Otsu's method - Automatic Histogram Based Image Thresholding

otsu.js

// Read https://en.wikipedia.org/wiki/Otsu%27s_method (added this since JS examples in wikipedia didn't work)

function otsu(histData /* Array of 256 greyscale values */, total /* Total number of pixels */) {
    let sum = 0;
    for (let t=0 ; t<256 ; t++) sum += t * histData[t];

    let sumB = 0;
    let wB = 0;
    let wF = 0;

    let varMax = 0;
    let threshold = 0;

    for (let t=0 ; t<256 ; t++) {
        wB += histData[t];               // Weight Background
        if (wB == 0) continue;

        wF = total - wB;                 // Weight Foreground
        if (wF == 0) break;

        sumB += t * histData[t];

        let mB = sumB / wB;            // Mean Background
        let mF = (sum - sumB) / wF;    // Mean Foreground

        // Calculate Between Class Variance
        let varBetween = wB * wF * (mB - mF) * (mB - mF);

        // Check if new maximum found
        if (varBetween > varMax) {
            varMax = varBetween;
            threshold = t;
        }
    }

    return threshold;
}