Turn a photo or scan into pure black and white with one cut-off value. Measure by brightness or a single colour channel, invert if needed, and save a crisp PNG.
Measures each pixel, compares it with the threshold you set, and makes it white if it is at or above and black if it is below. There are no greys in the result at all. By default the measurement is perceived brightness, the same weighting a black and white conversion uses, but you can measure a single channel instead, which behaves like shooting through a coloured filter. Transparent pixels in a PNG stay transparent.
A 2480 × 3508 phone photo of a printed page, slightly grey with a shadow across one corner. At the default 128 the shadowed corner turns solid black. At 165 the whole page is white, the text is black and the shadow is gone; saved as PNG the file is 96 KB instead of the 2.4 MB photo, and it prints cleanly. A hand-drawn pencil sketch scanned at 300 dpi needs the other direction: threshold 110 keeps the light strokes, and Invert produces white lines on black for a chalkboard look.
Threshold is the right tool for documents, signatures, line art, stencils, laser-cutter and vinyl-cutter input, QR codes recovered from photos, and anything that will be printed in one ink. It is the wrong tool for photographs you want to look like photographs, because every mid-tone is lost; Grayscale Image keeps them. For a poster-like result with a few tones use Posterize Effect, and if a scan is merely low in contrast rather than needing a hard cut, try Change Contrast first.
Threshold from 0 to 255; the chips jump to common values. Brightness is right for almost everything; Average is a plain mean of the channels; Red, Green and Blue isolate one channel to drop out coloured ink or highlighter. Invert swaps black and white. Save as PNG: it is lossless and tiny for two-colour images, whereas JPEG adds grey noise around every edge.
What value should I use for a scanned document? Start at 160 for a scan with a light grey background: the paper goes pure white and the ink stays black. If thin strokes break up, lower it towards 128; if the background is still speckled, raise it towards 190.
What do the Red, Green and Blue options do? They measure one channel instead of overall brightness, which acts like a colour filter. Blue makes yellow highlighter disappear from a scanned page; Red makes red stamps or corrections vanish while black text stays.
Can I get a smooth grey result instead of pure black and white? Not from this tool; that is its purpose. Use Grayscale Image for greys, or Posterize Effect for a few grey steps.
Is the image processed on a server? No. The brightness comparison runs in your browser and nothing is uploaded, so it is fine for private documents.
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