  Spectral synthesis calculations based on three-dimensional stellar atmosphere
models are limited by the affordable angular resolution of the radiation field.
This hampers an accurate treatment of rotational line broadening. We aim to
find a treatment of rotational broadening of a spherical star when the
radiation field is only available at a modest number of limb-angles. We apply a
combination of analytical considerations of the line-broadening process and
numerical tests. We obtain a method which is closely related to classical flux
convolution and which performs noticeably better than a previously suggested
procedure. It can be applied to rigid as well as differential rotation.
