How to get the data out of a .vtu file?

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1

I have some data from a 3D-Lidar scan in a.vtu file (a snapshot of a single frame) and I’d like to be able to access them in a programming environment (This is why I am using VTK). I am talking PRIMITIVE access (e.g. a std::array< vector3d > that ONLY contains the points (no intensity or anything like that) or something like that.)

I found this example: https://kitware.github.io/vtk-examples/site/Cxx/IO/ReadUnstructuredGrid/ that reads a .vtu file and renders the scene and that works without a problem. However, a class called “vtkXMLUnstructuredGridReader” just reads the data for itself and this data is passed to the renderer under the hood without me being able to access this data. I tried to get to the data by calling “getOutput” on the reader, but that leaves me with a vtkUnstructuredGrid object which I don’t know how to handle. I looked at the documentation (https://vtk.org/doc/nightly/html/classvtkUnstructuredGrid.html) and I am not entirely sure how to get to the 3D points.

2

You could have something that looks like below. I’m showcasing what you can do with arrays in VTK, and how to have a handle on that from the reader…

#include <vtkDataArray.h>
#include <vtkDataArrayRange.h>
#include <vtkNew.h>
#include <vtkPoints.h>
#include <vtkUnstructuredGrid.h>
#include <vtkXMLUnstructuredGridReader.h>

#include <algorithm>
#include <vector>

int main()
{
  vtkNew<vtkXMLUnstructuredGridReader> reader;
  const char* fileName = "a.vtu";
  reader->SetFileName(fileName);
  reader->Update();

  vtkUnstructuredGrid* ug = vtkUnstructuredGrid::SafeDownCast(reader->GetOutputDataObject(0));
  vtkPoints* points = ug->GetPoints();
  if (points)
  {
    // Here are my points
    for (vtkIdType pointId = 0; pointId < points->GetNumberOfPoints(); ++pointId)
    {
      double p[3];
      points->GetPoint(pointId, p);
      // Do something
    }
    // Here's a handle to a vtkDataArray holding the points.
    vtkDataArray* pointsArray = points->GetData();
    if (pointsArray)
    {
      // Interfacing the point array with ranges on which you can call STL algorithms
      auto pointsRange = vtk::DataArrayTupleRange<3>(pointsArray);
      using ConstTupleRef = typename decltype(pointsRange)::ConstTupleReferenceType;
      using ConstCompRef = typename decltype(pointsRange)::ConstComponentReferenceType;
      for (ConstTuplesRef point : pointsRange)
      {
        for (ConstCompRef comp : point)
        {
           // Here I have access to the coordinates.
        }
      }

      // Copying into a std::vector
      // Look at vtkArrayDispatch to have a type dependent access.
      std::vector<double> dest(points->GetNumberOfPoints() * 3);
      std::copy(pointsRange.begin(), pointsRange.end(), dest.begin());
    }
  }
  return EXIT_SUCCESS;
}

Edit: I didn’t try to compile this code, but it’s compile-ready near enough.

1 Like
3

Thanks, this was very helpful