In graphic arts, the term trapping has a different meaning in prepress and in printing.
Trapping in prepress
Trapping is the technique that is used to compensate for registration issues of successive colors or images. It ensures that there are no ugly gaps or overlaps in the printed result. Misregistration can occur because of:
- Mechanical inaccuracies in an imagesetter or CtP system
- Printing plates or film stretching
- Inaccuracy in copying film to either film or plates
- Instability of the printing press
- Dimensional instability of the paper or medium that is printed on
Not all printing processes suffer from misregistration. It is fairly common not to worry about trapping with various digital printing techniques. Some companies that have invested in high quality four or eight color offset printing presses also prefer not to trap files. Since trapping actually introduces artifacts in the job, they consider an occasional issue with misregistration less of an issue than the visual impact of trapping all jobs.
How to trap
There are different techniques used to trap layout elements.
- As a general rule, lighter colors should be spread into darker colors. This minimizes the visual effect of trapping.
- A spread or fattie means that an element is slightly enlarged or ‘spread’ so that it overlaps the background.
- A choke or skinny means that the size of an element is slightly reduced so that its background slightly overlaps the object.
In the days when film was used to prepare printing plates, trapping was done photographically. Now that all content is digital, software takes care of trapping. Most prepress workflow systems include a trapping option. The trapping is either done on intermediate files, such as print-ready PDFs, or it is done within the RIP or Renderer.
How much trapping is needed?
As a general rule, the width of traps should equal half a dot. That means creating traps that are 1/300 inch (0.08 mm) wide for a 150 lpi screen ruling. If one of the colors involved is black, traps are often up to twice as wide since the effect is less visible with such a dark color.
Trapping in printing
Trapping is the technique of printing one ink on top of another one. Poor trapping leads to inks that do not adhere properly. This can cause the ink to form little beads or it can be rubbed off too easily. Wet trapping refers to ink that is being laid down on another ink that has not yet dried. Dry trapping obviously involves an underlying ink that has already dried.