Trapping

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About trapping

Where colors printed from separate plates overlap or adjoin one another, press misregistration can cause gaps between colors on the final output. To compensate for potential gaps between colors in artwork, print shops use a technique called trapping to create a small area of overlap (called a trap) between two adjoining colors. You can use a separate, dedicated trapping program to create traps automatically, or you can use Illustrator to create traps manually.

There are two types of trap: a spread, in which a lighter object overlaps a darker background and seems to expand into the background; and a choke, in which a lighter background overlaps a darker object that falls within the background and seems to squeeze or reduce the object.

Spread (object overlaps background) compared to choke (background overlaps object)

When overlapping painted objects share a common color, trapping may be unnecessary if the color that is common to both objects creates an automatic trap. For example, if two overlapping objects contain cyan as part of their CMYK values, any gap between them is covered by the cyan content of the object underneath.

Trapping type can present special problems. Avoid applying mixed process colors or tints of process colors to type at small point sizes, because any misregistration can make the text difficult to read. Likewise, trapping type at small point sizes can result in hard-to-read type. As with tint reduction, check with your print shop before trapping such type. For example, if you are printing black type on a colored background, simply overprinting the type onto the background may be enough.

Create a trap

The Trap command creates traps for simple objects by identifying the lighter-colored artwork—whether it’s the object or the background—and overprinting (trapping) it into the darker artwork. You can apply the Trap command from the Pathfinder panel or as an effect. The advantage of using the Trap effect is that you can alter the trap settings at any time.

A. Area of overprinting B. Area of knockout C. Background color D. Foreground color
What the Trap command does

A. Area of overprinting B. Area of knockout C. Background color D. Foreground color 

In some cases, the top and bottom objects may have similar color densities so that one color is not obviously darker than the other. In this case, the Trap command determines the trap based on slight differences in color; if the trap specified by the Trap dialog box is not satisfactory, you can use the Reverse Trap option to switch the way in which the Trap command traps the two objects.

  1. If the document is in RGB mode, choose File > Document Color Mode > CMYK Color to convert it to CMYK Mode.
  2. Select two or more objects.
  3. Do one of the following:
    • To apply the command directly to the objects, choose Window > Pathfinder, and choose Trap from the panel menu.

    • To apply the command as an effect, choose Effect > Pathfinder >Trap. Select Preview if you want to preview the effect.

  4. Set trap options, and click OK.

Trap options

Thickness

Specifies a stroke width between 0.01 and 5000 points. Check with your print shop to determine what value to use.

Height/Width

Specifies the trap on horizontal lines as a percentage of the trap on vertical lines. Specifying different horizontal and vertical trap values lets you compensate for on‑press irregularities, such as paper stretch. Contact your print shop for help in determining this value. The default value of 100% results in the same trap width on horizontal lines and on vertical lines. To increase the trap thickness on horizontal lines without changing the vertical trap, set the Height/Width value to greater than 100%. To decrease the trap thickness on horizontal lines without changing the vertical trap, set the Height/Width value to less than 100%.

Height/Width set to 50% (left) compared to 200% (right)
Height/Width set to 50% (left) compared to 200% (right)

Tint Reduction

Reduces the tint of the lighter color being trapped; the darker color remains at 100%. This option is useful when trapping two light-colored objects, where the trap line may show through the darker of the two colors, resulting in an unsightly dark border. For example, if you trap a light yellow object into a light blue object, a bright green border is visible where the trap is created. Check with your print shop to find out what percentage of tint is most appropriate given the type of press, inks, paper stock, and so on being used.

Tint reduction value
Tint reduction value of 100% (trap contains 100% of lighter color) compared to tint reduction value of 50% (trap contains 50% of lighter color)

Traps With Process Color

Converts spot-color traps to equivalent process colors. This option creates an object of the lighter of the spot colors and overprints it.

Reverse Traps

Traps darker colors into lighter colors. This option does not work with rich black—that is, black that contains additional CMY inks.

Precision (as effect only)

Affects how precisely an object's path is calculated. The more precise the calculation, the more accurate the drawing and the more time is required to generate the resulting path.

Remove Redundant Points (as effect only)

Removes unnecessary points.

Create a spread or choke

For more precise control of trapping and for trapping complex objects, you can create the effect of a trap by stroking an object and setting the stroke to overprint.

  1. Select the topmost object of the two objects that must trap into each other.
  2. In the Stroke box in the Tools panel or the Color panel, do one of the following:
    • Create a spread by entering the same color values for the Stroke as appear in the Fill box. You can change the stroke’s color values by selecting the stroke and then adjusting its color values in the Color panel. This method enlarges the object by stroking its boundaries with the same color as the object’s fill.

    Object stroked with fill color
    Object stroked with fill color

    • Create a choke by entering the same color values for the Stroke as appear in the lighter background (again, using the Color panel); the Stroke and Fill values will differ. This method reduces the darker object by stroking its boundaries with the lighter background color.

  3. Choose Window > Stroke.
    Object stroked with background color
    Object stroked with background color

    A. Overprinted stroke creates choke trap B. Fill creates knockout C. Area of trap D. Area of knockout 

  4. In the Weight text box, enter a stroke width of between 0.01 and 1000 points. Check with your print shop to determine what value to use.

    For example, a stroke weight of 0.6 point creates a trap of 0.3 point. A stroke weight of 2.0 points creates a trap of 1.0 point.

  5. Choose Window > Attributes.
  6. Select Overprint Stroke.

Trap a line

  1. Select the line to be trapped.
  2. In the Stroke box in the Tools panel or the Color panel, assign the stroke a color of white.
  3. In the Stroke panel, select the desired line weight.
  4. Copy the line, and choose Edit >Paste In Front. The copy is used to create a trap.
  5. In the Stroke box in the Tools panel or the Color panel, stroke the copy with the desired color.
  6. In the Stroke panel, choose a line weight that is wider than the bottom line.
  7. Choose Window > Attributes.
  8. Select Overprint Stroke for the top line.
    Copy of stroke used for spread trap
    Copy of stroke used for spread trap

    A. Overprinted top stroke creates spread trap B. Bottom stroke creates knockout C. Area of knockout D. Area of trap 

Trap a portion of an object

  1. Draw a line along the edge or edges that you want to trap. If the object is complex, use the Direct Selection tool  to select the edges to be trapped, copy them, and choose Edit > Paste In Front to paste the copy directly on top of the original.
    Drop shadow with a trap (left) is based on the line drawn where the object and its drop shadow meet (right).
    Drop shadow with a trap (left) is based on the line drawn where the object and its drop shadow meet (right).

  2. In the Stroke box in the Tools panel or the Color panel, select a color value for the Stroke to create either a spread or a choke.
  3. Choose Window > Attributes.
  4. Select Overprint Stroke.
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