unity3d 碰撞中不检测图片透明部分怎么弄啊

[FrankWang]

Ray ray = Camera.main.ScreenPointToRay(Input.mousePosition);//从摄像机发出到点击坐标的射线
RaycastHit hitInfo;
if (Physics.Raycast(ray, out hitInfo))
{
//Debug.DrawLine(ray.origin, hitInfo.point);//划出射线，在scene视图中能看到由摄像机发射出的射线
GameObject gameObj = hitInfo.collider.gameObject;
//Debug.Log(gameObj.name);
selectObj = gameObj;
Vector3 Mousepoint = Camera.main.ScreenToWorldPoint(Input.mousePosition);
MouseDownPostion = new Vector3(gameObj.transform.position.x - Mousepoint.x, gameObj.transform.position.y - Mousepoint.y, 0);
isMouseDown = true;
}

用的collider box来检测碰撞，但是目标是sprite，包含透明通道，所以想点在透明的部分就不被碰撞检测到该怎么办或者提供下思路呢

[tianmao111]

算出MouseDownPostion向量与世界中的图片的焦点，再通过焦点获取图片上的Teature2d.GetPixcel(x,y).alpha是否==0