Accessing Template values in Eigen using EIGEN_MATRIXBASE_PLUGIN

Question

I am currently trying to adapt the Eigen API to support the following function:

matrix.similarity(similarityTransformationMatrix);

which simply does the following:

similarityTransformationMatrix * matrix * similarityTransformationMatrix.transpose();

My previous implementation only allows to take square matrices for both:

inline MatrixBase< Derived > similarity( const MatrixBase< Derived >& m) const {
  MatrixBase< Derived > t = m;
  t.transposeInPlace();
  return m*(*this)*t;
}

I need instead to be able to call this on a (square, but check for cols = rows not required) matrix, and a similarityTransformation of a different dimension. My problem is that the return matrix is not of the type of either of those, but it can be derived:

Matrix< double, similarityTransformation.rows(), similarityTransformation.rows()>

Could you give me pointers how to implement this? I didn't try implementing it as Eigen::Matrix but I'd rather stick to MatrixBase if possible, but MatrixBase didn't want to accept multiple arguments for the return type of the form

MatrixBase< OtherDerived::Scalar, OtherDerived::RowsAtCompileTime, OtherDerived::RowsAtCompileTime>

ggael ggael · Accepted Answer · 2013-11-05T12:27:34

First of all, you should notice that you cannot create object of type MatrixBase<...>. MatrixBase is just an abstract base class. In your case, t and the return type of similarity should be a Matrix<...> object.

Second, the template parameters of Matrix<...> must be known at compile time. This means you cannot use similarityTransformation.rows() for that purpose but use TYPEOF_similarityTransformation::RowsAtCompileTime.

Third, your similarity() method must be templated to accept any other Eigen's expression.

Here is a proposal:

template<typename OtherDerived>
Matrix<Scalar, OtherDerived::RowsAtCompileTime, OtherDerived::RowsAtCompileTime>
similarity(const MatrixBase<OtherDerived>& S) const
{
  return S * ( this->derived() * S.adjoint() );
}

This example could be further extended to support mixing reals with complexes... (only Scalar has to be changed to something more complicated for type promotion..)

Accessing Template values in Eigen using EIGEN_MATRIXBASE_PLUGIN

1 Answers