Why Object having NormalMap material can do mirror in Unity 5

4  case (make 3 real-axis look like match  right hand rule)  

它們都做了奇數次的鏡射(odd times mirror)

3 case (make 3  real-axis still  match left hand rule)

它們都做了偶數次的鏡射(even times mirror)

在奇數次的鏡射的情況(in  odd times  mirror)

要再多改變一次tangent的方向(swap y again)

Unity乘上unity_WorldTransformParams.w應該就是這個原因

但Unity怎麼計算出unity_WorldTransformParams.w？

我猜可能像這樣...

方法(1)

transform  has  LossyScale(the global scale of object)

unity_WorldTransformParams.w=sign(LossyScaleX*LossyScaleY*LossyScaleZ)

方法(2)

transform has   right , up , forward

但是right,up ,forward並不會反應出scale設為負值後的方向改變

所以必須乘上lossyScale

(也就是光看right,up,forward看不出3軸有沒有match right hand rule )

檢查cross(real_right ,real_up)和real_forward 是不是同向，決定[鏡射因子]

Vector3 real_right=LossyScaleX*right;

Vector3 real_up=LossyScaleY*up;

Vector3 real_forward=LossyScaleZ*forward;

unity_WorldTransformParams.w=sign(dot(cross(real_right,real_up),real_forward));