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I believe it used to be the position of the top-left corner of the checkerboard in the first calibration image, but it may have changed. You can visualized it by writing a few lines of code that project point (0,0,0) (in calibrated scene coordinates) in all the calibration images, then plotting its projected image coordinates on top of the image themselves.

You should really not depend on it being anything meaningful, and instead locate a separate feature in 3D and roto-translate the reference frame to it after calibration.

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