Trap trap function and its application in common DTP software

Trap (Trapping) is the last process in the prepress process. Its function is to make up for the blank gap caused by non-absolute registration in printing or the color difference caused by the color separation step of the image. Trap printing is generally invisible in the prepress production process, and the traditional output proofing in the DTP process is a means of checking 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 the substrate in printing are the theoretical and practical reasons for the existence of the trapping process. In the prepress process, the work of knocking out, overprinting, spreading and choke of CMYK color separation information (film dots) in the four-color step is what we usually call Trap.

As described above, trapping is the first function of trapping. The continuous tone image will also have a tone step when the screen is separated. Under normal circumstances, RIP will trap the color separation when hanging on the net. Although Photoshop provides the Trapping function, it is generally not recommended. Unless you are pretty sure that your brain is as sophisticated as a computer.

Experienced graphic designers know that the effect of pulling gradients in Photoshop is better than that in AI or FH. Of course, in AI or FH output, the file precision should be set higher than 2400DPI. I think the reason is that RIP supports different traps when bitmap and vector description languages ​​are connected to the Internet. (Example: a gradient: from c80% m30% y20% to m10% y60% bk20%, you need to make a trap, otherwise inaccurate overprinting during printing may lead to gradient color leakage.)

Trap is a process after the page group version, so it is a wise choice to output film or zinc plate. (Therefore, the DTP software that can be released has trapping options during the release. For example, in the FH release option, there is a full-color overprint of a certain color version (generally a black version), and there is also a certain color version. The option to start all overprinting from a certain percentage tone.) But generally the currently used RIP only has limited trapping function. The dedicated trapping software needs to be equipped with a dedicated workstation, such as TRAPWISE of Creole Race Angel. The complexity caused by a series of factors such as post-printing processes involved in trapping makes it often cost time and money in many cases, but may not necessarily have a good effect. In the future fully digital process flow, the trap filter system developed by ADOBE based on POSTSCRIPT LEVEL 3 is expected to solve this problem at the RIP level.

Nonetheless, thanks to our computer software writer base and object programming ideas, we can use the corresponding functions of DTP software to do some simple and cheap trapping. For example, set reasonable trap preset parameters in commonly used page layout software, and manually trap vector graphics elements in vector software. Before doing trapping, let's talk about the basics of trapping.

Trap basics

In the pre-press color separation process, the mutual relationship of the four-color film is generally completed by the back-end RIP.

Color separation basically has two relationships: overprint or knockout. (If the color block of c100% m50% is reflected on the four-color film, it can also be regarded as c100% overprinting m50%).

The rest of the yk version is hollow. In the four-color printing process, the black solid area of ​​the printing plate can generally be overprinted with other three-color inks. (In special cases, other three-color inks may be overprinted from 95% of black, depending on the specific situation).

Therefore, in all software, we can set bk100% page elements as overprint. But if there are black page elements straddling other graphic information. We generally cannot choose overprint.

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

When dealing with large areas of black ground, we often add c30% to the black. Because black ink is an inorganic ink, it lacks gloss. In this case, trapping is also required (the trapping parameters are set in the imposition software).

If there are some opaque color versions in the document: such as gold, then it can be overprinted.

But if there is black on the gold, then we cannot overprint the black, we can only knockout. Also make a spread on the edge. Make a few lines (0.1-0.2pt) on the edge of the black or gold version, which should generally be greater than the printing overprint accuracy. Then make an overprint on this wireframe separately.

Generally speaking, the black between the strong coverage and the spot color is the most safe way to trap, because sometimes you do n’t know which color to print first.

This principle is used in all manual traps in pre-press software. Among them, the trapping function of QuarkXpress is the strongest and most professional.

The following is a brief introduction for each software one by one, in which the numerical setting and related term explanations are not specific and in-depth. Because the actual production situation of each company is different, it can only be used flexibly under the principle.

Successful trapping requires the basic knowledge and careful design of color separation and plate making, as well as the mutual relationship between each page element (object) in the layout. The following is an introduction to commonly used software. (For PC version users, please refer to the corresponding settings)

u The trap setting of page layout software:

Quarkxpress, Pagemaker and Indesign2.0 all have built-in trapping functions. 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, color blocks and borders.

2. Objects generated by the page software that overlap with the imported bitmap image.

In other words, for objects imported in imposition software, EPS or bitmap. It should have done trapping correctly.

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

u The trap setting of vector production software:

In summary, trapping is the process of deconstructing the color information of the full layout, and then making the corresponding color compensation at the color step. The trap is based on the concept of trapping, we use related software functions to trap some simple page elements. The implementation of a more comprehensive trap is based on the program level or RIP level.

From this perspective, we can only look forward to the development of better trapping software and the development of other related processes and materials in order to get rid of the trapping of the prepress industry.

Drawings: Drawing software group version software dedicated trapping software purpose manual or automatic creation of simple trapping traps created in the group version software generally trapping the PS file after rasterization of the trapping elements isolated elements no nothing no overlap with a single element to create Simple manual trap setting Automatic trap setting in software Overlap automatic trapping with multiple elements Use the built-in Trap tool software's automatic trap setting and Trap tool automatic trapping to overlap imported elements Use overprint stroke software at the edge of the element to enable trapping for foreign objects (not commonly used, which is more difficult to control) Allows users to have more control over imported EPS objects cannot be trapped. Generally speaking, automatic trapping cannot be done without leaking but time Unexpectedly imported bitmaps cannot be trapped. Generally speaking, the traps cannot be missed or the functions are limited.

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