Lift a signature off a scan

This page opens on the ink path rather than the guided selection, and the two sliders on step 2 change accordingly. The first asks where paper stops being paper. The second asks how gradual the crossing is — a wide crossing keeps the soft grey shoulder around a ballpoint line, a narrow one gives you a harder, blacker signature that will look crisper at small sizes and blockier at large ones.

The paper level is not assumed. It is read out of your scan: the tool takes the bright end of the picture’s own brightness distribution as the sheet and the dark end as the ink, then measures every pixel between them. A yellowed page, a grey phone photograph and a bright flatbed scan therefore all start in roughly the right place without you telling the page anything about them.

Scan or photograph the page with even light and nothing resting on it — a shadow across one corner is the single most common reason one half of a signature comes out clean and the other half comes out with the paper attached. If the light is uneven anyway, set the threshold for the darker half and take the rest off with the brush on step 3, which works here exactly as it does on a photograph.

On step 4, crop tight to the subject. A signature occupying six percent of an A4 scan produces a PNG mostly made of nothing, and a document that places it will place the whole rectangle, invisible margins included. The tight crop trims to the ink’s own bounding box, and the breathing-room slider puts back as much margin as you actually want.

Why is there a separate way of doing this?

Because a signature is not an object standing in front of a wall — it is a stain on the wall. A subject solve looks for a region with a boundary, finds the whole sheet of paper, and hands back a rectangle. Measuring darkness against the paper instead treats each pixel as ink coverage, which is both closer to the truth and much kinder to the tapering end of a pen stroke.

The portal I am filling in demands a white background.

Then it wants the opposite of this page and you should not send it a transparent PNG. Forms that specify a plain sheet behind the signature are specifying a colour that must be present; a file with no colour there is usually flattened to black by whatever the form uses to process it, which looks like a corrupt upload. Save a copy with a solid ground from somewhere that offers one.

My scan is grey and blotchy rather than white.

Raise the first slider. The paper level is estimated from the bright end of the scan's own histogram, so a dim scan still finds its paper, but shadow near the fold or a grey cast can sit close enough to the ink that some of it survives. Push where paper stops upward until the page goes clean, then check the thin strokes have not gone with it.

Can I do a stamp, or a drawing?

A rubber stamp works well — it is ink on paper by any other name. A pencil drawing works if the pencil is dark enough to separate from the sheet. A printed logo on glossy paper usually does not, because the glossy sheet reflects unevenly and the paper estimate has no single level to find.

• Getting a signature off a scanSix steps, and why a subject model is the wrong instrument for ink.
• Make a picture transparentThe same four steps, with the file-format question answered on the way out.
• Cut the subject outOpen a photo with a clearly separable subject, let colour and contrast propose the edge, correct it by hand, save a transparent PNG.
• Why a transparent PNG can still look whiteTransparency is a channel, and every app you paste into decides what sits behind it.