title: Using openCV to implement SIFT in image
body: <p>I have tried to implement SIFT with openCV and I have refer to these links <a href="http://fahmifahim.com/2012/12/11/opencv-sift-implementation-in-opencv-2-4/" rel="nofollow">link1</a> and <a href="http://docs.opencv.org/doc/tutorials/features2d/feature_description/feature_description.html" rel="nofollow">link2</a>. Besides, I have also read the paper about SIFT written by Lowe. I have some problems about the code in link1 and link2.</p>
  
  <ol>
  <li><p>cv::SiftFeatureDetector detector( 0.05, 5.0 ); cv::SiftDescriptorExtractor extractor( 3.0 );</p>
  
  <p>I can't totally understand the parameter in the above function. If I modify the first function to  cv::SiftFeatureDetector detector( 0.05, 10.0 ); , there is a running time OpenCV Error:Assertion failed &lt; firstOctave>=-1 %% actualNLayers&lt;=nOctaveLayers >. </p>
  
  <p>In addition, I don't realize the parameter in the SiftDescriptorExtractor extractor( ). I know there is a distances ratio in keypoints matching, but the range is [0,1].</p></li>
  <li><p>I want to modify the method which I use to match to picture, so I need to extract the descriptor and the dominant orientation of each keypoint. How do I extract extract each keypoint's descriptor and dominant orientation? </p></li>
  </ol>
  
  <p>Thank you very much for your reply.</p>