Non-planar camera calibration input

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Why is it mandatory to provide an initial guess when calibrating from a non-planer rig? Does the initial guess affect the output result considerably? Why (such difference between planer and non-planar case)?

I am trying to calibrate a camera using some 3D->2D correspondences (need not be non-planar) I found in some way. But the results I am getting is really bad with very high reprojection error. Any one successfully used opencv to calibrate their camera using non-planar rig and using something other than their provided chessboard methods?

Thanks!