  We analyze the transverse single-spin asymmetry in direct photon production
from proton-proton collisions, denoted $A_N^\gamma$, within collinear twist-3
factorization. We provide a calculation of the contribution due to
quark-gluon-quark correlations in the unpolarized proton as well as summarize
previous studies on those effects in the polarized proton. Both soft-gluon
poles and soft-fermion poles are considered. From this complete result we then
estimate $A_N^\gamma$, including error bands due to uncertainties in the
non-perturbative inputs, at kinematics relevant for planned measurements of
this observable at the Relativistic Heavy Ion Collider. We find $A_N^\gamma$
can allow for a "clean" extraction of the Qiu-Sterman function, which could
lead to a definitive solution to the so-called "sign mismatch" crisis. Since we
use the Sivers function extracted from semi-inclusive deep-inelastic scattering
to develop our input for the Qiu-Sterman function, this reaction can also make
a statement about the process dependence of the Sivers function.
