Contrast + Binary does not handle low light well

[drbsoftware]

The combination of contrast and binary does not really handle low-light images.

A better approach is in https://gist.github.com/tobytailor/421369, which is to find the minimum and the maximum and use the average as a threshold.

I replaced them with a single function:

function contrastBinary(data) {
    var min = 127 * 3;
    var max = 128 * 3;
    for (var i = 0, len = Image.width * Image.height * 4; i < len; i += 4) {
        var val = data[i] + data[i + 1] + data[i + 2];
        if (val < min) {
            min = val;
        } else if (val > max) {
            max = val;
        }
    }
    var threshold = (max + min) / 2;
    for (var i = 0, len = Image.width * Image.height * 4; i < len; i += 4) {
        ave = (data[i] + data[i + 1] + data[i + 2]);
        if (ave < threshold) {
            data[i] = data[i + 1] = data[i + 2] = 0;
        } else {
            data[i] = data[i + 1] = data[i + 2] = 255;
        }
        data[i + 3] = 255;
    }
}

[EddieLa]

Added this an option in version update.