问如何将3d世界加速到屏幕(unity3d)

EN

public void  Shoot ()
{
    //bullet_target
    var layerMask = (1 << 8);
    layerMask = ~layerMask;
    Vector3 bullet_target = Vector3.zero;
    Ray ray = Camera.main.ScreenPointToRay( new Vector3(screen_width_mid, screen_height_mid, 0) );
    RaycastHit hit;
    GameObject hit_game_object;
    Transform hit_transform;
    if (Physics.Raycast (ray.origin, ray.direction * 1000 ,out hit, 1500.0f , layerMask) ) 
        {
            bullet_target = hit.point;
            hit_game_object = hit.transform.gameObject;
            hit_transform = hit.transform;
        }else
        {
            hit_game_object = null; 
        }
        if(hit_game_object!=null)
            {               
                if( hit_game_object.tag=="head" )
                    {
                        //blah blah do any stuf
                    }
            }
        }
}