Hi,

I would like to create the following two overloads for an OpenGL helper function:

void GLVertex(const Eigen::Vector2f &vec) { glVertex2f(vec.x(),vec.y()); }
void GLVertex(const Eigen::Vector3f &vec) { glVertex3f(vec.x(),vec.y(),vec.z()); }

I quickly discovered that calling e.g. GLVertex(a+b) doesn't work because the type of a+b is a CwiseBinaryOp.
I understand that I should instead be defining these functions as receiving MatrixBase<Derived>. But how can I write these two functions using MatrixBase<Derived> but still let the compiler figure out which one to call??

Thank you

You can use a helper struct with a default template argument and partial specialization to choose between the different functions:

Code: Select all

template <typename Derived, int RowsAtCompileTime = Derived::RowsAtCompileTime> struct GLVertex_impl;

template <typename Derived> struct GLVertex_impl<Derived, 2>
{
  static void run(const Eigen::MatrixBase<Derived> &vec) { glVertex2f(vec.x(),vec.y()); }
};
 
template <typename Derived> struct GLVertex_impl<Derived, 3>
{
  static void run(const Eigen::MatrixBase<Derived> &vec) { glVertex3f(vec.x(),vec.y(),vec.z()); }
};
 
template <typename Derived> void GLVertex(const Eigen::MatrixBase<Derived> &vec) { GLVertex_impl<Derived>::run(vec); }


I think boost::enable_if can also be used for this, but I don't have experience with it.

A different approach is to always evaluate the expression. The .eval() member function takes an expression (like a CwiseBinaryOp) and evaluates it into a Matrix.

Code: Select all

void GLVertex_impl(const Eigen::Vector2f &vec) { glVertex2f(vec.x(),vec.y()); }
void GLVertex_impl(const Eigen::Vector3f &vec) { glVertex3f(vec.x(),vec.y(),vec.z()); }

template <typename Derived> void GLVertex(const Eigen::MatrixBase<Derived> &vec) { GLVertex_impl(vec.eval()); }


You can also have a look at the "unsupported/Eigen/OpenGLSupport" file, to see how we dealt with that issue. This file contains many wrappers to OpenGL functions.

Thanks. Too bad it requires such awkward wrappers, but glad to see that there are some ready-made OpenGL wrappers available for my use.