获知新运动方向是向右偏还是向左偏

这段代码来自Unity官方案例中的 Stealth Project。

（涉及到通过向量积来判断顺时针还是逆时针旋转，然后使用sign()判断向量积与up向量的点积正负号）

float FindAngle (Vector3 fromVector, Vector3 toVector, Vector3 upVector)
	{
		// If the vector the angle is being calculated to is 0...
		if(toVector == Vector3.zero)
			// ... the angle between them is 0.
			return 0f;
		
		// Create a float to store the angle between the facing of the enemy and the direction it's travelling.
		float angle = Vector3.Angle(fromVector, toVector);
		
		// Find the cross product of the two vectors (this will point up if the velocity is to the right of forward).
		Vector3 normal = Vector3.Cross(fromVector, toVector);
		
		// The dot product of the normal with the upVector will be positive if they point in the same direction.
		angle *= Mathf.Sign(Vector3.Dot(normal, upVector));
		
		// We need to convert the angle we've found from degrees to radians.
		angle *= Mathf.Deg2Rad;

		return angle;
	}