utils/general_utils.py

import torch

def quaternion_raw_multiply(a: torch.Tensor, b: torch.Tensor) -> torch.Tensor:
    """
    From Pytorch3d
    Multiply two quaternions.
    Usual torch rules for broadcasting apply.

    Args:
        a: Quaternions as tensor of shape (..., 4), real part first.
        b: Quaternions as tensor of shape (..., 4), real part first.

    Returns:
        The product of a and b, a tensor of quaternions shape (..., 4).
    """
    aw, ax, ay, az = torch.unbind(a, -1)
    bw, bx, by, bz = torch.unbind(b, -1)
    ow = aw * bw - ax * bx - ay * by - az * bz
    ox = aw * bx + ax * bw + ay * bz - az * by
    oy = aw * by - ax * bz + ay * bw + az * bx
    oz = aw * bz + ax * by - ay * bx + az * bw
    return torch.stack((ow, ox, oy, oz), -1)

# Written by Stan Szymanowicz 2023
def matrix_to_quaternion(M: torch.Tensor) -> torch.Tensor:
    """
    Matrix-to-quaternion conversion method. Equation taken from 
    https://www.euclideanspace.com/maths/geometry/rotations/conversions/matrixToQuaternion/index.htm
    Args:
        M: rotation matrices, (3 x 3)
    Returns:
        q: quaternion of shape (4)
    """
    tr = 1 + M[ 0, 0] + M[ 1, 1] + M[ 2, 2]

    if tr > 0:
        r = torch.sqrt(tr) / 2.0
        x = ( M[ 2, 1] - M[ 1, 2] ) / ( 4 * r )
        y = ( M[ 0, 2] - M[ 2, 0] ) / ( 4 * r )
        z = ( M[ 1, 0] - M[ 0, 1] ) / ( 4 * r )
    elif ( M[ 0, 0] > M[ 1, 1]) and (M[ 0, 0] > M[ 2, 2]):
        S = torch.sqrt(1.0 + M[ 0, 0] - M[ 1, 1] - M[ 2, 2]) * 2 # S=4*qx 
        r = (M[ 2, 1] - M[ 1, 2]) / S
        x = 0.25 * S
        y = (M[ 0, 1] + M[ 1, 0]) / S 
        z = (M[ 0, 2] + M[ 2, 0]) / S 
    elif M[ 1, 1] > M[ 2, 2]: 
        S = torch.sqrt(1.0 + M[ 1, 1] - M[ 0, 0] - M[ 2, 2]) * 2 # S=4*qy
        r = (M[ 0, 2] - M[ 2, 0]) / S
        x = (M[ 0, 1] + M[ 1, 0]) / S
        y = 0.25 * S
        z = (M[ 1, 2] + M[ 2, 1]) / S
    else:
        S = torch.sqrt(1.0 + M[ 2, 2] - M[ 0, 0] -  M[ 1, 1]) * 2 # S=4*qz
        r = (M[ 1, 0] - M[ 0, 1]) / S
        x = (M[ 0, 2] + M[ 2, 0]) / S
        y = (M[ 1, 2] + M[ 2, 1]) / S
        z = 0.25 * S

    return torch.stack([r, x, y, z], dim=-1)