title: How to locate an image in another image while ignoring transparent pixels
body: <p>I'm trying to find out if a parent image contains a sub-image somewhere. Not a pixel perfect match, but within some error threshold. However, my sub-image has a transparent background which I am not trying to match.</p>
  <p>Let's say my sub-image is a ring with a transparent center. The parent image can contain whatever it wants to inside that transparent center, I just want to be able to locate that ring.</p>
  <p>I've tried messing around with opencv's matchTemplate, but whenever I start adding transparent pixels the similarity starts going down.</p>
  <p>What's the right way to do this?</p>