Canvas animation with requestAnimationFrame and mouse tracking for games

gistfile1.js

var Animator = (function() {
    var container,
        canvas, ctx,
        w, h,
        offsetLeft, offsetTop,
        lastX, lastY;
    
    var init = function(el, options) {
        container = el;
        
        w = options.w || 600;
        h = options.h || 400;
        
        canvas = $("<canvas width='"+w+"' height='"+h+"'>");
        ctx = canvas[0].getContext('2d');
        
        canvas.mousemove(onMouseMove);
        
        $(container).append(canvas);
        
        animate();
    }
    
    var getCursorPosition = function(e) {
        if(offsetLeft == undefined) {
            offsetLeft = 0;
            for(var node=$(canvas)[0]; node; node = node.offsetParent) {
                offsetLeft += node.offsetLeft;
            }
        }
        if(offsetTop == undefined) {
            offsetTop = 0;
            for(var node=$(canvas)[0]; node; node = node.offsetParent) {
                offsetTop += node.offsetTop;
            }
        }
        
        var x = e.pageX - offsetLeft;
        var y = e.pageY - offsetTop;
    
        return { x: x, y: y };
    }
    
    // requestAnimFrame shim layer by Paul Irish
    var requestAnimFrame = (function(){
        return  window.requestAnimationFrame       || 
                window.webkitRequestAnimationFrame || 
                window.mozRequestAnimationFrame    || 
                window.oRequestAnimationFrame      || 
                window.msRequestAnimationFrame     || 
                function(callback, element){
                    window.setTimeout(callback, 1000 / 60);
                };
    })();
    
    var animate = function(time) {
        requestAnimFrame(animate);
        
        canvas[0].width = canvas[0].width;
        
        // do some drawing here
        ctx.beginPath();
        ctx.arc(lastX-4, lastY-4, 8, 0, Math.PI * 2, true);
        ctx.closePath();
        ctx.fill();
    };
    
    var onMouseMove = function(e) {
        var pos = getCursorPosition(e);
        
        lastX = pos.x;
        lastY = pos.y;
    }
    
    return {
        init: init
    }
})();

var options = {
    width: 600,
    height: 400
}

$(function() {
    Animator.init(document.body, options)
});