Trapping process plays an important role in color printing process

In the process of color printing and copying, it is often the case that texts and figures are superimposed on one another. When these objects (characters, graphics and images) are superimposed on one another, the ones that are on the front will cover the back, and the ones on the print will usually be on the front. The object is called the foreground, and the object behind it is called the background. If the colors of the upper and lower “layers” are different, it will require us to do precise alignment in the subsequent imposition and printing process, otherwise we will appear white in the overprinting part. Or color overlapping phenomenon, which ultimately affect the output image.

However, due to equipment and process reasons, people cannot achieve true absolute registration at all, even in the era of digital printing. Because the process of high-speed transfer of paper in the printing press, generally will produce a slight shift and stretch, in addition, the paper after absorbing fountain solution and ink will also have a certain degree of deformation, these are brought to the color product sets Quasi-problem.

In order to avoid the influence of the deformation of printing materials or the relative displacement of materials to the printing quality, people apply trapping control (also called trapping white) to the overlap of various objects to solve this problem, that is, before printing. The computer software is used for proper processing during production to adjust the overprinting range of each color block so as to avoid the phenomenon of missing white edge. This process is called trapping process in printing.

Trapping principle
The simplest way to understand the trapping principle is to make the foreground color and the background color overlap each other by means of inward and outward expansion. This ensures that there is no white border between two different colors. The outer expansion corresponds to the enlargement of the foreground object size, the background patch size remains unchanged, and the outer edge of the foreground block is overprinted on the hollow background. The indent corresponds to the fact that the foreground color block keeps the same size, and the background hollow portion is reduced, so that the foreground color block is superimposed on the narrowed edge of the background color block cutout. Usually in the trapping process, the use of inner shrinkage or outer enlargement mainly depends on the contrast between the foreground color and the background color. In general, the following principles are followed in the outer expansion: the background color is not expanded, the foreground color is not expanded, and the light color is not expanded. Color, there is a flat screen without expanding the ground. The reason is that the visual effects of background color, light color, and flat screen on people are smaller than those of foreground color and dark color. Otherwise, people can easily detect changes in the shape of the object.

What is the outer pixel value? This mainly depends on the properties of the printing materials, the accuracy of the printing equipment, and the printing method. Due to the improvement of machine processing technology, the precision of the general printing press is very high. Therefore, it is not difficult to guarantee the printing of the registration book, but The factors of printed materials are not controllable by the printing personnel, and trapping values, such as newsprint and plastic printing materials, can be appropriately increased for easily deformable substrates. As for the coated paper with a smaller expansion ratio, the trapping value can be appropriately reduced. For trapping accuracy of the old equipment due to machine wear and other reasons, the trapping value can also be increased appropriately. In addition, the trapping value can also be appropriately increased for the rotating equipment.

In fact, in the pre-press production process, it is not necessary to set the trapping value. In many cases, there is no need to set the trapping value at all. The trapping function has a good effect only when the file has an obvious two-color transfer connection, and the contrast is very strong. However, trapping in a low-contrast place easily leads to blurring of the edges of the image. Therefore, there is continuous Transitional tone or gradient graphics generally do not require trapping.

Trapping method

Usually, trapping can be implemented in two ways. The first is related desktop software in desktop publishing, such as Adobe Photoshop, Adobe Illustrator, Macromedia Freehand, and Adobe Pagemaker, Indesign, and QuarkXpress. Trapping is available. One can apply trapping effects at various object landings with the aid of the "Trap" function. The second is the use of professional trapping software. Currently, a large number of printing process providers and some output device providers provide professional trapping software. Such professional trapping software usually handles Postscript files, such as Adobe's Adobe Trapwise. Process Postscript files on application workstations (just like the RIP in the imagesetter), convert some of the image information into pixels and dots in the file, and then make trapping settings on all adjacent objects. The current digital processes generally support the PDF file format, so there are a large number of PDF trapping processing tools on the market, such as Agfa's In-Rip Trapping and Heidelberg's.
Supertrap and Supertrap Plus trap processing tools.

Regardless of which software is used to process the trapping, the principle is mainly implemented in two ways. That is, the object is expanded by external expansion or contraction, or a color band is calculated from the edge pixel values ​​intersected by the color blocks. Color value and location. Specifically, the first method includes layering and shifting. The layering method traps elements at different layers by trapping or expanding the elements. The shift method is performed by moving the inflection point in the color patch. Retract or extend, used in gradient-related traps