Calculate the average color of a canvas

average-color.js

function avgColor(canvas){ // where canvas is an HTMLCanvasElement
    var tr = 0, tg = 0, tb = 0;
    var idata = canvas.getContext("2d").getImageData(0,0,canvas.width,canvas.height).data;
    for (i=0;i<idata.length;i+=4) { // red
        tr += idata[i];
    }
    for (i=1;i<idata.length;i+=4) { // green
        tg += idata[i];
    }
    for (i=2;i<idata.length;i+=4) { // blue
        tb += idata[i];
    }
    tr = tr/(idata.length/4);
    tg = tg/(idata.length/4);
    tb = tb/(idata.length/4);
    return [tr,tg,tb];
}