title: Circular Region of Interest in Opencv before thresholding
body: <p>I'm trying to detect a circular object in the middle of my images. Here is a sample image:</p>
  
  <p><img src="https://i.imgur.com/EtnkWLr.jpg" alt=""></p>
  
  <p>The left half is the greyscaled and Gaussian blurred input image; the right half is the same image after Otsu thresholding. The tiny silver of shadow on the lower left corner is leading the Otsu threshold astray. Is there any way to set a circular region of interest so the corner noises can be avoided?</p>