that. pixel centers are integers.
and yes, for a perfect pinhole, the principal point is (w-1)/2, (h-1)/2, i.e. if the camera has 2x2 pixels, it sits on the corners between all those four.
IDK if these things are documented properly anywhere but it’s the fundamental assumption.
it also explains “funny” behavior by resize(). it maps whole areas from pixel corner to pixel corner, not just putting the top left and bottom right pixel centers on top of each other.
4x4 example, resized to +1 width +1 height (and then magnified for display):
the aruco module is an entirely different matter. I wouldn’t quite trust it right now. lots of changes happening right now. some changes broke compatibility previously acceptable calibration patterns.
many people lack the attention to detail to care about these things. this is an uphill battle.