Lasso tool in html5 canvas

I'm trying to build a freeform lasso tool to clip an image inside canvas. I'm using fabric.js to draw the shape. This is my attempt, many thanks to who can take some time for suggestions :)

<script src="http://ift.tt/1tznvBx" ></script>
<script src="http://ift.tt/k72nu2"></script>

<canvas id="c" width="500" height="500"></canvas>


<script>
    var canvas = document.getElementById('c');
    var ctx = canvas.getContext('2d');

    var img = document.createElement('IMG');

    img.onload = function () {

        var OwnCanv = new fabric.Canvas('c', {
        isDrawingMode: true
        });

        OwnCanv.freeDrawingBrush.color = "purple"
        OwnCanv.freeDrawingBrush.width = 4


        ctx.clip();

        ctx.drawImage(img, 0, 0);
    }

    img.src = "http://ift.tt/1EY2Kze";
</script>