Trapping in screen printing is the deliberate overlap between two adjoining ink colors, added so that when the press drifts slightly out of registration, no white gap shows between them. It is one of those prepress steps that is invisible when done right and glaringly obvious when skipped — a thin white sliver running along every color boundary on the finished shirt. Understanding trapping, and specifically choke versus spread, is core to preparing multi-color separations that survive real presses.
The reason trapping exists is simple: no press registers perfectly. Screens flex, garments shift on the platen, and manual presses hold looser than automatics. A fraction of a millimeter of movement is enough to open a visible gap between two colors. Rather than chase impossible registration, printers build in a small, controlled overlap that absorbs the drift. This guide explains what trapping does, how choke and spread differ, and how much trap to add for manual versus digital work.
Why Misregistration Forces a Trap
When you print one color next to another, the two screens have to land their ink in exactly the right relationship. In practice they never do, quite. The sources of misregistration stack up:
- Garment movement — the shirt shifts slightly on the platen between colors.
- Screen instability — mesh flexes and frames move under squeegee pressure.
- Film and exposure error — small inaccuracies creep in during film-to-screen steps.
- Manual press slop — hand presses simply hold looser registration than automatics.
If two colors are set to butt exactly against each other and the press drifts, the gap where no ink lands shows the substrate color — a white line on a white shirt is hidden, but on a dark garment it is obvious. Trapping fixes this by overlapping the colors slightly so there is always ink where the two meet, even when registration is off.
Choke vs Spread: The Two Moves
Trapping used to be called "chokes and spreads," and those are still the two operations. Both create an overlap; they differ in which color moves and in which direction.
- Spread — you expand a shape outward into the neighboring color. The shape grows, pushing its edge out under the adjacent ink. Spreads are applied to the lighter of two colors so it tucks under the darker one.
- Choke — you pull an outer color inward within a shape. The background color is pinched or "choked" so the top color's edge overlaps it. Chokes are used when a darker foreground sits over a lighter background.
The governing principle: keep the lighter color moving, and let the darker color hide the overlap. A light element on a dark ground gets spread outward; a dark element on a light ground gets a choke on that light ground. In both cases the darker ink covers the overlap zone, so the eye never sees the doubled color.
| Situation | Move | What happens |
|---|---|---|
| Light color on dark background | Spread the light color | Light edge grows outward under the dark ink |
| Dark color on light background | Choke the light background | Light edge pulls in; dark color overlaps it |
| Underbase white on dark shirt | Choke the white | White pulls in so top colors cover its edge |
How Much Trap to Add
Trap width is a balance. Too little and misregistration still opens a gap; too much and the overlap itself becomes a visible muddy line. The right amount depends on your press and your file resolution.
For digital files, work at 300 DPI and think in pixels:
- Tight, well-registered press — around 2 pixels is often enough.
- Average manual setup — 3-4 pixels is a safe starting point.
- Loose press, wood frames, difficult garments — 4 pixels or more may be needed.
Because the trap is measured in pixels, the physical width scales with resolution — 3px at 300 DPI is a different real size than 3px at 150 DPI. Always set trap in the context of the file's actual DPI. If you are unsure whether your file is genuinely at print resolution, our guide to DPI for fabric printing covers checking effective resolution before you trap.
The honest answer to "how much" is: as little as your press allows. A great printer holding close registration needs only a hair of trap; a looser setup needs more. When in doubt, ask the shop that will run the job — they know their own registration tolerance.
Trapping the Underbase
On dark garments you print a white underbase first so the colors on top stay bright. That underbase edge is a classic trapping problem: if the white extends to the exact edge of the colors, any misregistration shows a white halo around the print.
The standard fix is to choke the underbase:
- Build the underbase to match the color art.
- Choke (pull in) the white a couple of pixels from the color edges.
- Let the top colors overprint that pulled-in white boundary.
Now the top inks always cover the edge of the white, and no halo shows even if the press drifts. The amount you choke follows the same logic as any trap — enough to cover your press's registration slop, no more. Underbase handling is part of a full separation workflow; see our color separation for screen printing guide for how the base and top colors fit together.
Manual vs Digital Trapping
Historically, trapping was done photographically at the film stage. Today it is almost always applied in software during separation, but the intent is identical.
- Vector art — add a small overprinting stroke to the shape, or expand the compound path, so the edge overlaps the neighbor.
- Raster art — expand or contract the color selection by the trap amount, then apply, which grows or shrinks that color's coverage.
- Separation tools — dedicated screen-print separation software can apply choke and spread automatically across a whole multi-color file.
Whichever tool you use, trap only where two colors share an edge. A color sitting on a blank shirt with no adjoining ink needs no trap. Intentional knockouts stay as knockouts. Blanket-trapping every element just creates muddy overlaps and extra ink where none was needed.
Avoiding Overtrapping
It is worth stressing the opposite failure. Under-trapping leaves white gaps; over-trapping leaves a dark, muddy third color along every shared edge where the two inks double up. On fine detail — small text, thin linework — an aggressive trap can close up gaps and thicken strokes until the art looks clumsy.
Keep overtrapping in check:
- Match trap width to your press, not to a maximum "just in case" value.
- Reduce trap on fine detail and small type.
- Preview the trapped separation at print scale before you output film.
- Remember every added pixel of trap is a pixel of doubled ink someone will see.
Trapping is a checklist item on any multi-color job, right alongside registration marks and screen counts. Working it into a repeatable prepress routine — like the one in our rotary screen printing design checklist — keeps it from being an afterthought at the film stage.
A Worked Example
It helps to walk one design through. Say you have a two-color print: a light cream motif sitting directly on a dark navy ground, printed on a black shirt with a white underbase. Three trapping decisions come up, and each follows the same rule.
- Cream motif vs navy ground. Cream is lighter, so it moves — spread the cream outward a couple of pixels. The navy overprints the spread edge, so the eye never sees the overlap. If the press drifts, the cream simply extends a hair under the navy instead of leaving a gap.
- Underbase white. The white base sits under both colors on the black shirt. Choke the white in from the color edges so the cream and navy always cover its boundary. No white halo peeks out around the print.
- Fine detail inside the motif. If the cream has thin linework, ease off the spread there — an aggressive trap would thicken the lines and close small gaps.
That is the whole logic in miniature: lighter color moves, darker color hides the overlap, underbase gets choked, fine detail gets a lighter touch. Once you can reason through a job like this, trapping stops being a mysterious prepress step and becomes a series of small, predictable decisions.
Where Trapping Fits in Separations
Trapping is not a standalone task — it lives inside the color separation workflow and only makes sense once the colors are already split into screens. The order matters:
- First, separate the art into clean per-color channels, one per screen.
- Build any underbase from those separations.
- Then apply chokes and spreads at the shared edges between channels.
- Finally, output film with registration marks so the press can align the trapped colors.
Get the separation right and trapping is a quick pass over the boundaries. Get it wrong — colors bleeding into each other, no clean channels — and there is nothing solid to trap against. The two steps are joined at the hip, which is why our color separation guide treats registration and overlap as part of the same job rather than an add-on.
What to Read Next
Trapping is one stage of preparing a clean multi-color separation. To see where it fits, read our color separation for screen printing guide, then confirm your file is genuinely at print resolution with DPI for fabric printing. When you are ready to build and trap separations for the press, do it in the Texloom Studio screen-print workspace. For a formal definition of trapping across the wider print industry, PrintWiki's entry on trapping is a reliable outside reference.
