toDataURL and Canvas zoom level in Fabric.js

https://groups.google.com/forum/?fromgroups#!topic/fabricjs/WQ0jegaIFCY

HTML

<script src="https://cdnjs.cloudflare.com/ajax/libs/fabric.js/2.4.1/fabric.min.js"></script>
<button id="btnImgNormal" class="large"><i class="icon-picture"></i> Normal image</button>
<button id="btnImgResSlice" class="large"><i class="icon-picture"></i> Slice by 2 Hack</button>
<button id="btnImgResStep" class="large"><i class="icon-picture"></i> steps Hack</button>
<button id="btnImgResHermite" class="large"><i class="icon-picture"></i> Fast Hermite</button>
<br/>
<canvas id="c" width="600" height="600"></canvas>

CSS

canvas {
    border-width: 1pz;
    border-style: solid;
    border-color: #000;
}

JavaScript

// button Zoom In
$("#btnImgNormal").click(function () {
    imag.src = '';
    imag.onload = function () {
        var theImage = new fabric.Image(imag, {top:5, left:5});
       theImage.scaleX = 1/6;
       theImage.scaleY = 1/6;
       canvas.add(theImage);
       canvas.renderAll();
    }
    imag.src = src;
});
// button Zoom Out
$("#btnImgResSlice").click(function () {
    imag.src = '';
    imag.onload = function () {
       var theImage = new fabric.Image(imag, {top:5, left:225});
       theImage.resizeFilter = new fabric.Image.filters.Resize({resizeType: 'sliceHack'});
       theImage.scaleX = 1/6;
       theImage.scaleY = 1/6;
       canvas.add(theImage);
       theImage.applyResizeFilter(null, function() {canvas.renderAll();});
    }
    imag.src = src;
});

// button Reset Zoom
$("#btnImgResStep").click(function () {
    imag.src = '';
    imag.onload = function () {
       var theImage = new fabric.Image(imag, {top:155, left:5});
       theImage.resizeFilter = new fabric.Image.filters.Resize({resizeType: 'stepHack'});
       theImage.scaleX = 1/6;
       theImage.scaleY = 1/6;
       canvas.add(theImage);
       theImage.applyResizeFilter(null, function() {canvas.renderAll();});
    }
    imag.src = src;
});

$("#btnImgResHermite").click(function () {
    imag.src = '';
    imag.onload = function () {
       var theImage = new fabric.Image(imag, {top:155, left:225});
       theImage.resizeFilter = new fabric.Image.filters.Resize({resizeType: 'hermite'});
       theImage.scaleX = 1/6;
       theImage.scaleY = 1/6;
       canvas.add(theImage);
       theImage.applyResizeFilter(null, function() {canvas.renderAll();}); 
    }
    imag.src = src;
});

var src = "http://upload.wikimedia.org/wikipedia/commons/thumb/a/a4/Equus_quagga_%28Namutoni%2C_2012%29.jpg/1280px-Equus_quagga_%28Namutoni%2C_2012%29.jpg";
// canvas
var canvas = new fabric.Canvas('c');
canvas.backgroundColor = "#FFFFFF";
var imag = new Image();
imag.crossOrigin = "anonymous";