You are right!, right now I am only performing the regression tests in 'test_imwarp.py' and these tests use the 'demons' step only (actually, ANTS only performs the demons step). A much better way to test the steps was suggested by Dr. Demian Wasserman: compare the analitical energy gradients against the numerical (finite-differences) ones. I still need to implement that, but for now I will add new regression tests to cover the Gauss-Newton step.