Trapping: the overlap that saves your registration

What it actually does

Two neighbouring inks meet edge to edge in the artwork. On press the screens shift by a fraction of a millimetre, and a strip of bare garment opens along the join. It reads worse than almost any other fault, because the eye goes straight to a light line.

Trapping deliberately spreads one of the inks into the other by a set width. Whether the screens move or not, there is already ink under the join, and no substrate shows.

It works on the separation masks, never on the source artwork, and it runs between the tone masks and the halftone. Your file is not touched.

Which ink should spread

Screen inks are opaque and go down in sequence, so the last ink over a spot is the one you see. Take ink A, printed first, and B, printed second.

Spread A under B and that strip gets A with B on top. It still looks like B, so the design is unchanged, but if the screens shift there is already ink underneath. Free insurance.

Spread B into A instead and the strip looks like B where the artwork wanted A. The visible boundary has moved by the trap width, and that shows.

So the ink to spread is the earlier one, which is what the print-order rule does. The default is the other one: the darker colour laps onto the lighter. That is deliberate. Read together with printing light to dark it means the spreading ink is the later, upper one and the boundary really does move, but a dark fringe over light is far less visible than a light gap showing the garment.

Both rules are available, and a custom one besides. If your process differs, the rule lives in the shop profile rather than in the program.

Where it stops being free

The argument that it still looks like B, so nothing changed, holds exactly while the covering ink is solid.

Where that area is screened at 60 percent coverage, the upper plate leaves 40 percent of it open and whatever is underneath shows through. The trap becomes visible instead of the bare garment. That is physics, not a defect: trapping into a screened region always changes appearance.

That is why there is a limit on the neighbour's tone: trapping can be restricted to edges solid enough for the neutrality argument to apply.

Where a trap does damage, and what the program does about it

An overlap can wreck artwork in two ways.

• A thin neighbour. If the shape next door is narrower than the trap width, spreading covers it completely. The detail is gone.
• A thin line spreading. If the thin line is the one that spreads, it thickens on both sides and can more than double in width.

Both are found geometrically: a feature narrower than twice the trap radius does not survive the test. Those regions are skipped rather than merely flagged, and the skipped area goes into the report.

That is on purpose: the rule is applied where it is safe, not everywhere by default.

When a trap is impossible

Some boundaries cannot be trapped in either direction. Then the opposite remedy applies: open a gap between the inks wide enough that a shift cannot close it. The default profile puts that at 0.3 mm, wider than an ordinary minimum gap, because it has to survive not just the mesh but two screens disagreeing.

The program will not open that gap for you. Opening it means removing part of the artwork, and it does not do that without being asked. So the report names the figure and leaves the decision where it belongs.

Why the preview makes trapping look worse than it is

The preview composites the plates in perfect register, because that is all it can do. It has no model of a press drifting.

So it sees the ink trapping adds and never the bare garment trapping prevents. Any colour difference figure from the preview will read against trapping, and that is expected.

On the reference artwork the mean difference is 1.14 with no trapping, 1.15 with the print-order rule and 1.53 with dark over light. Read naively that says do not trap at all. What the column does not contain is the run where every join shows garment the moment the screens move.

Dark over light scores worse precisely because it shifts the boundary on purpose. That is the trade the shop made: a quarter millimetre of dark fringe instead of white gaps.

FAQ