SEM images of the six representative ROIs, including (a) a flat surface, (b) a curved surface, (c) a vesicular surface, (d) a lineated surface, (e) a rectangular surface, and (f) a rough surface. In panels (a)–(b) and (d)–(f), features with preferred directionality are denoted by solid thick arrows. In panels (a)–(f), protruded and dented features are marked by solid thin arrows and dashed thin arrows, respectively. These six ROIs correspond to Nos. 9, 11, 21, 32, 35, and 42 in Table A1.