Microstructures illustrations. (a) Boolean schemes of spheres generated in a cube where straightforward definition of propagation direction for tortuosity calculations is possible. (b) real γ-alumina sample, imaged by electron tomography (resolution around 1 nm·vx⁻¹, vx standing for voxel), where a proper propagation direction is delicate to define. Volume (a) was generated using [35] and volumes (a) and (b) were rendered using [35].