Contour plots illustrating different properties of octree grids (left panels) and k-d tree grids (right panels), corresponding as a function of the δ_max and τ_max threshold values. The solid lines in each panel correspond to lines of equal numbers of grid cells, whereas the dashed lines correspond to iso-quality contours, corresponding to the density quality metric Δρ (top panels) and the optical depth quality metric Δτ (bottom panel).