vtkPolyDataToPointCloud does not color cloud

The function vtkPolyDataToPointCloud allows the user to convert a mesh to a colored point cloud; but the color information is not correctly handled. The result is a black point cloud.

Test code

#include <iostream>
#include <pcl/common/common.h>
#include <pcl/io/vtk_lib_io.h>
#include <pcl/io/vtk_io.h>
#include <pcl/io/ply_io.h>
#include <pcl/visualization/pcl_visualizer.h>
#include <vtkPolyDataReader.h>

typedef pcl::PointXYZRGBA PointT;
typedef pcl::PointCloud<PointT> PointCloudT;

int
main (int argc,
      char** argv)
{
  if (argc != 3)
  {
    PCL_ERROR("Usage: %s input.ply output.ply", argv[0]);
    return -1;
  }
  pcl::PolygonMesh mesh;
  vtkSmartPointer<vtkPolyData> polydata = vtkSmartPointer<vtkPolyData>::New ();
  PointCloudT::Ptr cloud (new PointCloudT);
  pcl::visualization::PCLVisualizer viewer ("PCL visualizer");

  if (pcl::io::loadPolygonFilePLY (argv[1], mesh) < 0)
    return -1;

  pcl::io::mesh2vtk (mesh, polydata);
  viewer.addPolygonMesh (mesh, "mesh");
  //viewer.addModelFromPolyData(polydata, "mesh");
  viewer.spin ();
  viewer.removeShape ("mesh");

  pcl::io::vtkPolyDataToPointCloud (polydata, *cloud);
  // FIXME: Alpha is set to 0 (or not set at all)
  for (size_t i = 0; i < cloud->size (); ++i)
    cloud->points[i].a = 255;

  for (size_t i = 0; i < cloud->size () / 100; ++i)
    std::cout << cloud->points[i] << std::endl;

  viewer.addPointCloud (cloud);
  viewer.setBackgroundColor (0.2, 0.2, 0.2);
  viewer.spin ();

  return 0;
}

Result is a black cloud (alpha = 0) without the FIXME comment.

#include <iostream>
#include <pcl/common/common.h>
#include <pcl/io/vtk_lib_io.h>
#include <pcl/io/vtk_io.h>
#include <vtkPolyDataReader.h>
#include <pcl/visualization/pcl_visualizer.h>

int
main (int argc,
      char** argv)
{
  pcl::visualization::PCLVisualizer viewer ("PCL visualizer");

  pcl::PolygonMesh mesh;
  pcl::io::loadPolygonFilePLY(argv[1], mesh);
  viewer.addPolygonMesh(mesh);
  viewer.setBackgroundColor(0.2, 0.2, 0.2);
  while (!viewer.wasStopped())
    viewer.spinOnce();

  pcl::io::saveVTKFile("/home/victor/temp.vtk", mesh);
  vtkSmartPointer<vtkPolyDataReader> reader = vtkSmartPointer<vtkPolyDataReader>::New();
  reader->SetFileName("/home/vitemp.vtk");
  reader->Update();
  vtkSmartPointer<vtkPolyData> polydata = reader->GetOutput();

  viewer.addModelFromPolyData(polydata); // To check that the conversion until here is fine

  pcl::PointCloud<pcl::PointXYZRGBA> cloud;
  pcl::io::vtkPolyDataToPointCloud(polydata, cloud);
  std::cerr << "Cloud:\n" << cloud << std::endl;

  pcl::PCDWriter pcdwriter;
  pcdwriter.write<pcl::PointXYZRGBA>(argv[2], cloud);
  return 0;
}

Start at the vtkPolyDataToPointCloud entry point and run the provided C++ reproducer using the mesh-to-VTK conversion and PCLVisualizer checks. Trace how color and alpha are transferred into the PointXYZRGBA cloud; done means the converted cloud retains its source colors and displays correctly without the FIXME alpha workaround.