Clean Up a Photographed Receipt

How it works

Your photograph is resampled with lanczos filtering to about four megapixels — more than the document page uses, because receipt print is considerably smaller and needs the pixels.

Then a morphological closing with a fifteen pixel kernel estimates what the paper looks like with no printing on it, including whatever shading the curl and your own shadow put across it. That estimate is subtracted, leaving only the marks that were too small to survive it. The result is inverted back to dark on white, contrast is raised, and a light sharpening pass restores the edges.

No model, no sampler, no prompt. The same photograph always produces the same output.

Thermal paper is running out

This is the part worth acting on, and it has nothing to do with image processing.

A till receipt has no ink. The paper carries a colorless dye and a developer in a coating, and the print head applies heat where the characters should be, which melts the two together and turns them black. That is the entire mechanism, and it explains everything about how receipts behave.

The reaction is not finished when the receipt is printed. Anything that supplies heat continues it: a warm car, a sunny windowsill, a radiator, a pocket. Friction does it too, which is why a receipt rubbed against a card in a wallet goes gray in patches. Some plasticisers in wallets and folders dissolve the coating outright.

The end state is a uniformly gray slip with nothing on it. There is no restoration for that anywhere, from anyone, because the dye has already reacted across the whole sheet — the contrast between character and background is gone rather than hidden.

Which makes the practical advice unusually clear. Photograph receipts on the day you get them. A good capture of a fresh receipt is a permanent record. A perfect tool applied to a two year old one in a glovebox is not.

Why the settings differ from a document page

The mechanism here is the same background subtraction used for photographed documents, with two adjustments that follow from the medium.

The kernel is smaller. That number sets the boundary between what counts as ink and what counts as background — anything wider is treated as paper and subtracted. Receipt characters are small and often condensed, so a kernel sized for A4 body text would classify some of them as background and delete them.

There is a sharpening pass at the end. Thermal printing produces softer characters than a laser printer does, because heat spreads sideways in the coating and the head has a coarse dot pitch. A document scan usually needs no help at the edges; a receipt does.

The curl is a real advantage here

Receipts are never flat, and for once that works out.

A strip of thin paper rolls, so under any light there is a smooth brightness gradient down its length, plus a darker band where it lifts away from the surface. That is a slowly varying signal across a large area — precisely the kind of thing background subtraction is built to remove, and precisely the kind of thing a brightness threshold cannot handle at all.

What the method cannot do is recover text the curl physically hid from the camera. If the paper folded over on itself, those characters were never photographed and no processing brings them back. Flatten the receipt under glass before shooting it, then let this deal with whatever shading remains.

Publication gate

This page ships once the workflow has been run against a fresh curled receipt, a partly faded one, a receipt with a rubbed gray patch, and one photographed under mixed lighting, with every total and date compared against the paper original.