Extract a Signature onto Transparency

How it works

Your image is scaled so its total area is around two megapixels. A morphological closing with a thirty one pixel kernel estimates the paper with no writing on it, and subtracting that estimate leaves a map of the strokes — bright where there is ink, black everywhere else, with the paper's tone and any shadow across it already canceled out.

Contrast is raised on that map, and then it is used twice. One copy is inverted to give dark strokes on white. The other is read as a mask and attached to the first as its alpha channel.

The result is a PNG where the strokes are opaque and the paper is not there at all. No model, no sampler, no prompt: the same photograph always gives the same file.

Removing paper rather than whitening it

The distinction between this and a cleanup page is the whole point.

Whitening the background gives you a signature that looks correct on a white page and wrong everywhere else. Drop it onto a form with a printed rule and the rule disappears under a white block. Put it on a colored letterhead and it arrives inside a pale box. The image is still a rectangle; it is only that the rectangle happens to match one particular background.

Transparency removes the rectangle. What gets drawn is the strokes and nothing else, so the line underneath continues through the gaps in your handwriting the way it does when you actually sign a page.

Reusing the ink map as the alpha channel is what makes that possible, and it is a convenient property of the subtraction: the same measurement that says how dark the ink is at a pixel is exactly the measurement of how opaque that pixel should be.

Soft edges are a feature

A signature does not have hard boundaries. A pen deposits more ink in the middle of a stroke than at its edge, and the paper wicks it outward, so every line fades over a small distance.

Approaches that cut a signature out with a threshold destroy that. Each pixel is either fully ink or fully gone, and the strokes come back with jagged, aliased edges that read as a cutout however clean the shape is.

Here the alpha follows the ink continuously. A pixel with half the ink density becomes half opaque, so the stroke edges fade against whatever they are placed on. This is why the result composites convincingly over a color rather than only over white.

What comes along with the ink

The method keeps everything narrower than its kernel, and it has no idea which of those things you wanted.

Ruled lines on notebook paper are the usual problem. A printed rule is roughly the width of a pen stroke, so it is classified as ink and arrives in your transparent file alongside the signature. The same goes for a grid on graph paper, a fold crease that caught the light, a stray dot of ink, and print showing through from the other side of a thin sheet.

None of this can be filtered here, because nothing distinguishes a rule from a stroke by size, and size is the only criterion in play.

The fix is at capture time rather than in processing. Plain, unlined, reasonably thick paper and a pen that lays down a definite line. That takes twenty seconds and produces a better result than any amount of cleaning afterwards.

Check it over black

One habit worth adopting, because the failures of this page are specifically invisible on white.

Leftover paper tone comes out as a faint gray haze at low opacity. A halo around the strokes shows up as a soft light ring. Both of these sit on a white page looking like nothing at all, and then appear plainly the moment the signature is placed on a colored or dark background — usually in the document you have already sent.

Open the download, put a solid black layer behind it, and look. Anything you can see there is something that will show up somewhere eventually.

Publication gate

This page ships once the workflow has been run against pen on plain paper, signatures on ruled and graph paper, a photograph with a shadow across the sheet, and a fine ballpoint line, with every output composited over black and over a mid-tone color to check for haze and haloing.