title: html5: copy a canvas to image and back
body: <p>I implemented a zoom in and out function on a canvas element.
  it works by scaling the canvas, translating it, and then redraw the whole scene again.
  the problem is that it takes a lot of time to redraw everything because i got a lot of things on my canvas.</p>
  
  <p>I need a way to copy the canvas to an image object and than copy the image back to the canvas without loosing quality. what are the specific methods to copy canvas to a javascript variable, and to to copy this variable back to the canvas later?</p>
  
  <p>I'll be glad if you write down the code because I couldn't find any good explanation over the internet.</p>
  
  <p>thanks,</p>