title: optimum hessian threshold for SURF feature extraction in opencv + Minimum descriptors matching
body: <p>Currently I am working on face recognition project where I am using Fisherfaces/LDA to filter out the images on a broader level and then using SURF to verify the output from LDA. What would be a good Hessian threshold which should be passed to <code>CvSURFParam()</code>? The sample code provided by OpenCV has 500. Is it optimum?</p>
  
  <p>Also, what would be minimum number of descriptors that should be matched so that my code can confirm the identity? Will 50% suffice?</p>
  
  <p>I'm using OpenCV 2.4 with C++ on Windows.</p>