Image Threshold Tool
Turn any image into two colours. Every pixel darker than the threshold becomes the dark colour and every lighter pixel the light colour – ideal for signatures, stamps, sketches, logos and scanned documents. The automatic setting uses Otsu’s method to find the best threshold, dithering simulates grey tones with dots, and the light colour can become transparent so a photographed signature turns into a clean overlay.
- Files stay on your device
- No sign-up
- Free to use
Threshold
Pixels darker than the threshold become the dark colour, the rest the light colour. With a transparent background, a photographed signature or line drawing becomes a clean overlay – save it as PNG or WebP.
Output
How to use Image Threshold Tool
- Drop one or more images onto the page.
- Click Automatic or move the threshold slider.
- Choose colours, dithering and transparency.
- Download the result as PNG or WebP.
Image Threshold Tool features
Otsu’s method
Automatic, data-driven threshold.
Dithering
Error diffusion for grey tones.
Custom colours
Any dark and light colour.
Transparency
Make the background see-through.
Batch
Apply the same settings to up to 50 images.
Private
Processed in your browser; nothing is uploaded.
When to use Image Threshold Tool
- Extracting a signature from a photo of paper.
- Cleaning scans of documents and forms.
- Preparing line art and stamps for printing or cutting.
- Creating high-contrast graphic artwork.
Image Threshold Tool FAQ
How do I make a signature with a transparent background?
Photograph the signature on white paper in good light, drop it here, click Automatic, tick “Make the light colour transparent” and download as PNG. Place it in documents with the Sign PDF tool.
What is Otsu’s method?
An algorithm that looks at the histogram of brightness values and chooses the threshold that best separates them into two groups, by maximising the variance between the groups.
What does dithering do?
Instead of hard black and white areas, it distributes black and white dots so that from a distance they look like grey tones. It suits photos; for text and line art, leave it off.
Why are some lines broken?
Lines lighter than the threshold disappear. Move the slider up to keep faint lines, or improve the lighting of the photo.
Which format should I download?
PNG keeps sharp edges and transparency. JPG adds compression artefacts around edges and cannot be transparent.
Are my images uploaded?
No. The images are processed in your browser and never leave your device.
Thresholding in practice
Thresholding, or binarisation, is the simplest form of image segmentation: each pixel is classified as foreground or background by its brightness alone. It is the first step in many document workflows, from OCR to signature extraction, and a classic graphic effect.
Choosing the threshold by eye works, but the best value depends on the lighting. Otsu’s method computes it from the image itself. It considers every possible threshold, splits the pixels into dark and light groups, and picks the value where the two groups are most clearly separated. For a page of text or a signature on paper, it lands reliably between the ink and the paper.
Photos contain gradual tones that a hard threshold destroys. Floyd–Steinberg error diffusion keeps track of the rounding error of each pixel and passes it on to its neighbours, so areas of mid-grey become a mix of black and white dots in the right proportion – the look of newspaper photos and early computer graphics.
Transparency makes the result directly usable. A signature on a transparent background can be placed over any document, a stamp over a letterhead, and line art over coloured backgrounds, without white boxes around them.
Everything runs locally; scanned documents and signatures never leave your device.