  We have written a 3D radiative transfer code that computes emerging spectra
and intensity maps. We derive from radiative hydrodynamic (RHD) simulations of
RSG stars carried out with CO5BOLD (Freytag et al. 2002) observables expected
for red supergiant stars (RSG) especially for interferometric observations,
with emphasis on small scale structures. We show that the convection-related
surface structures are detectable in the H band with today's interferometers
and that the diameter measurement should not be too dependent on the adopted
model. The simulations are a great improvement over parametric models for the
interpretation of interferometric observations.
