White halo around a logo: the edge is a measurement, not a look
The pale line that shows up around a cut-out logo on a dark garment is left over from the pixels that used to sit between the artwork and its old background. Those pixels are still partly the old colour, and on cotton they read as a rim. Whether that rim matters is a question about width in millimetres, and width in millimetres is something you can work out before the film goes through.
| A 2 px edge band printed at 300 DPI | 0.17 mm — below what the eye separates at arm's length |
|---|---|
| The same 2 px band at 200 DPI | 0.25 mm |
| The same 2 px band at 150 DPI | 0.34 mm — now a visible rim on black |
| Where the rim is worst | dark substrate, light artwork, hard-edged shapes |
| Where the rim disappears | artwork printed on a substrate close to the halo's own colour |
Why the rim widens when you print larger
The band is a fixed number of pixels; the millimetres it occupies depend entirely on how far you stretch those pixels. Print the same cut-out at half the resolution and the rim doubles in width without a single pixel changing. This is the same arithmetic that governs resolution, applied to an edge instead of to detail.
Two rims that look alike and are not
One comes from partly transparent pixels carrying the old background colour. The other comes from the alpha channel itself being cut too generously, leaving a band of near-transparent pixels that the press still lays ink into. The first is a colour problem and survives being darkened; the second is a shape problem and only goes away by trimming the alpha.
- Colour rim: the pixels are opaque but the wrong colour
- Alpha rim: the pixels are the right colour and should not be there at all
- The checker reports which one it measured rather than naming both
When trimming stops being safe
Every millimetre trimmed off the edge is a millimetre removed from the artwork. On a logo with a thin outline, a trim wide enough to clear the rim also eats the outline, and the result is a shape the customer did not approve. At that point the honest answer is a file with a real transparent background rather than a rescued one.
FAQ
- Can the halo be removed by making the background transparent again?
- Not by itself. Transparency decides which pixels print; it does not change the colour of the ones that do. A rim made of off-colour opaque pixels is still there after the background is dropped.
- Does printing smaller fix it?
- Often, because the band is measured in pixels and shrinks in millimetres as you reduce the print width. The trade is that everything else shrinks with it.
- Why does the same file look clean on the screen?
- A backlit display at roughly 100 pixels per inch renders a 2 px band as a fraction of a millimetre and lights it from behind. Ink on cotton spreads slightly and is seen in reflected light, which is a harsher test of the same edge.