Overprint, Trapping, and Registration

Full-color printing lays down one ink at a time - cyan, then magenta, then yellow, then black - each from its own plate. The illusion of a single continuous image depends on all four landing in register: perfectly aligned, to within a fraction of a millimeter. On a press running thousands of sheets an hour, on paper that swells with humidity and stretches under tension, that alignment is never exact.

The consequences show up wherever colors meet. If a magenta shape sits next to a cyan one and the plates drift apart, a sliver of bare white paper appears in the seam - a gap the eye catches instantly. Drift the other way and the inks overlap into an unwanted dark line. The craft of prepress is anticipating this slip and designing the artwork so it survives - with trapping, and by choosing between overprint and knockout at every boundary.

Start with the problem itself. Two colored shapes meet at an edge, each printed from a different plate. Nudge one plate out of register and watch the seam: a white gap opens on one side, and where the plates instead overlap, a dark fringe forms. The registration marks in the corner - the little crosshairs - are the press operator's early warning that the plates have drifted.

The fix is trapping: before printing, you slightly overlap the two colors so a small misregistration lands inside the overlap instead of opening a gap. Enlarging the lighter color into the darker is a spread; shrinking it is a choke. The overlap creates a thin transitional seam - usually a bit darker - but that's far less noticeable than a bright white line. Turn trapping on and slip the plates again.

When one object sits on top of another color, you choose how they interact. Knockout removes the ink beneath the object so it prints on bare paper - clean, accurate color, but the edge now depends on registration. Overprint prints the object straight over the underlying ink - no registration worry at the edge, but the two inks mix. Small black text is almost always set to overprint (so it stays crisp); a colored object usually knocks out. Watch black text both ways over a colored background.

Overprinting isn't just a registration trick - it's how process printing builds most of its colors in the first place. Each ink is a subtractive filter: cyan removes red, magenta removes green, yellow removes blue. Stack two and you get a secondary - cyan over yellow makes green, magenta over yellow makes red, cyan over magenta makes blue. This is exactly why an accidental overprint changes color. Build the overprints.

The terms you'll meet at the boundary between design and press:

Six questions on registration, trapping, overprint, and knockout. Instant feedback, no scores recorded - a wrong answer comes with a short explanation.

Registration and trapping sit in the middle of the print pipeline - here's what surrounds them.