I'm using such a tool from ROS/OpenCV in order to perform the camera calibration. The procedure ends up providing: camera matrix, distortion parameters, rectification matrix and projection matrix. As far as I know the projection matrix contains the intrinsic parameter matrix of the camera multiplied by the extrinsic parameters matrix of the matrix. The extrinsic parameter matrix itself provides the roto-translation of the camera frame with respect to the world frame. If these assumptions are correct...how the projection matrix is computed by Opencv? I,m not defining any world frame!