title: Q matrix for the reprojectImageTo3D function in opencv
body: <p>I am doing a project in opencv to detect obstacle in the path of a blind person using stereo calibration. I have calculated the disparity map correctly. Now to find the distance of obstacle from the camera, I want its 3D coordinates [X,Y,Z] , which I am guessing can be found by reprojectImageTo3D(), but I dont have the Q matrix to use in this function because the Q matrix I am getting from stereoRectify() is coming null probably because I used pre calibrated images. Although I do have the inrinsic and extrinsic parameters of my camera.
  So my question is that how can I manually create the Q matrix to directly use in the function reprojectImageTo3D(), if I know the focal length, baseline and everything else about my camera? What is the basic format of the Q matrix?</p>