Quaternion camera. How do I make it rotate correctly?

Question

I have a 3D camera with its current rotation stored as a quaternion, and I'm having trouble rotating it correctly. I want the camera to rotate incrementally around its local axes based on mouse movement each frame (first-person-shooter-style), but the rotation is wrong. It kind of works, but the camera seems to "roll" around its forward axis when it shouldn't.

I update the rotation each frame with this function:

void Camera::rotate(const Quat& q)
{
    // m_rot is the current rotation
    m_rot = m_rot * q;
}

Here's my quaternion multiplication function:

Quat Quat::operator*(const Quat &rhs) const
{
    // quaternion elements in w,x,y,z order
    Vector4d res;

    res[0] = m_q[0]*rhs.m_q[0] - m_q[1]*rhs.m_q[1] -
             m_q[2]*rhs.m_q[2] - m_q[3]*rhs.m_q[3];
    res[1] = m_q[0]*rhs.m_q[1] + m_q[1]*rhs.m_q[0] +
             m_q[2]*rhs.m_q[3] - m_q[3]*rhs.m_q[2];
    res[2] = m_q[0]*rhs.m_q[2] - m_q[1]*rhs.m_q[3] +
             m_q[2]*rhs.m_q[0] + m_q[3]*rhs.m_q[1];
    res[3] = m_q[0]*rhs.m_q[3] + m_q[1]*rhs.m_q[2] -
             m_q[2]*rhs.m_q[1] + m_q[3]*rhs.m_q[0];

    return Quat(res);
}

Am I doing something wrong, or is this some kind of floating-point error thing?

Answer 1

Figured out the problem. For a mouse-controled first-person camera like the one I'm going for, I want to rotate around the local x-axis to look up and down, but the global y-axis for looking side to side.

So this is correct for the x-axis:

m_rot = m_rot * q;

But I need to do this for the y-axis:

m_rot = d * m_rot;