Unity中差乘判断目标是否在左边或右边

使用差乘判断左右一般是比较差乘的y，小于0是左，大于0是右。特殊情况可以用其他分量来比较

默认情况：

var cross = Vector3.Cross(lhsObject.transform.position, rhsObject.transform.position);
if (cross.y > ) //side
if(cross.y < )//another side

用forward方向,从而对有旋转的对象进行比较：

void OnCollisionEnter(Collision other) {
     Vector3 direction = other.contacts[].point - transform.position;
     var cross = Vector3.Cross(transform.forward, direction);
     if (cross.y < ) print ("Left");
         else print("right");
 }

用Transform的right就可以比较是否在前后，很常用，对于赛车游戏的路点等都会用到

y>0 a在b后，y<0 a在b前

var dir = a.position - b.position;
var cross = Vector3.Cross(b.right, dir.normalized).y > ;
if (cross) return;

论坛上还看见一个demo，加深理解(得到forward,up方向，取得right,left)：

// original vector:
  var forward: Vector3 = Vector3(0.5, 0.7, 0.8);
  // up direction:
  var up: Vector3 = Vector3(0.0, 1.0, 0.0);
  // find right vector:
  var right = Vector3.Cross(forward.normalized, up.normalized);
  var left = -right;