Trapping
Trapping is a term from reproduction technology with two meanings: contour trapping and ink trapping .
Contour trapping
Contour trapping - to German and overfilling - refers to a method and a method within the reproduction process for all printing processes, which is necessary for a high quality printed product.
If several colors have to be printed one on top of the other in a printing process, the substrate (paper) must always be perfectly aligned in the printing system ( registration in English ). Otherwise, white borders or holes, so-called flashes , can appear at the boundary between two printing inks . In the figure above, the undesired speed cameras can be clearly seen on the left a, here the white paper shines through. To understand: The color of the background is 100% cyan, while the color of the letter a is 100% magenta. Since the colors on the print sheet were not printed on top of each other, but slightly offset, a small area in between remains unprinted. On the other hand, the colors cyan and magenta overlap on the other side of the letter, which results in a shade of blue.
In practice it is impossible to always guarantee perfect alignment, so trapping is used. To put it simply, a thin line is placed around the lighter object, here the "a", so that the boundary surfaces overlap slightly. In this case - as can be seen in the figure on the right "a" - this means that three printing colors (cyan, magenta, yellow) are printed on top of each other at the interface, so that small alignment errors are compensated as long as the shift is not greater than the overlap area is. The size of the overlap depends on the printing technique used. While a very precise alignment is possible in modern offset printing , more overfilling is required in screen printing in order to effectively prevent flashes.
There are different ways of implementing a trapping, both with regard to the method and with regard to the point in time within the production process. DTP applications such as Adobe InDesign , Scribus or QuarkXPress offer built-in trapping engines that automatically perform trapping according to certain presettings. In addition, most graphics programs offer the user the option of manual trapping with the options of overprinting , overfilling and underfilling , some of which are also used for certain optical effects. However, the trapping information carried out in DTP applications is often not reliable as part of a production process because incompatibilities (e.g. with the supported Postscript versions) lead to incorrect trapping information or data loss. In addition, QuarkXPress does not pass on a composite file , which makes corrections to the later PDF file more difficult. In practice, therefore, trapping is usually carried out in the last instance, i.e. in the Raster Image Processor (RIP) of the printing plate output or the workflow system. This will mostly achieve the best result. Especially in packaging printing and when using highly opaque colors (e.g. metallic colors), trapping should be reserved for the specialists of the printing company.
Ink trapping
Ink trapping describes the ability of a subsequent printing ink to accept ink during wet-on-wet printing , i.e. H. Overprint of several, not dried inks. The colors below generally have a lower tack to promote acceptance.