Curved, glossy, outdoors: where printed QR codes meet the real world

Wrapping a code round a bottle

A camera sees a flat projection of a curved surface. Where the bottle turns away, the modules are foreshortened: a module on a part of the surface tilted at angle θ appears cos θ as wide as it really is. At the centre of the code nothing is lost; at the edges, a lot can be.

The angle at the edge is half the code’s width divided by the bottle’s radius, in radians. A 30 mm code on a 66 mm can (radius 33 mm) reaches about 26° at its edges, where modules appear 90% as wide. The same code on a 33 mm vial reaches about 52°, and the edge modules appear 61% as wide — narrower than the decoder expects from the centre of the same symbol.

Edge angle = (width ÷ 2) ÷ radius, in radians. Appearance = cos(edge angle). Geometry, not measurement.

Reduce the wrap, not the code

The fix that follows from the geometry is to make the code narrower relative to the curve — place it on the flattest part of the pack, run it along the bottle’s height rather than round it, or use a payload short enough that a smaller code still has generous modules. Making the whole code smaller to keep it on a flat strip is fine if the module width stays sensible; the print size calculator shows it.

Gloss hides modules at the angle people hold phones

A glossy laminate reflects light the way a mirror does. When a light source, the code and the camera line up, part of the code turns white in the camera’s image, and the modules under the reflection are gone. People tend to hold a phone square to a code, under ceiling lights — which is exactly the arrangement that produces the reflection.

A matte or satin finish scatters the light instead, so there is no single angle where the code disappears. If the surface has to be glossy, a matte patch under the code solves it.

A reflection is not damage the code can correct — it is a hole the camera cannot see past.

Sunlight takes the contrast out

Pigments fade under ultraviolet light, and they do not fade evenly: coloured inks usually go first. A code printed in a brand colour that passed a contrast check on the day it was printed can fail the same check after months in a window. Black carbon-based inks hold up best, and a UV-resistant laminate slows the rest.

For anything outdoors, check contrast against the faded state you expect rather than the fresh one, and replace codes on a schedule rather than when someone notices.