pcl::PCLPointCloud2 usage - c++

Question

I'm confused with when to use pcl::PointCloud2 vs pcl::PointCloudPointCloud
For example, using these definitions for pcl1_ptrA, pcl1_ptrB and pcl1_ptrC:
pcl::PointCloud<pcl::PointXYZRGB>::Ptr pcl1_ptrA(new pcl::PointCloud<pcl::PointXYZRGB>); //pointer for color version of pointcloud
pcl::PointCloud<pcl::PointXYZRGB>::Ptr pcl1_ptrB(new pcl::PointCloud<pcl::PointXYZRGB>); //ptr to hold filtered Kinect image
pcl::PointCloud<pcl::PointXYZRGB>::Ptr pcl1_ptrC(new pcl::PointCloud<pcl::PointXYZRGB>); //ptr to hold filtered Kinect image
I can invoke the following PCL functions:
pcl::VoxelGrid<pcl::PointXYZRGB> vox;
vox.setInputCloud(pcl1_ptrA);
vox.setLeafSize(0.02f, 0.02f, 0.02f);
vox.filter(*pcl1_ptrB);
cout<<"done voxel filtering"<<endl;
cout<<"num bytes in original cloud data = "<<pcl1_ptrA->points.size()<<endl;
cout<<"num bytes in filtered cloud data = "<<pcl1_ptrB->points.size()<<endl; // ->data.size()<<endl;
Eigen::Vector4f xyz_centroid;
pcl::compute3DCentroid (*pcl1_ptrB, xyz_centroid);
float curvature;
Eigen::Vector4f plane_parameters;
pcl::computePointNormal(*pcl1_ptrB, plane_parameters, curvature); //pcl fnc to compute plane fit to point cloud
Eigen::Affine3f A(Eigen::Affine3f::Identity());
pcl::transformPointCloud(*pcl1_ptrB, *pcl1_ptrC, A);
However, if I instead use pcl::PCLPointCloud2 objects, e.g.:
pcl::PCLPointCloud2::Ptr pcl2_ptrA (new pcl::PCLPointCloud2 ());
pcl::PCLPointCloud2::Ptr pcl2_ptrB (new pcl::PCLPointCloud2 ());
pcl::PCLPointCloud2::Ptr pcl2_ptrC (new pcl::PCLPointCloud2 ());
This function works:
pcl::VoxelGrid<pcl::PCLPointCloud2> vox;
vox.setInputCloud(pcl2_ptrA);
vox.setLeafSize(0.02f, 0.02f, 0.02f);
vox.filter(*pcl2_ptrB);
But these do not even compile:
//the next 3 functions do NOT compile:
Eigen::Vector4f xyz_centroid;
pcl::compute3DCentroid (*pcl2_ptrB, xyz_centroid);
float curvature;
Eigen::Vector4f plane_parameters;
pcl::computePointNormal(*pcl2_ptrB, plane_parameters, curvature);
Eigen::Affine3f A(Eigen::Affine3f::Identity());
pcl::transformPointCloud(*pcl2_ptrB, *pcl2_ptrC, A);
I am having trouble discovering which functions accept which objects. Ideally, wouldn't all PCL functions accept pcl::PCLPointCloud2 arguments?

Answer 1

pcl::PCLPointCloud2 is a ROS (Robot Operating System) message type replacing the old sensors_msgs::PointCloud2. Hence, it should only be used when interacting with ROS. (see an example here)
If needed, PCL provides two functions to convert from one type to the other:
void fromPCLPointCloud2 (const pcl::PCLPointCloud2& msg, cl::PointCloud<PointT>& cloud);
void toPCLPointCloud2 (const pcl::PointCloud<PointT>& cloud, pcl::PCLPointCloud2& msg);
Extra Information
fromPCLPointCloud2 and toPCLPointCloud2 are PCL library functions for conversions. ROS has wrappers for those functions in pcl_conversions/pcl_conversions.h that you should use instead. These will call the right combinations of functions to convert between the message and templated format.

Answer 2

As a follow-up for what Albert said, For ROS Hydro and later (if you are using ROS), PCL has done a work to remove all its dependencies from ROS. What this means is the pcl has created a set of classes which are almost identical to the corresponding ROS messages, in order to minimize API breakage for pcl users. PointCloud users using "now depracated" sensor_msgs::PointCloud2 are now asked to use pcl_conversions package, this package implements conversions from/to sensor_msgs::PointCloud2 to/from) pcl::PointCloud and should include :
#include <pcl_conversions/pcl_conversions.h>
and the ROS code should be modified as the following :
void foo(const sensor_msgs::PointCloud2 &pc)
{
pcl::PCLPointCloud2 pcl_pc;
pcl_conversions::toPCL(pc, pcl_pc);
pcl::SomePCLFunction(pcl_pc);
...
}
Also, pcl is no longer packaged by the ROS community as a catkin package, so any packages which directly depend on pcl should instead use the new rosdep rules libpcl-all and libpcl-all-dev and follow the PCL developer's guidelines for using PCL in your CMake. Generally this means that a package.xml will change like this:
find_package(PCL REQUIRED)
...
include_directories(${PCL_INCLUDE_DIRS})
...
target_link_libraries(<YOUR_TARGET> ${PCL_LIBRARIES})
More on the migration rules can be found here, here and here.