title: How to convert an OpenCV cv::Mat into a float* that can be fed into Vlfeat vl_dsift_process ?
body: <p>How to convert an OpenCV <code>cv::Mat</code> into a <code>float*</code> that can be fed into <code>Vlfeat vl_dsift_process</code>?
  I cannot understand how <code>vl_dsift_process</code> works on a one-dimensional float array.
  The official samples only demonstrate how to use MatLab API.
  It seems that there are no samples about C API, and the only clue is the MEX file in the source package.</p>