Adobe ACROBAT 3D Manual page 616

Trapping with black and rich black
The value you type for Black Color in the New Trap Preset dialog box determines what
Acrobat considers to be a solid black and a rich black. A rich black is any black color that
uses a support screen--adding 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 will
cause 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,
placement 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.
Note: Don't be concerned that the Black trap width setting will be too wide for trapping
thin elements, such as black keylines around images. In those cases, the trapping engine
automatically overrides the Black trap width setting and limits the trap to half the width of
the thin element.
To set the trap width for colors next to black:
1. In the Trap Presets dialog box, click Create to create a preset, or select a preset and click
Edit to edit it. (See
2. In the Trap Width section, for Black, enter a distance (in points) for how far you want
other colors to spread into black, or for how far you want support screens choked back
under black. Typically, the Black trap width is set to be 1.5 to 2 times the value of the
Default trap width.
3. For Black Color and Black Density, set values. (See
Note: To use black trapping features, a color area must use an ink with a neutral density
greater than or equal to the Black Density, and the ink must occur in percentages greater
than or equal to the Black Color.
Using trap presets to specify trap settings.)
Setting trap thresholds.)