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Registration (Trapping)

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Trapping Type

Trapping type can present special problems. Avoid applying mixed process colours or tints of process colours 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 coloured background, simply overprinting the type onto the background may be enough.

To trap type, you can add the stroke below the fill in the Appearance palette, and set the stroke to overprint (or set it to Multiply blending mode).

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Trapping Two Light Colours

When trapping two light-coloured objects, the trap line may show through the darker of the two colours, 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.

To prevent the trap line from showing through, you can specify a tint of the trapping colour (in this example, the yellow colour) to create a more pleasing effect. 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.

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Common Colours

When overlapping painted objects share a common colour, trapping may be unnecessary if the colour 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.

Note:

When artwork does contain common ink colours, overprinting does not occur on the shared plate.

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

You can create both spreads and chokes in Adobe Illustrator.

It is generally best to scale your graphic to its final size before adding a trap. Once you create a trap for an object, the amount of trapping increases or decreases if you scale the object.

Converting strokes to filled objects

The Outline Stroke command traces the outline of all stroked paths within the selected artwork and substitutes a filled object with the same width as the original stroked path. This commands lets you modify the outline of an object more than you could if it were only a stroke.

The Outline Stroke command is useful when you prepare artwork for trapping colour separations.You can apply it as a filter or as an effect.

To create stroke-shaped objects from strokes. Do one of the following:

• To apply the command as a Path command, select the artwork to process. The artwork can contain filled and stroked paths; however, only stroked paths become stroke-shaped objects. Then choose Object >Path >Outline Stroke.
• To apply the command as an effect, select an object or group in the artwork. Then choose Effect >Path >Outline Stroke.

The outline replaces the original artwork and is selected.

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Using the Trap command

The Trap command creates traps for simple objects by identifying the lighter artwork--whether it's the object or the background--and overprinting (trapping) it into the darker artwork.

Note:

The Trap command is only available when you are working on CMYK documents.

Function of Trap command
A. Area of overprint
B. Area of knockout
C. Background colour
D. Foreground colour

You can apply the Trap command as a Pathfinder command or as an effect.

In some cases, the top and bottom objects may have similar colour densities so that one colour is not obviously darker than the other. In this case, the Trap command determines the trap based on slight differences in colour; 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.

Light object and dark background: Trap command creates spread.
Dark object and light background: Trap command creates choke.

To create a trap using the Trap command:

1. Select two or more objects, choose Window >Pathfinder, and select Trap from the palette's fly-out menu.
2. In the Thickness text box, enter a stroke width of between 0.01 and 5000 points. Check with your print shop to determine what value to use.
3. Enter a value in the Height/Width% text box to specify the trap on horizontal lines as a percentage of the trap on vertical lines.
4. • 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% and 200%
5. Enter a Tint Reduction value to change the tint of the trap. The default value is 40%.
6. • The Tint Reduction value reduces the values of the lighter colour being trapped; the darker colour values remain at 100%. The Tint Reduction value also affects the values of custom colours.
   • With tint reduction value of 100%, trap contains 100% of lighter colour. With tint reduction value of 50%, trap contains 50% of lighter colour.
7. Select additional trapping options as required:
8. • Traps with Process colour if you want to convert spot colour traps to equivalent process colours. This option creates an object of the lighter of the spot colours and overprints it.
   • Reverse Traps to trap darker colours into lighter colours. This option does not work with rich black--that is, black that contains additional CMY inks.
9. Click OK to create a trap on the selected objects. Click Defaults to return to the default trapping values.

Trapping by overprinting

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.

To create a spread or choke by overprinting:

1. Select the topmost object of the two objects that must trap into each other.
2. In the Stroke box in the toolbox or the colour palette, do one of the following:
3. • Create a spread by entering the same colour values for the Stroke as appear in the Fill option. You can change the stroke's colour values by selecting the stroke and then adjusting its colour values in the colour palette. This method enlarges the object by stroking its boundaries with the same colour as the object's fill.
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     Object stroked with 1-point stroke of fill colour
     A. Stroke creates spread trap
     B. Fill creates knockout
     C. Area of trap
     D. Area of knockout
   • Create a choke by entering the same colour values for the Stroke as appear in the lighter background (again, using the colour palette); the Stroke and Fill values will differ. This method reduces the darker object by stroking its boundaries with the lighter background colour.
   
   Object stroked with 1-point stroke of background colour
   A.Stroke creates choke trap
   B. Fill creates knockout
   C. Area of trap
   D. Area of knockout
4. Choose Window >Stroke.
5. In the Stroke palette, in the Weight text box enter a stroke width of between 0.6 and 2.0 points.
6. 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. Check with your print shop to determine what value to use.
7. Choose Window >Attributes.
8. Select Overprint Stroke.

In a spread, the lighter object traps into (overprints) the darker background. In a choke, overprinting the stroke causes the lighter background to trap into the darker object.

To 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, copy it, and choose Edit >Paste in Front to paste the copy directly on top of the original.
2. 
   Drop shadow with a trap. The line drawn
   at the intersection of two shapes creates a trap.
3. In the Stroke box in the toolbox or the colour palette, select a colour value for the Stroke to create either a spread or a choke.
4. Choose Window >Attributes.
5. Select Overprint Stroke.

[ Source: Illustrator Help ]