I am asking which point is the reference point of the Reprojection-Matrix Q or rather the xyz-data. There are to my mind two possible options:
The origin of the xyz-data lies in the optical center of the right of the two physical cameras.
The origin of the xyz-data lies in the optical center of the right virtual and rectified camera. The right camera with horizontal epipolar lines.
The reason why im asking is, that I have with a baseline of ca. 2 inch a constant negative offset in the z-distances data of nearly 1 inch. I have calculated with stereoRectify with CV_CALIB_ZERO_DISPARITY and alpha =1 the rectification maps. When I look at the rectified images, there is a black edge around them. It looks like the cameras are virtually shifted a little bit backwards, possibly exactly 1 inch. If this assumption that the origin is also shifted was true, I had an explanation for the negative offset, because I have measured the distance to the physical camera for comparison, not the virtual camera.