What is trapping in printing, and when does it matter?

Trapping is the small, deliberate overlap of two neighbouring inks so that when the printing plates don't line up perfectly, no thin white gap appears between the colours. Layout by Zaweba has trap presets you can assign to page ranges and it produces trapped separations when printing, so you can see and control what happens at those edges rather than leaving it entirely to chance.

It sounds like a tiny detail and on screen it is invisible. On paper, it is the difference between a crisp red headline on a blue panel and one with a hairline of bare paper peeping out along one side.

Why gaps appear at all

A colour press lays down each ink from its own plate, one after another, as the paper races through. Paper stretches and shifts a little, and the plates are never in perfect register to the thousandth of a millimetre. Where two colours simply butt up against each other, a shift of a fraction of a millimetre opens a gap on one side and makes an overlap on the other. The gap shows as a white line, and the eye finds it straight away.

Trapping deals with that by spreading the lighter colour slightly into the darker one, or choking the darker area slightly. The overlap is so small it is hard to see, because the darker ink dominates where they meet, but it means a small shift no longer opens a gap.

How Layout handles it

In Layout, trapping is set up through trap presets. A preset holds the decisions a prepress operator would make: the trap width, a separate width for black, the step limit (how different two colours must be before a trap is made), how black is recognised by colour and density, whether a sliding trap is used where a colour runs into a gradient, the strength of the trap colour, where the trap is placed and how corners are treated. You assign presets to ranges of pages, so a section with fine type on coloured panels can be treated differently from a section of photographs.

Ink densities, which matter when deciding which colour is the darker, are set in the ink manager. When you print with separations, Layout produces trapped separations, and Export > Separations writes a greyscale TIFF for each plate so you can look at them closely. Show traps lets you see the traps on screen.

When you need to think about it

Many jobs never need you to touch trapping. Photographs don't need it, since there are no hard edges between flat inks. Text and shapes that overprint on black don't need it either. Where it does matter is flat colour meeting flat colour: logos, coloured type on coloured panels, packaging and anything with spot inks laid next to process colours.

The bigger question is who does it. Many print shops trap files themselves in their own workflow and would rather you sent untrapped artwork. Others expect trapping to be done before the file arrives. That is a question for the printer, asked early. If they trap, leave it to them. If they want you to, Layout's presets give you the controls.

Overprinting, trapping's close relation

Overprinting is a cousin of trapping. Black text is often set to overprint, which means it prints on top of the colour beneath rather than knocking a hole in it, so misregistration can't show. Layout's colour settings include overprinting of black, and the Separations preview has overprint preview so you can see the effect before it reaches the press, along with an ink limit warning for total area coverage.

It is easy to treat trapping as something mysterious. It is simply a way of building a tiny margin for error into the places where the press is least forgiving. Knowing where those places are in your own artwork is most of the battle.