When printing a document it is often necessary to generate separations for each ink that is used in the particular printing process being used. The separations can either be made in PTC ALD using a print control stream, or may be generated later from a composite PostScript or PDF file by the printer or another piece of software. When generating the separations, each element on the page will normally print solid on its own plate, and cut a hole on the other plates (knockout mode).

Unfortunately when printing the page in several passes, one ink at a time, you may encounter slight errors in the alignment which may leave white edges around text on a colored background. To overcome this problem, you can use the technique of overprinting. When overprinting, the holes are not cut out on the other color plates, so the object is simply printed over the top.

To control individual objects, any color can explicitly specify its own overprint mode that overrides the default set with <?tovpr>, for example:

When enabling overprint on individual items in this way, the color itself can be specified in any of the acceptable forms, e.g. {0,0,0,100}[1] (CMYK),50/80 Green+5[1] (HSB), %50 Beige[1] (named color).