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.
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.
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.
- 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.
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.
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.
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.
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.
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 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.
Specifies the trap width in points for trapping all colors except those involving solid black. The default value is 0p0.25.
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.
(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.
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.
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.
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.
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.
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%.
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.
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.
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.
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 theTrap 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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
Don’t alter the default trapping sequence without first consulting with your prepress service provider.