How could i edit previous drwant skecth?

[chinmoydutta]

I am using below code for populating the previously saved sketch and trying to edit it, but it got refresh as soon as I start sketching over. May I know how to get this working properly!

var ctx = document.getElementById('colors_sketch').getContext('2d');
var img = new Image;
img.onload = function(){
ctx.drawImage(this,0,0);
$('#colors_sketch').sketch();
};
img.src = 'data:image/png;base64,iVBORw0KG.....'