smoother rotations techniques

// use a damped spring to move v0 towards target given a current velocity, // time over which the spring would cover 90% of the distance from rest; // and dt, the change in time.template<typename T>inline void damp_spring(T& v0, const T& target, T& vel, float time_90, float dt){	const float c0 = dt * 3.75f / time_90;	if(c0 >= 1.0f)	{		v0 = target;		vel = T(0.0f);		return;	}	const T delta = target - v0;	const T force = delta - 2.0f * vel;	v0 += vel * c0;	vel += force * c0;}void Frame::spring_facing(float dt, float spring){	vec4 z(m_ornt.z_axis());	damp_spring<vec4>(z, m_target_z, m_zvel, spring, dt);		// now make sure that normalizing z preserves the y coord.	// linearly interpolating the z-axis is an approximation, 	// and the y distortion is most noticeable.	const float D = 1.0f - z.y * z.y;	const float L = z.x * z.x + z.z * z.z;	const float mult = sqrtf(D / L);	z.x *= mult;	z.z *= mult;	m_ornt.face(-z);}