title: Is it possible to create transparent canvas
body: <p>I am developing a paint brush application in javascript using processing.js
  It is using a canvas object. I want to keep an image at the background of the canvas. Draw something in the foreground. And while saving i need to get only foreground data.</p>
  
  <p>For that we need to make canvas object transparent so that background image is visible.</p>
  
  <p>I don't see any option to make the canvas transparent. How do I do that?</p>