  Direct photon production in central Pb+Pb collisions at CERN-SPS energy is
calculated within the relativistic microscopic transport model UrQMD, and
within distinctly different versions of relativistic hydrodynamics. We find
that in UrQMD the local momentum distributions of the secondaries are strongly
elongated along the beam axis initially. Therefore, the pre-equilibrium
contribution dominates the photon spectrum at transverse momenta above $\approx
1.5$ GeV. The hydrodynamics prediction of a strong correlation between the
temperature and radial expansion velocities on the one hand and the slope of
the transverse momentum distribution of direct photons on the other hand thus
is not recovered in UrQMD. The rapidity distribution of direct photons in UrQMD
reveals that the initial conditions for the longitudinal expansion of the
photon source (the meson ``fluid'') resemble rather boostinvariance than
Landau-like flow.
