Trapping function and its application in commonly used dtp software

Trapping is the last step in the prepress process. Its function is to compensate for gaps in the printing caused by non-absolute registration or chromatic aberrations at the color separation steps of the image. Trapping is generally not visible during the prepress production process. Traditional output proofing in the dtp process is a means of testing the process.

The halftone reproduction of image reproduction (the visual mixing of four-color dots in the human eye) and the non-absolute registration of substrates in printing are the theoretical and practical reasons for the trapping process. In the prepress process, the cmyk color separation information (film dots) is a knockout, overprint, spread, and choke to be done in the four-color step portion. India.

Trapping is the first function of trapping as described above. Consecutive tone images also have tonal steps at the time the screen is separated. Normally, rip will trap the color when it is connected to the trap. Although Photoshop provides trapping, it is generally not recommended. Unless you are quite sure that your head is as sophisticated as a computer.

Experienced graphic producers know that the gradient effect in the Photoshop lira is better than the lira gradient in the ai or fh. Of course, in the ai or fh output file accuracy should be set above 2400dpi. I think the reasoning behind this is the different trapping support for rip on bitmaps and vector description languages. (Example: A gradient: c80%m30%y20% to m10%y60%bk20% would require a trap, otherwise overprinting may result in gradient color leakage.)

Trapping is a process after the page group edition, so it is a wise choice when exporting a film or a zinc plate. (So ​​the dtp software that can send and dispatch has trapping options when publishing. For example, in the fh option, there is a full-page overprint (usually black version) option in the color and there is a certain color version. Start all the overprint options from a certain percentage.) However, the current active rip has only a limited trapping function. The dedicated trapping software needs to be equipped with a dedicated workstation, such as the trapwise of the Cleopatra Angel. The complexity caused by trapping involves a series of factors such as post-printing processes, which often result in time and money in many cases, but do not necessarily have good results. In the future all-digital process flow, the postscriptlevel3 trapping system developed by adobe is expected to solve this problem at the rip level.

Nonetheless, thanks to the programming ideas of our computer software writer base and object, we can use the corresponding functions of dtp software to do some simple and cheap trapping. For example, in the commonly used page layout software, a reasonable trapping preset parameter is set, and the vector graphic element is manually trapped in the vector software. Before trapping, we first talk about the relevant foundation for trapping.

Trapping foundation

In the prepress separation process, the interrelationship of four-color film is generally completed by the back-end rip.

Separation basically has two relationships: overprint or knockout. (For example, c100% m50% color patches are reflected on four-color film, and can also be regarded as c100% overprint m50%).

The rest of the yk version is hollow. In the four-color printing process, other three-color inks can generally be superimposed on the black solid plate of the printing plate. (In special cases, other three-color inks are superimposed from the black 95%, depending on the circumstances.)

So in all software we can set overprint for bk100% page elements. But if there is a black page element straddling other graphic information. We generally cannot choose overprint.

We can only knockout the element separately (or set the black to bk100%y1%) or set the black to rich black, that is, add other component colors in black, such as black 100%c60 %. To avoid the visual density difference due to ink overprint.

When dealing with large areas of black ground, we often add c30% in black. Because black ink is inorganic ink, it lacks luster. Trapping is also required in this case (setting trap parameters in the imposition software).

If the document has some shades of strong hiding power, such as gold, it can be overprinted.

But if there is black on gold, then we can not overprint the black election, we can only knockout. At the same time, make a spread at the edge. Larger lines (0.1-0.2pt) at the edge of the black or gold plate should generally be larger than the accuracy of the overprint. Then make an overprint for this wireframe alone.

Generally speaking, it is safest to do trapping between covered blacks and spot colors, because sometimes you don't know what to print.

This is the principle in all the manual trapping of prepress software. Among them, quarkxpress's trapping function is the strongest and most professional.

The following is a brief introduction to each software, one by one, in which the relevant values ​​and related nouns are not specifically deepened due to limited space. Because each company's actual production situation is different, it can only be used flexibly under the principle of mastery.
Successful trapping requires the basic knowledge and careful design of color separation, and it also requires a good understanding of the interrelationship between each page element (object) in the layout. The following describes the commonly used software. (Pc version user please refer to the corresponding settings)
u Trapping software settings:

Quarkxpress, pagemaker, and indesign2.0 all have built-in trapping capabilities. Among them, quark's trap function is the most comprehensive. They only trap the following information:

1. Elements generated in the page software. Such as text, lines, patches, and borders.

2. The object generated by the page software that overlaps with the imported bitmap image.

This means that for objects imported in imposition software, eps or bitmaps. It should have been trapped correctly.

For the former we will talk about how to make traps in vector production software. For the latter, I think everyone already has a concept. Using trapping in photoshop will sharpen the subtle levels of the image.

Trapping in quark can be controlled in three ways:

1. Automatically trap: In the quark output, the trapping is based on the setting value in the “Trap Preset Parameters” dialog box.

To access trap parameters, select the document under the Edit menu and click the Trapping tab. Care should be taken not to open any quark document at this time. Trapping panel see below:

2. Color-to-color trapping: This trap is created directly in the color definition, and these values ​​automatically override the auto trapping value.

Under Edit, select the color dialog box, select the color you want to edit, and then click the Trapping button. The Trapping Information dialog box appears. See the following figure:

3. Object-to-object trapping: This trap uses quark's trapinformation floating panel to make trap settings for individual files.

You can set trapping settings for individual objects by selecting the Trappinginformation panel under the Window menu. Please refer to the following figure for details:

Automatic Trapping in pagemaker:

Automatic trapping in indesign software:

u Trapping settings for vector production software:

Automatic Trapping in illustrator:

In summary, trapping is to deconstruct the color information of the entire layout, and then to make a corresponding process of color version compensation at the color step. Trapping is the trapping of some simple page elements based on the concept of trapping and using related software functions. More comprehensive trapping is based on program level or rip level.

From this point of view, we can only expect the development of better trapping software and the development of other related processes and materials for printing. Only then can we get rid of the pitfalls of the prepress industry.

Purpose To create a simple trap manually or automatically Create a trap in the build software Normally trap the ps file after rasterizing

Isolated elements

Overlapping with individual elements Creating a simple manual trapping software Automatic trapping Presets Auto trapping

Overlap with multiple elements Automatically traps presets and traps automatically traps with the built-in trap utility

Overlays with imported elements Use traps in the edge of the element to open traps for foreign objects (unused, harder to control) Allows the user more control

Imported eps object cannot be trapped. Normally, traps cannot be automatically trapped but time is not expected.

Imported bitmaps cannot be trapped. Generally speaking, they cannot be leaked or functions are limited.