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

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