using useExtrinsicGuess in solvePnPRansac?

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Hi,

I have doubts about the proper use of solvePnPRansac: I know the intrinsic parameters, the 3D points in the scene and their projections on an image, also I know the rotation vector (rvec) but I don't know the traslation vector (tvec). How can I specify that I only want to use rvec as an extrinsic guess?

Thanks, Raúl