incidence angle from translation vector [closed]

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Using OpenCV 3.2 for a local positioning system utilizing aruco.

Have successfully extracted 3d angles/matrices between camera and identified markers. But I also need to calculate the angle to the camera caused by translation of the marker in x and y. Using typical methods in OpenCV, it is straightforward to get this translation vector.

But I am stuck on how to straightforwardly go from the translation vector to angle of incidence.

I have searched/read widely, but have not found any commentary on this. I am not a 3d expert by any means and it is more than possible I have just overlooked something.

My own instinct at this point is to simply translate 0, 0, 0 in model coords to a camera location, and estimate the angle of incidence geometrically. There are some obvious problems with this, and I suspect it will be less than satisfactory.

So - my question is: Is there a straightforward way of going from the translation vector returned by methods like solvePnP() or estimatePoseSingleMarkers() to the angle of incidence to the camera caused by the translation? This is NOT the angle of pose - these come (reasonably) directly from these functions - but these angles not reflect changes in the angle of incidence due to the translation - the incorporate only the actual rotation of detected object.

Any help greatly appreciated,

Seth