hwo to use pcl to achieve that discretizing the 3D point cloud into a 2D grid over the xy plane, not use the voxelgrid? - point-cloud-library

Question

I want to project the 3D point cloud into a 2D grid over the xy plane, each grid cell size is 20cm*20cm, how to achieve it effectively?
NOT use VoxelGrid method, because I want to retain every point and deal with them in the next step(Gaussian kernel every column and use EM to deal with each grid)

Answer 1

As discussed in the comments, you can achieve what you want with OctreePointCloudPointVector class.
Here is an example how to use the class:
#include <pcl/point_cloud.h>
#include <pcl/io/pcd_io.h>
#include <pcl/octree/octree_pointcloud_pointvector.h>
using Cloud = pcl::PointCloud<pcl::PointXYZ>;
using CloudPtr = Cloud::Ptr;
using OctreeT = pcl::octree::OctreePointCloudPointVector<pcl::PointXYZ>;
int main(int argc, char** argv)
{
if(argc < 2)
return 1;
// load cloud
CloudPtr cloud(new Cloud);
pcl::io::loadPCDFile(argv[1], *cloud);
CloudPtr cloud_projected(new Cloud(*cloud));
// project to XY plane
for(auto& pt : *cloud_projected)
pt.z = 0.0f;
// create octree, set resolution to 20cm
OctreeT octree(0.2);
octree.setInputCloud(cloud_projected);
octree.addPointsFromInputCloud();
// we gonna store the indices of the octree leafs here
std::vector<std::vector<int>> indices_vec;
indices_vec.reserve(octree.getLeafCount());
// traverse the octree leafs and store the indices
const auto it_end = octree.leaf_depth_end();
for(auto it = octree.leaf_depth_begin(); it != it_end; ++it)
{
auto leaf = it.getLeafContainer();
std::vector<int> indices;
leaf.getPointIndices(indices);
indices_vec.push_back(indices);
}
// save leafs to file
int cnt = 0;
for(const auto indices : indices_vec)
{
Cloud leaf(*cloud, indices);
pcl::io::savePCDFileBinary("leaf_" + std::to_string(cnt++) + ".pcd", leaf);
}
}
You can see the output by calling pcl_viewer:
pcl_viewer leaf_*.pcd
See sample output

Answer 2

You can achieve this using https://github.com/daavoo/pyntcloud with the following code:
from pyntcloud import PyntCloud
cloud = PyntCloud.from_file("some_cloud.ply")
# 0.2 asumming your point cloud units are meters
voxelgrid_id = cloud.add_structure("voxelgrid", size_x=0.2, size_y=0.2)
voxelgrid = cloud.structures[voxelgrid_id]
You can learn more about VoxelGrid here:
https://github.com/daavoo/pyntcloud/blob/master/examples/%5Bstructures%5D%20VoxelGrid.ipynb

Answer 3

What do you mean with 2D grid over the xy plane? Do you still want the z value to be the original value, or do you want to project the point cloud to the XY plane first?
Keep Z value
If you want to keep the Z values, just set the leaf size for Z of VoxelGrid to infinite (or a very large number).
pcl::VoxelGrid<pcl::PCLPointCloud2> sor;
sor.setInputCloud (cloud);
sor.setLeafSize (0.01f, 0.01f, 100000.0f);
sor.filter (*cloud_filtered);
Project Cloud to XY plane first
Project a cloud to the XY plane is nothing else than setting the Z value for each point to 0.
for(auto& pt : cloud)
pt.z = 0.0f;
Now you can do normal VoxelGrid on the projected point cloud.