Comparison of the grid quality metrics for octree (stars) and k-d-tree (dots) based grids of the spiral galaxy model, using different node subdivision stopping criteria. The blue lines correspond to a mass threshold only, the red lines lines to an optical depth threshold only, and the green lines to a combination of both criteria. The different panels represent the average number of cells crossed per path ⟨ N_cross ⟩, the density quality Δρ, and the optical depth quality metric Δτ as a function of the total number of cells in the grid, N_cells.