Fisheye Camera Calibration

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We're using the OcamCalib in Matlab to calibrate our fisheye camera. The OcamCalib c++ functions allow us to undistort the image very nicely. Does anyone know how we can now go from undistorted fisheye to bird's eye view? Alternatively, from cam2world (in the OcamCalib c++ functions) spherical coordinates to birds eye view.

We've been hitting our heads against the wall for a while now. Getting some form of birds eye view but not very accurate at all.