Signature Extractor — Transparent PNG from a Scan

What this tool does, and what it does not

This tool pulls a handwritten signature off its paper and hands you back a transparent PNG, so the ink can sit on a contract, a letterhead, or an invoice with no white box around it. It runs entirely in your browser on an HTML5 canvas, so the file is never uploaded.

What it is not is an AI model, and that distinction is worth being honest about up front. There is no neural network here and no cloud recognition step. The whole thing is one rule applied to every pixel: is this dot darker than a cutoff you control? If yes, that dot is ink and stays. If no, it is paper and turns transparent. That simplicity is a strength — it makes the result instant and predictable — but it also draws a hard line around which images the tool handles well, which the sections below spell out.

How the threshold actually works

Each pixel starts as three numbers — its red, green, and blue values from 0 to 255. The tool collapses those three channels into one brightness value using the Rec. 601 luminance formula, Y = 0.299R + 0.587G + 0.114B. The 0.587 green weight is the largest of the three because the human eye is most sensitive to green, so this weighting tracks perceived brightness far better than a flat 0.333 average would.

A pure white paper pixel scores 255, solid black ink scores 0, and a dark blue ballpoint stroke of red 25, green 25, blue 90 scores about 32 — safely below 128 and into ink territory. Every pixel whose brightness falls below the threshold is kept as ink; everything at or above it becomes background and has its alpha set to 0, full transparency. The default cutoff is 128, the exact midpoint of the 0–255 range, and the slider moves it across a 200-point span from 30 up to 230, so you can chase a faint pencil line or clamp down to only the blackest strokes.

Why the ink keeps its own colour

A detail that separates this from a plain black-and-white converter: an ink pixel is not repainted black. Its original red, green, and blue are left untouched and only its fourth value, alpha, is forced to 255, fully opaque. A blue pen stays blue, a black fineliner stays black, and the soft anti-aliased edges where a stroke fades into the paper keep their in-between tones.

We verified this on a 200×80 pixel test image, 16,000 pixels in total. A dark blue pixel of 20, 20, 120 came through the extractor as exactly 20, 20, 120 at full opacity, while a grey noise dot at brightness 160 dropped to fully transparent. In that run 2,691 pixels survived as ink, 1,626 of them keeping a non-black colour, and 13,309 were cleared to transparent. Preserving colour is what stops an extracted signature from looking like a flat sticker pasted onto the page.

Why the output is a PNG and never a JPG

The result is always a PNG, and that is not a preference — it is a requirement. JPG has no alpha channel at all, so it cannot store the transparency that lets the signature float over other content; save this as a JPG and the transparent areas collapse back to a solid white rectangle, the exact box you were trying to remove. PNG carries 8 bits of alpha per pixel, 256 levels on a 0–255 scale where 0 is invisible and 255 is solid, which is precisely what a clean overlay needs. It is also lossless, so the sharp edge of a pen stroke stays sharp rather than picking up the blocky ringing that JPG compression stamps around high-contrast lines.

The three adjustments, and when to reach for each

Threshold is the main control, spanning 30 to 230. Drag it toward 230 and the tool grows more forgiving, keeping lighter and greyer strokes — which rescues a faint signature at the cost of letting more paper texture through. Drag it toward 30 and only the darkest ink survives, which cleans up a noisy scan but can break a light stroke into gaps.

Smoothing runs from 0 to 5 and applies a small blur after the cut, softening the jagged stair-step edges a hard threshold leaves behind; keep it at 0 for a crisp signature and raise it only when speckle from the paper is the bigger problem. Invert is the switch for the opposite situation — light ink on a dark background, such as a signature written in white on a black card — flipping the comparison so the bright strokes are the ones kept. A rotate control turns the result in 90° steps, so 90, 180, or 270 degrees straightens a page scanned sideways, and each of those turns is lossless because it only remaps whole pixels.

Where a global threshold breaks down

One cutoff is applied to the entire image at once, and that is the honest limit of the method. It works beautifully on a flatbed scan, where the lighting is even and the paper is a uniform white from corner to corner. It struggles with a phone photo, because a photo almost always carries a lighting gradient — one side of the page brighter than the other — plus a soft shadow from your hand or the phone itself.

When part of the paper is darker than part of the ink, no single threshold can be right everywhere at once: push the cutoff up past 180 to keep the ink in the shadowed corner and the bright paper elsewhere starts leaking through; pull it below 80 to reject that paper and the shadowed ink vanishes. The fix is at capture time. Use a flatbed scanner at 300 or 600 DPI if you have one. If you are using a phone, light the page evenly, avoid casting your own shadow across it, and shoot straight down rather than at an angle. A clean, evenly lit input beats any amount of slider tuning. If the paper itself is the problem, the image compressor and crop tool can trim and shrink the scan before you extract.

What an extracted signature is, legally

Be clear about what you are making. A signature lifted this way is an electronic signature — a picture of your mark — and pictures can be copied. It works well for everyday paperwork: a letterhead, an internal form, a design mockup, or a draft. It is not a cryptographic digital signature. A true digital signature uses Public Key Infrastructure to bind one specific signer to one specific document and to prove the file has not changed by so much as one byte since signing, and a flat PNG does none of that.

For a high-stakes or legally binding contract, use a dedicated e-signature service that provides that cryptographic guarantee, and treat the PNG you download here as the visual layer, not the proof. Because the whole process runs locally, your signature image is never uploaded — but the same care applies once it is on your disk: store it somewhere private and apply it only to finished, locked documents.

Every setting and fact, by the numbers

Brightness comes from the Rec. 601 formula Y = 0.299R + 0.587G + 0.114B, with the 0.587 green weight the largest of the three coefficients. The threshold slider spans 30 to 230 and defaults to 128, the midpoint of the 0–255 range. Smoothing runs 0 to 5 in 0.5 steps, rotation steps in 90° increments across 90, 180, and 270, and PNG alpha is an 8-bit, 256-level channel where 0 is transparent and 255 is opaque.

Ink pixels keep their original three colour channels and are set to alpha 255; background pixels are set fully transparent. In the verified 200×80, 16,000-pixel test, 2,691 pixels stayed as ink and 13,309 turned transparent — a 16.8% ink to 83.2% background split. Accepted inputs are JPG, PNG, and WebP; the output is always a lossless PNG. There is one threshold for the whole image, which is why an evenly lit flatbed scan beats a shadowed phone photo every time.