Trapping is a term used in reproduction technology with two meanings: the contour trapping and ink trapping.
Contour trapping - in German also overfilling - refers to a method or a process within the process of reproduction for all print process, which is necessary for a high quality printed product.
When individually several colors in a printing process, and must be printed over one another in succession, then the substrate (paper) must always perfect in the printing plant based ( in English registration). Otherwise, at the boundary line of two printing colors white edges or holes, called Blitzer arise. In the picture above the unwanted Blitzer at the left a are clearly seen here shining through the white paper. To clarify: The color of the background is made of 100 % cyan, while the color of the letter a is made of 100% magenta. Because the sheet of both colors were not exactly printed over one another, but slightly offset, a small area in between remains unprinted. On the other side of the letter itself, however, overlap the colors cyan and magenta, with the result that this is a shade of blue is formed.
In practice, it is impossible to always guarantee a perfect alignment, so used to the trapping. Simply put is placed around the brighter object, here "a", a thin line, so that minimally overlap the interfaces. Shown in the figure on the right, " a" - - in this case leads to the fact that at the interface of the three colors (cyan, magenta, yellow) are printed over each other, so that small orientation errors are compensated for as long as the shift is not greater than the overlap region is. The amount of overlap depends on the printing technique used. While in modern offset printing very accurate alignment is possible, you need the screen printing a stronger trapping to prevent Blitzer effectively.
There are various ways to implement a trap, both in terms of the method as well as with respect to the time point within the production process. To provide desktop publishing applications such as Adobe InDesign, Scribus or QuarkXPress trapping built engines that make a trapping according to certain preferences automatically. Furthermore, most graphics programs offer the user the option of a manual trapping with the options overprinting, trapping and underfilling, some of which are also used for certain optical effects. However, the changes made in DTP applications trapping information in the context of a production process are often not reliable because it comes to faulty Trappinginformationen or data loss due to incompatibilities ( and others in the supported PostScript versions ). In addition, must be dispensed with QuarkXPress on the passing of a composite file, which makes corrections to the subsequent PDF file. Therefore, trapping is usually carried out in practice in the last instance, ie in the Raster Image Processor (RIP) of the pressure plate issue or the workflow system. Thus, the best result is achieved mainly. Especially in packaging printing and the use of highly opaque colors (eg metallics ) trapping should be left to specialists in exporting printing reserved.
Ink trapping refers to the ink trapping ability of a subsequent print color when wet-on -wet printing, that is, printing several, not dried inks. The following colors generally have a lower tack to promote the acceptance.