Overview of the procedure for converting a probability distribution over total (i.e. baryonic and dark) matter density fields into a probability distribution over monochromatic emission coefficient (MEC) fields. We show a fixed slice through ℛ, of 750 Mpc h⁻¹ × 750 Mpc h⁻¹ for the density and MEC fields, and a subregion of 150 Mpc h⁻¹ × 150 Mpc h⁻¹ for the Gaussian and percentile random fields (to more clearly illustrate the bijective mapping between the two). Left: BORG SDSS density ρ sample, in multiples of the current-day mean total matter density . Middle left: Gaussian random field 𝒵 sample using a squared exponential (SE) kernel with lengthscale l_SE = 2 Mpc. Middle right: the corresponding percentile random field 𝒫 sample. Right: the corresponding MEC j_ν sample at an emission frequency of ν = 150 MHz, with the weakly-constrained electron acceleration efficiency ξ_e divided out.