title: Are there any fast alternatives to SURF and SIFT for scale-invariant feature extraction?
body: <p>SURF is patented, as is SIFT. ORB and BRIEF are not patented, but their features are not scale-invariant, seriously limiting their usefulness in complex scenarios.</p>
  
  <p>Are there any feature extractors that can extract scale-invariant features as fast as SURF and are not so strictly patented as SURF and SIFT?</p>