A Simple Ray Acceleration Structure for Non-LTE Radiative Transfer

Abstract: We present a novel ray acceleration structure for radiative transfer outside of local thermodynamic equilibrium (non-LTE), leveraging techniques from computer graphics to improve computational efficiency. By applying mipmapping (local recursive spatial averaging) and sparse voxel grids, we exploit spatial coherence and sparsity in astrophysical models to accelerate the formal solution of the radiative transfer equation. We introduce a variance-limited mipmapping (VLM) scheme with tunable error control, and extend it to handle anisotropic emission via two methods: velocity interpolation, and so-called "Core and Voigt". Our approach integrates a hierarchical digital differential analyzer (HDDA) for efficient ray traversal, which, combined with the mipmapping scheme achieves an order of magnitude speedup with less than 0.5 % error in the 99.9th percentile of the level populations. These methods are implemented in the DexRT code and demonstrate significant performance gains in realistic solar atmospheric models.