title: Can i specify the FAST keypoints number i get when using opencv FastFeatureDetector
body: <p>i am using openCV FastFeatureDetector to extract fast keypoints from image.<br/>
  but the number of the FastFeatureDetector detect  is not a const number.<br/>
  i want set the  max keypoints number FastFeatureDetector get.<br/>
  Can i specify the FAST key points number i get when using openCV FastFeatureDetector<br/>
  How?</p>