title: How does the function cv::DescriptorMatcher::radiusMatchImpl() work?
body: <p>I have a computer vision course with OpenCV (C++) at my university, and I have a task to implement keypoints descriptor matcher by overriding functions cv::DescriptorMatcher::radiusMatchImpl() and cv::DescriptorMatcher::knnMatchImpl().</p>
  <p>So I get a target image (snapshot from web-camera) and then my program must to match this image descriptors with descriptors of frames from the web-camera stream. But I don't understand what is argument of the function cv::DescriptorMatcher::radiusMatchImpl() that corresponds to the target image?</p>
  <p>I have found its documentation and its desciption is:</p>
  <p><code>virtual void radiusMatchImpl (InputArray queryDescriptors, std::vector&lt; std::vector&lt; DMatch &gt; &gt; &amp;matches, float maxDistance, InputArrayOfArrays masks=noArray(), bool compactResult=false) CV_OVERRIDE</code></p>
  <p>If I understand correctly, <code>queryDescriptors</code> is cv::Mat that coressponds to the frame descriptors from the videostream. But where is the argument corresponding to the image descriptors with which we should compare this frame?</p>