Uniform background test. Left: Illustration of the distribution of background sources within the simulation box for 128 background emitters. The virtual background emitters are distributed along a spiral on a sphere centred on the centre of the simulation box; the point size corresponds to the distance from the viewer. Right: Incident radiation field at 13.6⁺ eV (HI-low) as experienced by gas particles in the uniform background test in a thin slice through the centre of the simulation box for the Haardt & Madau (2012) UVB at redshift 0 and 128 background sources. The dashed, dot-dashed, and dotted circles indicate where the radiation field is within 10%, 25%, and 50% of the expected central flux, respectively. The corresponding radii are, from the centre out, [0.26, 0.36, 0.43]×L_box.