Fisheye distortion correction using Omnidir namespace - change of perspective

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I have been able to correctly calibrate a 180 degrees FOV camera (fisheye), i.e. I have been able to extract the distortion and camera matrices using the omnidirectional model, so I used the omnidir::calibrate() function to extract the matrices and used the omnidir::undistortImage() function for undistorting the images.

Everything works well, but I would like to change the angle at which the undistort is done, i.e. the angle at which the camera is viewing when the image is undistorted. To get a better idea of what I am after please check this link: paulbourke.net/dome/fish2/ (4th image down - looking right by 40 degrees) something similar to that. I have tried changing the cx value in the Knew matrix but that doesn't have the desired effect.

Any help would be greatly appreciated.