function transform(img, limit) {
    if (img.width == 0 || img.height == 0)
        return null;
    var cnv = document.createElement('canvas');
    cnv.width = img.width;
    cnv.height = img.height;
    var ctx = cnv.getContext('2d');
    ctx.drawImage(img, 0, 0);
    var imgData = ctx.getImageData(0, 0, cnv.width, cnv.height);
    var data = imgData.data;
    var i = 0;
    var length = data.length;
    var span = cnv.width * 4;
    var ret = '';
    var red, green, blue, alpha, gray;
    for (;;) {
        red = data[i];
        green = data[i + 1];
        blue = data[i + 2];
        // alpha = data[i + 3];
        gray = Math.round(0.3 * red + 0.59 * green + 0.11 * blue);
        ret += 1 * (gray < limit);
        i += 4;
        if (i == length)
            break;
        if (i % span == 0)
            ret += '\n';
    }
    return ret;
}

var img = document.getElementsByTagName('img')[0];
var data = transform(img, 230);