title: Python opencv: Draw lines between points and find full contours
body: <p>I have a set of three points, in a triangle. eg: [[390  37], [371 179], [555 179]]</p>
  <p>After drawing the lines, <a href="https://stackoverflow.com/questions/50671524/how-to-draw-lines-between-points-in-opencv">How to draw lines between points in OpenCV?</a></p>
  <p>How do I find the full Contours between the lines I Just drew?</p>
  <p>I keep receiving error below:</p>
  <pre><code>    img = np.zeros([1000, 1000, 3], np.uint8)
      cv2.drawContours(img, triangle_vertices, 0, (0, 0, 0), -1)
      contours, hierarchy = cv2.findContours(img, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_NONE)
  </code></pre>
  <blockquote>
  <p>Error:  (-210:Unsupported format or combination of formats) [Start]FindContours supports only CV_8UC1 images when mode != CV_RETR_FLOODFILL otherwise supports CV_32SC1 images only in function 'cvStartFindContours_Impl'</p>
  </blockquote>