crop image using points

edit

I have converted one image into gray scale and hsv now want to get part of hsv from original image. Can someone help me in this?

cv::Mat source=[self cvMatFromUIImage: self.imgTaken];

cv::Mat hsv;

cv::cvtColor(source, hsv, cv::COLOR_BGR2HSV);
cv::Mat Img;

cv::cvtColor(hsv, Img, cv::COLOR_BGR2GRAY);