title: how card.io convert credit card image to clean binary image without any noise
body: <p>dose any one know how card.ioeprocess the image and remove the non-text background without noise. I'm making a credit card ocr engine like card.io. But it is very tough to remove the non-text background without any noise. thus, it cannot send to NN to make further process.
  So dose any have experience on this kind of problem.
  Thanks a lot</p>