Trapping settings in 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 what we usually call the knock out, overprint, spread, and choke work done in the four-color step section. Trapping.

Trapping is the first function of trapping as described above. Consecutive tone images also have tonal steps at the time the screen is separated. Under normal circumstances, RIP traps traps when they are trapped. Although Photoshop provides Trapping functionality, it is generally not recommended. Unless you are quite sure that your head is as sophisticated as a computer.

Experienced graphic producers know that gradients in Photoshop are better than ila or FH lira gradients. Of course, in the AI ​​or FH output file accuracy should be set above 2400 DPI. In my opinion, the reason is that RIP supports different trapping for 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 can send traps with trapping options when publishing. For example, there is a color full-page overprint (usually black version) option in the FH option, and there is a color version The option to start all overprints from a certain percentage scale.) 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 TRAPWISE from Creo Serb Angels. 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 POSTscript LEVEL 3 trapping system developed by ADOBE Company is expected to solve this problem at the RIP level.

Nonetheless, thanks to the programming ideas of our computer software writer base and objects, we can use the corresponding functions of DTP software to do some simple and inexpensive 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, the trapping function of QuarkXpress 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 users please refer to the corresponding settings)

Page layout software trap settings:

Quarkxpress, Pagemaker, and Indesign 2.0 all have built-in trapping features. 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.

That is to say, EPS or bitmaps for objects imported in imposition software. 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 sharpens the subtle levels of the image.

Trapping in Quark can be controlled in three ways:

1. Automatic Trapping: In Quark output, trapping is done according to the set 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 documents at this time.

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, then click the Trapping button. The Trapping Information dialog box appears
3. Object-to-object trapping: This trap uses the Quark Trap Information floating panel to make trap settings for individual objects in the file.

Select the Trapping Information panel under the window menu to make trapping settings for individual objects.

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. The more comprehensive implementation of 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.