Question about Surface Normal Calculation

edit

I am new in PCL framework. I want to know how PCL calculate surface normal. I used only three points to calculate surface normal, but failed to estimate surface normal. I collected the code from the link http://www.pointclouds.org/documentation/tutorials/normal_estimation.php#normal-estimation. I modied little. According to my understanding about surface normal, it should provide some value. Unfornately, it provided garbage value (-1.#ND). Here is my sample code and input file. Thanx in advance.

Input: 1 1 1 
         2 2 2 
         3 3 3 

Output: -1.#ND -1.#ND -1.#ND 
           -1.#ND -1.#ND -1.#ND 
           -1.#ND -1.#ND -1.#ND

Code:

int main(..) 
{ 
pcl::PointCloud<pcl::PointXYZ> cloud; 

  cloud.width    = 3; 
  cloud.height   = 1; 
  cloud.is_dense = false; 
  cloud.points.resize (cloud.width * cloud.height); 

  int x_i=1,y_i=1,z_i=1; 
  for (size_t i = 0; i < cloud.points.size (); ++i) 
  { 

    cloud.points[i].x = x_i; 
    cloud.points[i].y = y_i; 
    cloud.points[i].z = z_i; 
        x_i++; 
        y_i++; 
        z_i++; 

  } 

pcl::NormalEstimation<pcl::PointXYZ, pcl::Normal> ne; 
ne.setInputCloud (cloud.makeShared()); 
pcl::search::KdTree<pcl::PointXYZ>::Ptr tree (new pcl::search::KdTree<pcl::PointXYZ> ()); 
ne.setSearchMethod (tree); 
pcl::PointCloud<pcl::Normal>::Ptr cloud_normals (new pcl::PointCloud<pcl::Normal>); 
int k=3; 
ne.setKSearch (k); 
ne.compute (*cloud_normals); 
std::cerr << "Normal cloud size " << cloud_normals->points.size()<< std::endl; 

for (size_t i = 0; i < cloud_normals->points.size(); ++i) 
        std::cerr << "    " << cloud_normals->points[i].normal_x << " " << cloud_normals->points[i].normal_y << " " << cloud_normals->points[i].normal_z << std::endl; 

}