When I render with an equirectangular camera in Cycles, the image is formed around the camera's local axes, so if the camera is rotated to look down (0,0,0 in blender), the top and bottom poles of the sphere would be +y and -y respectively. The stereo separation however does not follow this. The two eyes are spaced in the x direction, and the whole set is rotated around the z axis to make the spherical stereo image. If the camera is rotated on its side, (90,90,0), the left and right images are separated "vertically" with respect to the camera.
I'm not experienced at all, so I'm not sure I describe this properly, but I think the stereo effect is done with a global orientation, while the equirectangular projection is done with the camera's orientation.
Is there some way to force the stereo effect to follow the camera's orientation? I'd like to have some control of where the poles would be, even if that produced images that wouldn't look aligned to the horizon, if that makes sense.
Alternatively, is there a SANE way to rotate the whole entire scene, environment and everything included, around the camera which would then remain horizontal?
Here are some images of what I'm talking about :
Camera is facing forward [90,0,0] : this is what I'd like the other orientations' stereo effect to look like.
Camera is facing down, [0,0,0] : The bottom cube shows rotation around the z axis, I'd like the stereo rotation around the camera's "top", the y axis in this instance.
Camera is rotated to its side, facing forwards [90,90,0]: again, weird, locally vertical separation, and the pole remains at the z axis instead of the camera's "top" x axis .
Oh, I've tested this in 2.79b, latest 2.79 and 2.8 beta, but it's the same.