title: detect whether the checkbox is checked using opencv
body: <p>i am trying to find if a checkbox([<img src="https://i.stack.imgur.com/6am9N.png" alt="Checked Checkbox">])<a href="https://i.stack.imgur.com/6am9N.png" rel="nofollow noreferrer">1</a></p>
  
  <p>is checked or not. till now i tried using the method used in <a href="https://www.pyimagesearch.com/2016/02/08/opencv-shape-detection/" rel="nofollow noreferrer">pyimagesearch</a>.i am trying to see if there are any triangles in the image. if there is at least one triangle it means that the checkbox is checked. The problem i found is that, sometimes the outer rectangle is being detected instead of triangles. how should i ignore rectangle and detect only triangles</p>