Consider two cameras with reference frames {1} and {2} respectively. As always, the reference frame of each camera is such that the X Y plane is parallel to the image plane and the Z-axis points in the direction of viewing. Assume we know the relative transform g21 = (R,T) SE(3). Additionally, assume the cameras are calibrated and normalized, so that the camera matrix K is the identity. Both cameras are looking at the same point p in 3D space, which has unknown coordinates X1 R3 in frame {1} and X2 R3 in frame {2}. We observe their image coordinates x1 and x2, written in 2D homogeneous coordinates