black (screening back blacks or rich blacks) and decrease the "Black Color" setting from its default of 100%. This adjustment compensates for dot gain and ensures the trapping engine applies the right trap width and placement to black objects.

When a color reaches the "Black Color" value, the "Black trap width" applies to all abutting colors, while rich black areas use the "Black trap width" as keepaway traps.

If support screens extend to the edge of a black area, any misregistration can create unwanted halos or distort edges. To avoid this, the trapping engine uses keepaways (holdbacks) for rich blacks. Keepaways keep support screens a specified distance from edges of reversed-out or light foreground elements, preserving sharpness. You control this distance by setting the "Black trap width" value.

Adjust 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.

When adjusting ND (Neutral Density) values, follow these guidelines:

• Metallic and Opaque Inks:
  • Metallic inks are usually darker than CMYK equivalents, and opaque inks obscure underlying colors.
  • For both metallic and opaque spot colors, set the ND values much higher than default to prevent spreading.
    
• Pastel Inks:
  • Pastel inks are lighter than process equivalents.
  • To ensure they spread into adjacent darker colors, set the ND value lower than default.
• Other Spot Inks:
  • Some spot colors, like turquoise or neon orange, can be significantly darker or lighter than their CMYK equivalents.
  • Compare printed swatches of actual spot inks to CMYK equivalents to determine the difference.
  • Adjust the spot ink's ND value higher or lower as needed.

Customize trapping for specialty inks

When working with certain inks, there are special trapping considerations. For instance, when using varnish, you want to avoid affecting trapping. On the other hand, if you have a completely opaque ink overprinting specific areas, traps for items underneath may not be necessary. To handle these situations, ink options are available. Generally, it's best to stick to the default settings unless your prepress service provider suggests changes.

Here's what to do:

1. From the All tools menu, select Use print production > Ink managers.
2. Select the ink that needs special treatment.
3. For "Type," choose from the following options:
   • Normal: Use for traditional process inks and most spot inks.
   • Transparent: Ideal for clear inks to ensure underlying items trap. Suitable for varnishes and dieline inks.
   • Opaque: Use for heavy, nontransparent inks. Prevents trapping of underlying colors but allows for trapping along the ink's edges. Suitable for metallic inks.
   • Opaque Ignore: Perfect for heavy, nontransparent inks that shouldn't trap underlying colors or along the ink's edges. Use for inks like metallic and varnishes, which may interact unfavorably with other inks.

Adjust the trapping sequence

The trapping sequence, also known as the trapping order, reflects the order in which inks are printed at the press but not how separations are produced at the output device.

For printing with multiple opaque colors like metallic inks, the trapping sequence is crucial. Opaque inks with lower sequence numbers spread under those with higher sequence numbers. This prevents the last applied ink from spreading while still creating effective traps.

To adjust the trapping sequence:

1. From the All tools menu, select Use print production > Ink managers.
2. In the inks list, the current trapping sequence is shown in the Sequence column.
3. Select an ink and type a new value for Trapping Sequence, then press Tab. The sequence number changes, and others adjust accordingly.
4. Repeat for as many inks as needed, then select Ok.