alphaluminosity.js

function alphaImageLuminosity(el) {
    if (!HTMLCanvasElement || !el.complete) return -1;

    var canvas = document.createElement('canvas');
    canvas.setAttribute('width', el.width);
    canvas.setAttribute('height', el.height);
    var c = canvas.getContext('2d');
    c.drawImage(el, 0, 0);

    var r = 0, g = 0, b = 0, total = 0;
    var pixels = c.getImageData(0, 0, el.width, el.height).data;

    // Look at every pixel's alpha value and only proceed if it is semi-transparent.
    for (var i = 3; i < pixels.length; i += 4) {
        if (pixels[i] < 255 && pixels[i] > 0) {
            total++;
            r += pixels[i - 3];
            g += pixels[i - 2];
            b += pixels[i - 1];
        }
    }

    if (!total) return 0;

    // Calculate average luminosity of all alpha pixels.
    // Using the median of all per-pixel luminosities would be better but is
    // computationally much more expensive and since this runs in the UI thread,
    // is not an option at this point.
    return Math.pow(r / total / 255, 2.2) * 0.2126 +
           Math.pow(g / total / 255, 2.2) * 0.7152 +
           Math.pow(b / total / 255, 2.2) * 0.0722;
}

im.onload = function() {
    console.warn(alphaImageLuminosity(im));
};