  Interpretation of resolved polarized images of black holes by the Event
Horizon Telescope (EHT) requires predictions of the polarized emission
observable by an Earth-based instrument for a particular model of the black
hole accretion system. Such predictions are generated by general relativistic
radiative transfer (GRRT) codes, which integrate the equations of polarized
radiative transfer in curved spacetime. A selection of ray-tracing GRRT codes
used within the EHT collaboration is evaluated for accuracy and consistency in
producing a selection of test images, demonstrating that the various methods
and implementations of radiative transfer calculations are highly consistent.
When imaging an analytic accretion model, we find that all codes produce images
similar within a pixel-wise normalized mean squared error (NMSE) of 0.012 in
the worst case. When imaging a snapshot from a cell-based magnetohydrodynamic
simulation, we find all test images to be similar within NMSEs of 0.02, 0.04,
0.04, and 0.12 in Stokes I, Q, U , and V respectively. We additionally find the
values of several image metrics relevant to published EHT results to be in
agreement to much better precision than measurement uncertainties.
