stereo calibration and 3d reconstruction

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My purpose is to calculate the relative distance of two points on an object in 3D space. First I do stereo camera calibration to get the intrinsic parameters of the two cameras and the rotation and transition matrix of the two cameras. So for the two points on the object, I know the image position in both images,intrinsic parameters of the two cameras, and also the elemental/fundamental marix of the two cameras, but I don't know the extrinsic parameters of the object with respect of each cameras, how can I　calculate the 3d coordinates on the object please?