  A reference implementation of a new method in isogeometric analysis (IGA) is
presented. It delivers low-cost variable-scale approximations (surrogates) of
the matrices which IGA conventionally requires to be computed by element-scale
quadrature. To generate surrogate matrices, quadrature must only be performed
on a fraction of the elements in the computational domain. In this way,
quadrature determines only a subset of the entries in the final matrix. The
remaining matrix entries are computed by a simple B-spline interpolation
procedure. We present the modifications and extensions required for a reference
implementation in the open-source IGA software library GeoPDEs. The exposition
is fashioned to help facilitate similar modifications in other contemporary
software libraries.
