title: Python/OpenCV how to deal with large number of SIFT feaure in .yml file?
body: <p>I'm using OpenCv and I took yaml to store SIFT Keypoints and Descriptors. 
  I have a database of 1659 pictures (.jpg, each picture around 95 KB). For each image, I created a .yml file with Keypoints and Descriptors. Now, for a single image, I ended up with 700 keypoint and descriptors resulting in a file of ca. 4MB and I would like to avoid using binary files.<br>
  My questions are:  </p>
  
  <blockquote>
    <ul>
    <li>How can I know if the number of features is adequate to the image?  </li>
    <li>There is any way to control the number of features? For example, setting a threshold for SIFT?  </li>
    <li>Now storing a numpy matrix into a yamil file using  <code>cv2.FileStorage.write</code>, OpenCv writes each number with a 16 significant digits (ex. 1.9705572128295898e+00). Is there a problem if I reduce the significant digits? For example to 4?  </li>
    </ul>
  </blockquote>