title: How to use ocl module in opencv
body: <p>I am trying to use ocl module in opencv. I am using Visual studio 2012</p>
  
  <p>I started by sample code for feature detection using surf. The code seems as follow:</p>
  
  <pre><code>SURF detector;
  SURF extractor;
  BFMatcher matcher;
  
  std::vector&lt;KeyPoint&gt; keypoints_1, keypoints_2;
  
  detector.detect( image1, keypoints_1 );
  detector.detect( image2, keypoints_2 );
  
  //-- Step 2: Calculate descriptors (feature vectors)
  Mat descriptors_1, descriptors_2;
  
  extractor.compute( image1, keypoints_1, descriptors_1 );
  extractor.compute( image2, keypoints_2, descriptors_2 );
  
  //-- Step 3: Matching descriptor vectors using FLANN matcher
  std::vector&lt; DMatch &gt; matches;
  matcher.match( descriptors_1, descriptors_2, matches);
  </code></pre>
  
  <p>The code works perfectly, Now I want to use SURF_OCL instead of SURF. What should I do?</p>
  
  <p>The following code doesn't work:</p>
  
  <pre><code>ocl::SURF_OCL detector;
  ocl::SURF_OCL extractor;
  </code></pre>
  
  <p>generate compile time error:</p>
  
  <pre><code>'ocl' : is not a class or namespace name
  'SURF_OCL' : undeclared identifier
  </code></pre>
  
  <p>What should I do to be able to use functions from ocl library?</p>