Camera pose and reflections using OpenCV's SovePNP

Question

I'm trying to use the function SolvePNP to estimate the relative position of a camera. Mi question is this, when choosing world coordinates, do I need to be careful in choosing them so that there can be no reflections when transforming them to camera coordinates? Or will OpenCV correct that for me?

Details: I'm filming a tennis court and was originally setting the world coordinate origin to be the centre of the court, with the x-axis pointing parallel to the net towards the left, the y-axis pointing forwards vertically on the court, and the z-axis pointing upwards. If I've understood correctly, SolvePNP will transform these coordinates to a system with origin at some point behind the top left corner of an image, with x-axis pointing downwards on the image, y-axis pointing to the right, and z-axis pointing forwards to the scene. However this transformation would definitely involve a reflection, must I swap the x and y axis of my world coordinates to avoid this or is it fine to leave them as they are? (Also, let me know if I'm making a big mistake and SolvePnp actually puts the origin at a point behind the centre of the image rather than one the top left corner...)

Answer 1

Assuming that you have a camera calibration matrix (and that such calibration was done assuming a right hand coordinate system all along), and correct correspondences between the tennis field features in the image and the CAD-features:

You need to select the reference frame in the tennis court such that is a right hand coordinate system, so that your solution from solvePNP provides the pose and position of the tennis field reference frame with respect to the camera coordinate system (by default a right hand coordinate system).

Hope it helps