title: opencv_annotation error message; 'opencv_annotation' is not recognized as an internal or external command, operable program or batch file
body: <p>I want to use OpenCV's integrated annotation tool as per <a href="https://docs.opencv.org/4.5.1/dc/d88/tutorial_traincascade.html" rel="nofollow noreferrer">https://docs.opencv.org/4.5.1/dc/d88/tutorial_traincascade.html</a>.</p>
  <p>The tutorial says this command will open up a window containing the images for you to annotate using your cursor.</p>
  <pre><code>opencv_annotation --annotations=/path/to/annotations/file.txt --images=/path/to/image/folder/
  </code></pre>
  <p>But when I try to use it:</p>
  <pre><code># Attempt 1
  opencv_annotation --annotations=C:\Users\my_user\po_images\pos_anno.txt --images=C:\Users\my_user\po_images
  
  # Attempt 2
  opencv_annotation --annotations=\Users\my_user\po_images\pos_anno.txt --images=\Users\my_user\po_images
  
  # Attempt 3   
  opencv_annotation --annotations \Users\my_user\po_images\pos_anno.txt --images \Users\my_user\po_images\
  </code></pre>
  <p>I get an error message:</p>
  <pre><code>'opencv_annotation' is not recognized as an internal or external command,
  operable program or batch file.
  </code></pre>
  <p>I've definitely got openCV installed:</p>
  <pre><code>C:\Users\my_user&gt;pip show opencv-python
  Name: opencv-python
  Version: 4.5.1.48
  </code></pre>
  <p>So where am I going wrong?</p>
  <p>I'm on Python 3.9.1, Windows 10.</p>
  <p>Thanks!</p>