"Rules" tab

In this tab you define rules that set how the trapper reacts when creating trap lines and when handling images.

"Trap" group

The parameters in the "Trap" group make it possible for you to control the trapping conditions. The following three conditions must be met in order to create trap lines:

​•​absolute difference between separations

The difference in screen percentage density between two separation colors must be greater than 5% in at least two separations.

​•​relative difference between separations

The difference in screen percentage density between two separation colors, namely the difference of the darker color relative to the lighter one, must be greater than the step limit entered by the user.

​•​Overall common density

The sum of the neutral density of all separations must be less than the common density limit.

Step Limit (%)

This parameter indicates the degree to which the separations of adjacent colors must vary before a trap is created.

A trap is created in those separations where the difference between adjacent colors is greater than the input value.

How this works:

​•​Lower values cause traps to be created even with slight differences in color. Accordingly, there are a greater number of traps.

​•​Higher values mean that traps are created only if the differences in color are greater. Accordingly, there are not as many traps.

Note: There is no trapping if the difference between abutting colors is less than 5 %.

The step limit ranges from 1% to 100%. The default value is 25%.

"Common Density Limit" parameter

A Common Density Limit is used to prevent trap lines being created in places where flashes are not visible due to the ink recipe. Spot colors are also included in this calculation, if necessary.

Definition of the Neutral Density:

"Neutral density" is the degree to which a color is found in common in each separation of two adjacent colors:

Each process color (CMYK) has a different ink strength. "Neutral Density" was defined to determine the ink strength of a process color, with paper white having a neutral density of 0.

Example of the neutral density for process colors:

The following formula can be used to calculate the neutral density (ND) in values less than 100%:

ND = –1.7 · log(1 – Color(1 – 10^{(–0.6 · D)}))

The neutral density of all separations is obtained from the sum of the neutral densities of the single separations.

How "Common Density Limit" works":

​•​Lower values mean that trap lines are not created if there are only slight differences in color. Accordingly, there are not as many trap lines.

​•​Higher values mean that trap lines are created only if there are greater differences in color. Accordingly, there are more trap lines.

The values for the Common Density Limit range from 0 to 10. The default value is 0.5.

Example:

Common Density Limit = 0.5, this means trap lines are not to be created if the common neutral density is greater than 0.5.

Color 1 -> 60% cyan and 20% magenta

Color 2 -> 40% cyan and 70% magenta

The neutral density value (ND) of cyan is 0.61, that of magenta 0.76.

A common neutral density of ND = 2.94 results based on the formula specified above for calculation of the neutral density (ND).

The common neutral density of both colors is less than the specified "Common Density Limit", i.e. a trap line will be created.

"Centerline Trap Limit (%)" parameter

The "Centerline Trap Limit" defines under what conditions a centerline trap will be created.

The default trap direction (objects with a darker color are trapped by lighter colors) is not mandatory between colors with a similar neutral density (ND).

The default is 100%, meaning that no centerline traps (centered symmetrically at the color transition) will be created.

Rare exception: Centerline traps will be created if the neutral density of the objects is identical.

How "Centerline Trap Limit" works":

The lower the Centerline Trap Limit entered, the more centerline traps will be created.

Theory:

A centerline trap is created if the neutral density of the lighter color is greater than the neutral density of the darker color multiplied by the "Centerline Trap Limit".

The values for "Centerline Trap Limit" range between 0% and 100%.

"Trap Color Scaling (%)" parameter

When required, Trap Color Scaling reduces the proportion (in percent) of single colors at trap lines and, by doing so, helps to make trap lines less noticeable (for example, this can be good in pastels).

The default value is 100% and means that there is no trap color scaling.

How "Trap Color Scaling" works":

The lower the Trap Color Scaling entered, the lighter the trap colors and the less visible the trap lines.

Theory:

Trap Color Scaling makes trap lines less visible by reducing the various separation colors according to the degree they are present in the color of the trap line. Trap Color Scaling only affects those parts of the trap color that originate from the lighter of the adjacent colors.

The values for Trap Color Scaling range between 0% and 100%.

​•​Example of Trap Color Scaling

Trap Color Scaling = 75%

The yellow separation in the darker color is 60%.

The yellow separation in the lighter color is 88%.

Without Trap Color Scaling (color reduction), the trap in the yellow separation would be 88%. With Trap Color Scaling, the difference (88 - 60 = 28) is now reduced by 75% (0.75 * 28 = 21) and this 21% is added to the separation with the lesser value (60 + 21=81).

Result:

By means of a 75% TrapColor Scaling, the trap color in the yellow separation was reduced from 88% to 81%.

"Spread White against Color" option

This option causes white objects, e.g. white text, on a colored background to be spread whereas colored objects, e.g. colored text, on a white background is not choked.

For a technical viewpoint this means:

When this option is enabled, (white) objects with a color percentage of 0% in all separations are spread against color objects lying below whose color percentage is > 0% in more than one separation.

In this process, the darkest separation, measured against neutral density, of the object lying below is kept.

"Keepaway Mode" option

You enable a special trapping function with this option that you can use for work related to packaging printing.

Normally, in trapping darker objects are trapped with the darker separations. If the Keepaway Mode is enabled, lighter objects are trapped by the lighter separations. This usually creates a white frame around every object.

This "White Framing" (cf. "White Framing") is useful if you want to keep inks away from each other, for example, when printing on metallic surfaces.

Note: You could also call this function "inverted trapping".

"Images" Section

Settings made here affect trapping at the transition points between graphics (vector data) and images (pixel data) and between several images as well as trapping within images.

"Trapping – Objects" option

If this option is checked, images are trapped to other objects. The image pixels are not replicated. Instead the trap lines are created by "overprinting" single color separations.

"Trapping – Images" option

If this option is checked, images are trapped to other images. Trapping within images is not supported.

"Direction" parameter

This parameter controls how trap lines are placed when images are trapped.

The following settings are available:

​•​Center

The trap lines are centered along the edge between the image and the adjacent object.

​•​Into Image

The trap lines are placed in the image.

​•​Into Object

The trap lines are placed in the adjacent object.

​•​Automatic

The trap direction is determined automatically on the basis of the color of the adjacent object and of the average color of that image area.

"Trap Complex Bitmaps, if Possible" option

Complex bitmaps (graphics where only area coverages of 0% and 100% occur per separation) with many details and a great number of colors are difficult to trap. When this option is enabled, the system checks whether it is possible for complex bitmaps in the documents you are editing to be trapped.

Sometimes trapping complex bitmaps (bitmaps with many small elements) may not be what you want. If required, you can disable the trapping of complex bitmaps by enabling this option.

"Convert Bitmaps to Graphics, if possible" option

You have to convert bitmaps without a large number of small elements into graphics (i.e. into vector graphics with variable area coverages in the separations) to be able to use the full trapping functionality. When this option is enabled, the system checks whether these kinds of bitmaps are present. These are then converted to graphics.