// http://ati.su/img/3515947f83d9212729cb36f8f1b7c5e9ec883fcd.jpeg
// http://ati.su/img/49ce64b5d6a2e3f1a7a34e876cf5946b376748a5.jpeg
var st = new Date().getTime();
var img = document.getElementsByTagName('img')[0];
// для начала представим изображение в виде массива оттенков серого
var data = rgb2l(img);
var hist = histogram(data);
var threshold = otsu(hist, data.length);

var canvas = document.createElement('canvas');
canvas.width = img.width;
canvas.height = img.height;
var context = canvas.getContext('2d');
var imageData = context.createImageData(img.width, img.height);
var j = 0;
var value;
for (var i = 0; i < imageData.data.length; i += 4) {
    value = 255 * (data[j++] > threshold);
    imageData.data[i] = value;
    imageData.data[i + 1] = value;
    imageData.data[i + 2] = value;
    imageData.data[i + 3] = 255;
}
context.putImageData(imageData, 0, 0);
open(canvas.toDataURL());

var et = new Date().getTime();
console.log(et - st + ' ms');

function rgb2l(img) {
    var canvas = document.createElement('canvas');
    canvas.width = img.width;
    canvas.height = img.height;
    var context = canvas.getContext('2d');
    context.drawImage(img, 0, 0, img.width, img.height);
    var imageData = context.getImageData(0, 0, img.width, img.height);
    var data = imageData.data;
    var ret = new Uint8ClampedArray(img.width * img.height);
    var r, g, b;
    var j= 0;
    for (var i = 0; i < data.length; i += 4) {
        r = data[i];
        g = data[i + 1];
        b = data[i + 2];
        ret[j] = Math.round(0.3 * r + 0.59 * g + 0.11 * b);
        ++j;
    }
    return ret;
}

function histogram(data) {
    var ret = new Uint32Array(256);
    for (var i = 0; i < data.length; ++i)
        ++ret[data[i]];
    return ret;
}

// http://www.labbookpages.co.uk/software/imgProc/otsuThreshold.html
function otsu(histogram, total) {
    var sum = 0;
    for (var i = 1; i < 256; ++i)
        sum += i * histogram[i];
    var sumB = 0;
    var wB = 0;
    var wF = 0;
    var mB;
    var mF;
    var max = 0;
    var between;
    var threshold = 0;
    for (var i = 0; i < 256; ++i) {
        wB += histogram[i];
        if (wB == 0)
            continue;
        wF = total - wB;
        if (wF == 0)
            break;
        sumB += i * histogram[i];
        mB = sumB / wB;
        mF = (sum - sumB) / wF;
        between = wB * wF * Math.pow(mB - mF, 2);
        if (between > max) {
            max = between;
            threshold = i;
        }
    }
    return threshold;
}