How to resize a Vector in Eigen3

Question

I'm wrapping two Eigen3 vectors in a templated LineSegment<T,dim> class. You might use it like this:

typedef LineSegment<double,2> LineSegment2d;
typedef LineSegment<double,3> LineSegment3d;
typedef LineSegment<int,3> LineSegment3i;

It contains a templated method to change the dimensions of the components. Here is the trimmed definition:

template<typename T,int dim>
struct LineSegment
{
public:
  template<int newDim>
  LineSegment<T,newDim> to() const
  {
    Eigen::Matrix<T,newDim,1> newp1;
    Eigen::Matrix<T,newDim,1> newp2;

    // TODO initialise newp1 and newp2 from d_p1 and d_p2

    return LineSegment<T,newDim>(newp1, newp2);
  }

  // ... other members ...

protected:
  Eigen::Matrix<T,dim,1> d_p1;
  Eigen::Matrix<T,dim,1> d_p2;
}

So my question is, how can I compose the return value, as shown above? This should support both increasing and decreasing the dimension.

I tried using the Eigen3 resize(int) method, but couldn't get it to work without seeing warnings about mixing matrix sizes.

Ultimately, this should work:

LineSegment2d ls2d;
LineSegment3d ls3d = ls2d.to<3>(); // increase dim
ls2d = ls3d.to<2>();               // decrease dim

I'm relatively new to C++ templates, and would appreciate a bit of explanation if this isn't just an API question and is related to templates.

Answer 1

Firstly, Eigen's resize method reallocates memory if the new number of elements is not the same as the old, both when growing and when shrinking, so you would lose data in this case

The following method uses .head<int>(), which is Eigen3's version of .start<int>(), plus some template programming so you don't have to check whether you're shrinking or growing:

#include <Eigen/Core>

template <bool COND, int A, int B>
struct IF
{
  enum { val = A };
};

template <int A, int B>
struct IF<false, A, B>
{
  enum { val = B };
};

template <int A, int B>
struct MIN : IF<A < B, A, B>
{
};

template <typename T,int dim,int newDim>
Eigen::Matrix<T,newDim,1> to(Eigen::Matrix<T,dim,1> p)
{
  Eigen::Matrix<int,newDim,1> newp =
    Eigen::Matrix<T,newDim,1>::Zero();

  newp.template head< MIN<dim,newDim>::val >() =
    p.template head< MIN<dim,newDim>::val >();

  return newp;
}

Using this, the following program:

#include <iostream>

int main()
{
  Eigen::Vector2i p_2i(1,2);
  Eigen::Vector3i p_3i(3,4,5);

  std::cout << to<int, 2, 3>(p_2i) << std::endl << std::endl;
  std::cout << to<int, 3, 2>(p_3i) << std::endl << std::endl;

}

Gives as output:

1
2
0

3
4