  Exact conditions for anti-Klein transmission zeros are found analytically
with a 4-component continuum approach which includes trigonal warping.
Anti-Klein tunneling occurs at oblique incidence on steps and barriers with
soft and hard walls as well as in the known case of normal incidence on a hard
step. The necessary energy and angle of incidence depend on the
crystallographic orientation of the step or barrier. At normal incidence on an
armchair step in unbiased bilayer graphene, anti-Klein tunneling occurs because
both the continuum and the tight binding Hamiltonians are invariant under layer
and site interchange. At oblique incidence, anti-Klein tunneling is
valley-dependent even in the absence of trigonal warping. An experimental
arrangement that functions both as a detector of anti-Klein tunneling and a
valley polarizer is suggested. There are cases where anti-Klein tunneling
occurs in the 4-component theory but not in the 2-component approximation.
