angle_axis_to_quaternion returns (w, x, y, z) instead of (x, y, z, w)

https://github.com/kornia/kornia/blob/b5418d8bac9e7bb01e66310d7d2b4227766c5a02/kornia/geometry/conversions.py#L682

def angle_axis_to_quaternion(angle_axis: torch.Tensor) -> torch.Tensor:
    r"""Convert an angle axis to a quaternion.
    The quaternion vector has components in (x, y, z, w) format.               # <-- says (x, y, z, w)
    # ...
    return torch.cat([w, quaternion], dim=-1)                                  # <-- does (w, x, y, z)

proposal:

return torch.cat([quaternion, w], dim=-1)

Similarly, in quaternion_to_angle_axis:

q1: torch.Tensor = quaternion[..., 0]
q2: torch.Tensor = quaternion[..., 1]
q3: torch.Tensor = quaternion[..., 2]

[schuhschuh]

I didn’t realize the convention was inconsistent with other quaternion related functions in kornia itself. In this case this should be flagged as bug maybe rather than enhancement?

[edgarriba]

@amonszpart @schuhschuh let’s follow ceres convention [w, x, y, z] - we should also explain somewhere in the docs quaternion definition. There was even a proposal from @versatran01 to have a separated module similar to pyquaternion.