  In the context of ultra-relativistic nuclear collisions, we present a fast
method for calculating the final particle spectra after the direct decay of
resonances from a Cooper-Frye integral over the freeze-out surface. The method
is based on identifying components of the final particle spectrum that
transform in an irreducible way under rotations in the fluid-restframe.
Corresponding distribution functions can be pre-computed including all
resonance decays. Just a few of easily tabulated scalar functions then
determine the Lorentz invariant decay spectrum from each space-time point, and
simple integrals of these scalar functions over the freeze-out surface
determine the final decay products. This by-passes numerically costly
event-by-event calculations of the intermediate resonances. The method is of
considerable practical use for making realistic data to model comparisons of
the identified particle yields and flow harmonics, and for studying the viscous
corrections to the freeze-out distribution function.
