message: need atleast 4 corresponding points to calculate homography
diff: @@ -374,6 +374,9 @@ cv::Mat cv::findHomography( InputArray _points1, InputArray _points2,
  return Mat();
  convertPointsFromHomogeneous(p, p);
  }
  + // Need at least 4 point correspondences to calculate Homography
  + if( npoints < 4 )
  + CV_Error(Error::StsVecLengthErr , "The input arrays should have at least 4 corresponding point sets to calculate Homography");
  p.reshape(2, npoints).convertTo(m, CV_32F);
  }
  