unity, 让主角头顶朝向等于地面法线（character align to surface normal）

计算过程如下：

1，通过由主角中心raycast一条竖直射线获得主角所在处地面法线，用作主角的newUp。

2，根据主角forward和newUp计算newForward。

3，使用Quaternion.LookRotation (newForward, newUp)获得主角新的rotation。

结果如图：

代码：

　　　　 //note, the code here suppose our character use a capsuleCollider and the floors‘ layerMask is "floor"..

　　　　　//..if yours‘ is not, you should make some change.

        RaycastHit hitInfo;
        Vector3 capsuleColliderCenterInWorldSpace=GetComponent<CapsuleCollider> ().transform.TransformPoint (GetComponent<CapsuleCollider>().center);
        bool isHit=Physics.Raycast (capsuleColliderCenterInWorldSpace,new Vector3(0f,-1f,0f),out hitInfo,100f,LayerMask.GetMask("floor"));

        Vector3 forward=GetComponent<Rigidbody>().transform.forward;
       
        Vector3 newUp;
        if (isHit) {
            newUp = hitInfo.normal;
        } else {
            newUp = Vector3.up;
        }
        Vector3 left = Vector3.Cross (forward,newUp);//note: unity use left-hand system, and Vector3.Cross obey left-hand rule.
        Vector3 newForward = Vector3.Cross (newUp,left);
        Quaternion oldRotation=GetComponent<Rigidbody>().transform.rotation;
        Quaternion newRotation = Quaternion.LookRotation (newForward, newUp);

　　　　　float kSoftness=0.1f;//if do not want softness, change the value to 1.0f
        GetComponent<Rigidbody> ().MoveRotation (Quaternion.Lerp(oldRotation,newRotation,kSoftness));