Problem with using cvPerspectiveTransform

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C++

My target is to know the 3D-coordinate(XYZ) with providing the two matching points from the left and right cameras.Someone says just using the function cvPerspectiveTransform() or cvreprojectImageto3D(),can someone tells me how to use the two functions?And where can I get the Tx from the reprojective matrix Q?
Thank you!