Image Threshold Tool – Convert to Pure Black & White

Image thresholding is a technique that converts a grayscale or color image into a binary (pure black and white) image. Each pixel is compared against a threshold value—pixels brighter than the threshold become white, and pixels darker become black. It's widely used in document scanning, edge detection, and computer vision.

Otsu's method automatically determines the optimal threshold by analyzing the image's histogram. It finds the threshold that minimizes the variance within each class (black and white pixels) or equivalently maximizes the variance between the two classes. It's excellent for images with bimodal histograms, like scanned documents.

For scanned text documents, a threshold between 180 and 210 usually works well. This range helps separate dark text from slightly off-white paper backgrounds. Otsu's auto mode often produces excellent results for document scans automatically.

Yes! Color photos are first converted to grayscale using a weighted luminance formula (30% red, 59% green, 11% blue), then the threshold is applied. The result is a stark, high-contrast black and white image—great for artistic effects, stencils, or graphic design projects.

A grayscale image contains up to 256 shades of gray (0=black to 255=white), while a binary image has only two colors: pure black and pure white. Thresholding converts grayscale to binary by splitting at a specific cutoff value. Binary images are much smaller in file size when saved as PNG.

The ideal threshold depends on your image. Use the histogram to see the distribution of brightness values. For images with two clear peaks (bimodal), place the threshold in the valley between them. For general photos, try values between 100-150. The Otsu auto mode can also determine the best value automatically.

All processing happens entirely in your browser using the HTML5 Canvas API. Your images never leave your device—no data is uploaded to any server. This ensures complete privacy and fast processing speeds.

The tool outputs PNG format, which is ideal for binary images since PNG's lossless compression excels with limited color palettes. A pure black and white PNG is extremely compact. You can also copy the result to your clipboard and paste it into any application that accepts images.