title: Keypoints detection in OpenCV with SIFT
body: <p>i have a problem with Keypoints detection of an image in OpenCV. 
  If i have for example a SIFT detector, when it detects an image it retrives 1000 keypoints but if i split the image in two "sub-image", and foreach SIFT detects keypoints, the sum is not 1000, but less. 
  For me the total keypoints number can be different but not too much!
  Thanks</p>