title: Filling in a single colour background in OpenCV
body: <p>I'm writing a program in OpenCV which takes a picture of parasite eggs and attempts to identify at least a large portion of them. My problem is that the input image I have the best results with has a large background. I've tried both filling in the background, and cropping it out, but when I do I get a worse selection of the eggs.</p>
  
  <p>My currently thought out solution is to use the image with the background and then fill it in. It felt like it would be easy because I just want to fill anything outside of that circle with black, but I'm not sure how to actually perform the action. If anyone could point be toward a method to use, or any suggestions that would be great.</p>
  
  <p>Here is a link to what the image looks like:</p>
  
  <p><img src="https://i.stack.imgur.com/QNJUY.png" alt="enter image description here"></p>
  
  <p>Thanks!</p>