For my project, I had a-priori knowledge of the camera's distortion parameters, so I added a CALIB_FIX_FX and CALIB_FIX_FY to the fisheye calibration to allow me to use that a-priori information. Similarly, I changed the behaviour of CALIB_FIX_K1...K4 to use the provided distortion coefficients if CALIB_USE_INTRINSIC_GUESS is set, instead of just setting them to 0. These changes are currently working on my dev machine.

Before I make a full PR with the relevant changes to documentation, could someone confirm that this is a valid change that could be accepted upstream? I want to make sure that there was not some rational for the previous behaviour that I was missing.