Signature Background Remover

A signature photographed on paper usually comes with a grey, shadowed background that looks wrong when you paste it into a contract, invoice or email. This tool measures how bright the paper is around every point, so shadows and uneven light are evened out, then separates ink from paper with an automatic threshold. The paper becomes transparent, the strokes keep smooth edges, the ink can be made pure black or pen blue, and the result is cropped to the signature. It works for stamps and seals too, and the image never leaves your device.

How to use this tool

1. Sign with a dark pen on plain white paper and take a well-lit photo or scan.
2. Choose the image; keep “Even out shadows” on for phone photos.
3. Leave the threshold on automatic, or set it by hand if faint strokes disappear or smudges remain.
4. Pick the ink colour, optional speck removal and the crop margin.
5. Download the transparent PNG and place it in your document.

Formula

ink opacity = clamp((T + s − n) ÷ 2s, 0, 1), where n = 255 × pixel brightness ÷ local paper brightness

The local paper brightness is the brightest value in each neighbourhood (wider than a pen stroke), smoothed; dividing by it cancels shadows and gradients. The automatic threshold T is Otsu's: the value that best splits the brightness histogram into two groups. s is the softness band (up to 32 brightness levels each side at 100% softness), which keeps anti-aliased stroke edges partly transparent instead of jagged.

Example

On a phone photo where the paper is 230 in the light and 160 in a shadow, a pen stroke of brightness 70 in the shadow gives n = 255 × 70 ÷ 160 ≈ 112, while the shadowed paper gives n ≈ 255. With an automatic threshold of around 60% (n ≈ 155), the stroke is solid ink and the paper — lit or shadowed — becomes fully transparent.