title: OpenCV Brisk not detecting any keypoints
body: <p>I have an image that I'd like to detect in a larger image (i.e. see if a similar version is present). Template matching is not accurate enough, as the items can be rooted and scaled, so Brisk seems a hopeful approach:</p>
  <p>However, when I try to create descriptors, I get no return:</p>
  <p>Here is my example image:</p>
  <p><a href="https://i.stack.imgur.com/TiODo.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/TiODo.png" alt="enter image description here" /></a></p>
  <pre><code>BRISK = cv2.BRISK_create()
  keypoints1, descriptors1 = BRISK.detectAndCompute(img, None)
  </code></pre>
  <p>returns</p>
  <pre><code> ([], None)
  </code></pre>
  <p>Any suggestions how I can get better results? Why does Brisk not detect any features? Changing threshold to 0 and adding octaves doesn't seem to help.</p>