code from Gianni Schena.  in his words:

This time is my turn to share a simple (yet simplistic) routine that I 
found of great help to show the relationship between the traditional FBP 
and iterative methods to my students when I present micro tomography as a 
NDE method.

The routine Grid2Glinear is a surrogate of your Glinear routine bur rather 
that building G from scratch it takes the back projection grids used by 
most FBP routines and >explodes< it into a G sparse matrix.  It is also 
quite fast. Just run the demo.