Getting sequential pointers for the coordinates of a vtkPoint data - c++

Question

I wrote the following function to store the (x, y, z) of a vtkPoint in an array of type double and size of 3*N, where N is the number of vertices (or points).
double* myClass::getMyPoints(void)
{
double* vertices = new double[this->m_numberOfVertices * 3];
for (vtkIdType ivert = 0; ivert < this->m_numberOfVertices; ivert++)
for (auto i = 0; i < 3; ++i)
this->m_points->GetPoint(ivert, &vertices[3 * ivert]);
return vertices;
}
where m_points is a member of myClass and is of type vtkSmartPointer<vtkPoints>.
This function does what I want and works just fine. I was wondering if there is an elegant way of getting the sequential pointers. I tried GetVoidPointer(), which looks like an elegant one-line code, to avoid the for loop but it does not get the coordinates correctly after the function returns vertices.
(double*)(m_points->GetData()->GetVoidPointer(0));
Could someone help me with this?

Answer 1

vtkPoints internally stores it's data as a float array instead of a double array. So you may need to modify your function to work with float* instead of double*. If we want to use double array for vtkPoints then we should call SetDataTypeToDouble() on the vtkPoints object.
#include <stdio.h>
#include <stdlib.h>
#include <vtkPoints.h>
#include <vtkSmartPointer.h>
int main(){
// Create data
auto N = 5;
vtkNew<vtkPoints> pts;
pts->SetDataTypeToDouble();
for(auto i=0; i < N; ++i)
pts->InsertNextPoint(rand()%100,rand()%100,rand()%100);
// Read using for loop
std::cout<< "Using for loop ... " << std::endl;
for( auto j=0; j < N; ++j ){
double p[3];
pts->GetPoint( j, p );
std::cout<< p[0] << "," << p[1] << "," << p[2] << std::endl;
}
// Read using GetVoidPointer()
std::cout<< "Using GetVoidPointer() ... " << std::endl;
auto data_ptr = (double*) pts->GetData()->GetVoidPointer(0);
for( auto k = 0; k < N; ++k )
std::cout<< *(data_ptr + 3*k) << ","
<< *(data_ptr + 3*k + 1) << ","
<< *(data_ptr + 3*k + 2) << std::endl;
return 0;
}
This gives result as follows:
Test that there are N = 5 tuples.
Using for loop ...
83,86,77
15,93,35
86,92,49
21,62,27
90,59,63
Using GetVoidPointer() ...
83,86,77
15,93,35
86,92,49
21,62,27
90,59,63