Trapping - used at the prepress stage of the printing process (in the manufacture of the layout), the reception of masking register defects. It consists in thickening the contours of the dies by a certain amount to prevent the formation of gaps at the junction of two regions of different colors due to color mismatch during multicolor printing.
Objects can be superimposed on each other in two ways: knockout (cutting out the background with the borders of the object) and overprint (overlaying the object with the background).
Trapping (masking) using the knockout method can be done in two ways: by expanding the border of the object and sealing the background (spreading) and superimposing the edge of the background on the object with its partial sealing (choking).
The use of trapping in the printing process is due to the transillumination of white paper in places of inaccurate registering, which may be due to deformation of the paper in the printing press, displacement of films or printing plates, etc., which is especially noticeable when colored lines and letters are printed on a colored background - between the two forms of the same color there are gaps in the form of white stripes. When printing halftone color images, trapping is extremely rare due to a possible violation of the color balance and tone distribution.
In most cases, lightening is used to create a trapping contour - a contour of the same but less saturated color is allowed around the object (except for black - here the stroke is only black).