title: Android custom view save visible screen to bitmap
body: <p>In my app the user manipulates a image, rotates, pinch, zoom etc. This is make within <code>onDraw()</code> <code>using canvas.translate()</code>, <code>canvas.rotate()</code> etc. Of course the image is big and after the manipulation of the image the user performs interactions with other Views. The thing is I don't want to redraw the whole big image over and over again only the visible part. So I'd like, after the user finishes operations, SAVE the current VISIBLE image on the screen to memory/bitmap/cache and redraw only that. </p>
  
  <p>How can I achieve this ?</p>