  An arbitrarily-shaped optical potential on a discrete photonic lattice, which
transversely drifts at a speed larger than the maximum one allowed by the light
cone of the lattice band, becomes reflectionless. Such an intriguing result,
which arises from the discrete translational symmetry of the lattice, is
peculiar to discretized light and does not have any counterpart for light
scattering in continuous optical media. A drifting non-Hermitian optical
potential of the Kramers-Kronig type is also an invisible potential, i.e. a
discrete optical beam crosses the drifting potential without being distorted,
delayed nor advanced.
