Stencil Maker

A stencil is a picture reduced to one colour and empty space, clean enough to cut from card or vinyl, spray through, or use as a silhouette on a T-shirt, cake or wall. This maker splits your image into dark and light parts with an automatic threshold, removes specks and tiny holes, and gives you either a filled silhouette or an outline. Download a transparent PNG, or an SVG whose outlines cutting machines and vector editors can use directly. It counts the enclosed islands that would fall out when you cut.

How to use this tool

1. Choose a photo or logo with clear contrast between subject and background.
2. Keep the automatic threshold, or switch to manual and move it until the shape looks right.
3. Choose whether the dark or the light parts form the stencil, and set the smoothing and speck size.
4. Pick a filled silhouette or an outline, the colour and the background.
5. Download the PNG, or the SVG for Cricut, Silhouette, laser cutters and editors.

Formula

stencil = brightness ≤ t (dark parts) · brightness = 0.299R + 0.587G + 0.114B

With the automatic setting t is Otsu's threshold: the split of the 256-level brightness histogram that maximises w₀w₁(μ₀ − μ₁)², the variance between the two groups. Smoothing is a box blur applied twice. Pieces and holes are 4-connected; pieces smaller than the limit are removed and enclosed holes smaller than it are filled. The SVG traces pixel edges with marching squares and straightens them with the Douglas–Peucker method, allowing the chosen deviation.

Example

A logo with a navy shape (brightness about 40) on a pale grey background (about 220) gets an Otsu threshold between the two brightness levels, so the navy shape becomes the stencil and the background drops out. If the logo contains the letter "A", its triangle hole is counted as 1 loose island: cut from a sheet, that piece would fall out unless you add a bridge.