camera.ScreenPointToRay()，camera.ScreenToWorldPoint()與Raycast()

1. camera.ScreenPointToRay(Input.mousePosition)，自camera.tranform.position起經過Input.mousePosition發射一條ray並返回ray
2. camera.ScreenToWorldPoint(Input.mousePosition+new Vector3(0, 0, (xxx)))，將屏幕坐標轉換為世界坐標，但必須賦z值，z值應為攝像機到該點所在平面的投影
3. 不要忘記distance參數，使用layermask可能與distance混淆

4. Ray ray = Camera.main.ScreenPointToRay(Input.mousePosition);               
               RaycastHit hit;
   
               if (Physics.Raycast(ray,out hit,layermask))
               {
                   Vector3 dir = hit.point - Camera.main.transform.position;                   //dir為由攝像機指向攝像機到射線打到的點的向量
                   Vector3 forward = Camera.main.transform.forward;                            //指向攝像機正前方的單位向量
                   Debug.Log("Camera.main.ScreenToWorldPoint(Input.mousePosition) " 
                       + Camera.main.ScreenToWorldPoint(Input.mousePosition)
                       + new Vector3(0, 0, (Vector3.Dot(dir, forward))));                      //Vector3.Dot(dir, forward)為dir與forward的點乘，幾何意義為dir在forward上的投影
                   Debug.Log("hit.point " + hit.point);
               }