Learning PCL - Basic data structure

In general, point clouds can be divided into organized and unorganized categories in terms their structure.

An organized point cloud dataset is the name given to point clouds that resemble an organized image (or matrix) like structure, where the data is split into rows and columns. Examples of such point clouds include data coming from stereo cameras or Time Of Flight cameras. The advantages of a organized dataset is that by knowing the relationship between adjacent points (e.g. pixels), nearest neighbor operations are much more efficient, thus speeding up the computation and lowering the costs of certain algorithms in PCL.

In unorganized point cloud, the points are placed in series without indexing. In PCL width and height attributes indicate the structure of that point cloud. If the height is 1, then it is a unorganized point cloud whose size is indicated by its width. For organized point cloud, the height * width is the number of points.

In order to declare and use the point cloud type in our own class, we need to define a pointer to the point cloud rather than a object itself. PCL uses boost::shared_ptr which is a little bit different from normal C++. Here is an example to declare a boost::share_ptr of a class:

struct MyClass{}; 

int main (int argc, char** argv) 
{ 
 boost::shared_ptr MyClassPtr; 
 MyClassPtr = boost::shared_ptr(new MyClass); 
 return(0); 
} 

Following the same route, we can define a boost::share_ptr to pcl::PointXYZ class in this way:

int main (int argc, char** argv) 
{ 
 pcl::PointCloud::Ptr CloudPtr; 
 CloudPtr = pcl::PointCloud::Ptr(new pcl::PointCloud); 
 return(0); 
} 

In the initialization of our class, we can 'initialize' the point cloud by:

 CloudPtr->resize(640*480);

It is also easy to access point in the point cloud with these snippets:

  // Output the (0,0) point
  std::cout << (*cloud)(0,0) << std::endl;

  // Set the (0,0) point
  (*cloud)(0,0).x = 1;
  (*cloud)(0,0).y = 2;
  (*cloud)(0,0).z = 3;
  // Confirm that the point was set
  std::cout << (*cloud)(0,0) << std::endl;

The result would be:
(0,0,0)
(1,2,3)