c++ - cv::goodFeaturesToTrack gives the corner vector size as zero -

i trying opencv cv::goodfeaturestotrack corner points. did following code. output corner vector size 0 corners capacity changed.

using namespace cv;  void main() {     mat img = imread("cameraman.tif",cv_load_image_grayscale);      std::vector<point2f> corners;      cv::goodfeaturestotrack(img,corners,10,0.01,10);      std::cout<<corners.size()<<endl; } 

the output 0