OpenCv: Extract portion of a Mat image

edit

Assume you got your point like, topLeft, topRight, bottomLeft, bottomRight.

Mat src;//Source image load here
Rect R(topLeft,bottomRight); //Create a rect 
Mat ROI = src(R) //Crop the region of interest using above rect

Here, mat will be your originalMat.

Rect roi = new Rect(40, 100, 100, 120);
Mat cropped = new Mat(mat, roi);