[GRASSLIST:2703] r.thin , where am I wrog?

[michele.rocc@libero.it]

Hi,

I tried to vectorize a simple map:
ported to pnm(ppm) with the gimp, from a jpg;
run r.thin, after 5 iterations had a thinned map;
but running r.line I had the message: not thinned properly...

do the map have to be  in a particular format to have a properly
thynned map? 

thanks for help,       Michele.