Javascript - Most repeats pixels on background-image

Question

You don't have access to the pixel data of a background image via JavaScript. What you will have to do is to create a new Image object and set the source to the background image URL. Afterwards, you will have to do these steps:

Create an in-memory canvas object
Draw the image on the canvas
Get the image data, iterate through all pixels and store the colors in an Object (key = color, value = amount of repitition)
Sort the array by the amount of repitition, then select the first value

Here, I created an example. This loads the JSconf logo and sets the body's background color to the most repetitive color.

// Create the image
var image = new Image();
image.crossOrigin = "Anonymous";

image.onload = function () {
    var w = image.width, h = image.height;

    // Initialize the in-memory canvas
    var canvas = document.createElement("canvas");
    canvas.width = w;
    canvas.height = h;

    // Get the drawing context
    var context = canvas.getContext("2d");

    // Draw the image to (0,0)
    context.drawImage(image, 0, 0);

    // Get the context's image data
    var imageData = context.getImageData(0, 0, w, h).data;

    // Iterate over the pixels
    var colors = [];
    for(var x = 0; x < w; x++) {
        for(var y = 0; y < h; y++) {
            // Every pixel has 4 color values: r, g, b, a
            var index = ((y * w) + x) * 4;

            // Extract the colors
            var r = imageData[index];
            var g = imageData[index + 1];
            var b = imageData[index + 2];

            // Turn rgb into hex so we can use it as a key
            var hex = b | (g << 8) | (r << 16);

            if(!colors[hex]) {
                colors[hex] = 1;
            } else {
                colors[hex] ++;   
            }
        }
    }

    // Transform into a two-dimensional array so we can better sort it
    var _colors = [];
    for(var color in colors) {
        _colors.push([color, colors[color]]);   
    }

    // Sort the array
    _colors.sort(function (a, b) {
        return b[1] - a[1]; 
    });

    var dominantColorHex = parseInt(_colors[0][0]).toString(16);
    document.getElementsByTagName("body")[0].style.backgroundColor = "#" + dominantColorHex;
};

image.src = "http://upload.wikimedia.org/wikipedia/commons/thumb/6/6a/JavaScript-logo.png/600px-JavaScript-logo.png";