title: How does BruteForce Feature Matching computes the "distance" value?
body: <p>I wrote an application which detects keypoints, compute their descriptors and match them with BruteForce in OpenCV. That works like a charme.</p>
  
  <p>But:
  How is the distance in the match-objects computed?</p>
  
  <p>For example: I'm using SIFT and get a descriptor vector with 128 float values per keypoint.
  In matching, the keypoint is compared with for example 10 other descriptors with the same vectorsize.
  Now, I get the "best match" with a distance of 0.723. </p>
  
  <p>Is this the average of every single euclidean distance of all floats of one vector to another?
  I just want to understand how this one value is created.</p>