Is there an algorithm for converting quaternion rotations to Euler angle rotations?

Question

Is there an existing algorithm for converting a quaternion representation of a rotation to an Euler angle representation? The rotation order for the Euler representation is

Answer 1

In a right-handed Cartesian coordinate system with Z axis pointing up, do this:

struct Quaternion
{
    double w, x, y, z;
};

void GetEulerAngles(Quaternion q, double& yaw, double& pitch, double& roll)
{
    const double w2 = q.w*q.w;
    const double x2 = q.x*q.x;
    const double y2 = q.y*q.y;
    const double z2 = q.z*q.z;
    const double unitLength = w2 + x2 + y2 + z2;    // Normalised == 1, otherwise correction divisor.
    const double abcd = q.w*q.x + q.y*q.z;
    const double eps = 1e-7;    // TODO: pick from your math lib instead of hardcoding.
    const double pi = 3.14159265358979323846;   // TODO: pick from your math lib instead of hardcoding.
    if (abcd > (0.5-eps)*unitLength)
    {
        yaw = 2 * atan2(q.y, q.w);
        pitch = pi;
        roll = 0;
    }
    else if (abcd < (-0.5+eps)*unitLength)
    {
        yaw = -2 * ::atan2(q.y, q.w);
        pitch = -pi;
        roll = 0;
    }
    else
    {
        const double adbc = q.w*q.z - q.x*q.y;
        const double acbd = q.w*q.y - q.x*q.z;
        yaw = ::atan2(2*adbc, 1 - 2*(z2+x2));
        pitch = ::asin(2*abcd/unitLength);
        roll = ::atan2(2*acbd, 1 - 2*(y2+x2));
    }
}