Vec3 vtx = hfield_vertex(u, v, calc_height(u, v, ptr), xsz, ysz, height);
/* calculating normal with forward differences:
slopes in x, z, axis */
Vec3 tangent = hfield_vertex(u + du, v, calc_height(u + du, v, ptr),
Vec3 vtx = hfield_vertex(u, v, calc_height(u, v, ptr), xsz, ysz, height);
/* calculating normal with forward differences:
slopes in x, z, axis */
Vec3 tangent = hfield_vertex(u + du, v, calc_height(u + du, v, ptr),