excanvas - aspect ratio

HTML

<!DOCTYPE html>
    <html>
      <head>
        <meta charset="utf-8" />
        <title>aspectratio test</title>
<!--[if IE]><script type="text/javascript" src="http://explorercanvas.googlecode.com/svn/trunk/excanvas.js"></script><![endif]-->
<!--[if lt IE 9]><script src="http://html5shim.googlecode.com/svn/trunk/html5.js"></script><![endif]-->
</head>
<body style="margin:0px;">
    <canvas id="myCanvas" style="position:absolute;left: 0px; top: 0px; z-index: 1;"></canvas>
</body>
</html>

JavaScript

function initCanvas(id){
        var canvas = document.getElementById(id);
        if (typeof window.G_vmlCanvasManager!="undefined") { 
            var canvas=window.G_vmlCanvasManager.initElement(canvas);
            var ctx = canvas.getContext('2d');
        }else{
            var ctx = canvas.getContext('2d');
        };
        return ctx;
    };

   function getWindowSize(typ) {
    var myWidth = 0, myHeight = 0;
    if( typeof( window.innerWidth ) == 'number' ) {
      //Non-IE
      myWidth = window.innerWidth;
      myHeight = window.innerHeight;
    } else if( document.documentElement && ( document.documentElement.clientWidth || document.documentElement.clientHeight ) ) {
      //IE 6+ in 'standards compliant mode'
      myWidth = document.documentElement.clientWidth;
      myHeight = document.documentElement.clientHeight;
    } else if( document.body && ( document.body.clientWidth || document.body.clientHeight ) ) {
      //IE 4 compatible
      myWidth = document.body.clientWidth;
      myHeight = document.body.clientHeight;
    }
    if(typ=="width"){
        return myWidth;
    }else{
        return myHeight;
    };
};
    

    function canvasFullScreen(id,imgSrc) {
        imageCanvas = document.getElementById(id);
        var image2d = initCanvas(id);
        function canvasFullScreenResize() {
            imageCanvas.width = 0;
            imageCanvas.height = 0;
            var windowWidth = getWindowSize("width"); 
            var windowHeight = getWindowSize("height"); 
            var yScale = windowHeight / img.height;
            var xScale = windowWidth / img.width;
            var newImgHeight = img.height * xScale;
            var newImgWidth = windowWidth;
            if (windowHeight >= newImgHeight) {
                newImgHeight = windowHeight;
                newImgWidth = img.width * yScale;
            };
            imageCanvas.width = windowWidth;
            imageCanvas.height = windowHeight;
            var diffX...