SparseQRMatrixQReturnType should inherit EigenBase and provide an evalTo member to be converted to a plain SparseMatrix.
Also, investigate what need to be done to be able to exploit more features of the Householder module.
An evaluation to a plain sparse matrix is provided with this changeset
Date: 2013-05-21 17:35:10
Summary: Fix bug 596 : Recover plain SparseMatrix from SparseQR matrixQ()
Affected #: 5 files
Let's differ the second part to 3.3:
> Also, investigate what need to be done to be able to exploit more features of
> the Householder module.
matrixQ() currently relies on SparseQR_QProduct which is implemented with dense RHS in mind. This makes matrixQ() quite inefficient. The solution is to extend SparseQR_QProduct to handle sparse RHS using the same approach than during the factorization. There is a nice opportunity for code sharing here.
see also: http://forum.kde.org/viewtopic.php?f=74&t=117500&p=293605#p292860