if (argparser_keyword_present(pp, "-matrix"))
      { /* Parse the nine entries of the projective matrix, row order: */
        r3x3_t A;
        int i, j;
        for (i = 0; i < 3; i++)
          { for (j = 0; j < 3; j++)
              { A.c[i][j] = argparser_get_next_double(pp, -BIG, +BIG); }
          }
        o->M.dir = A;
        r3x3_inv(&A, &(o->M.inv));
      }
    else if (argparser_keyword_present(pp, "-points"))
      { hr2_point_t ip[4], op[4];
        /* Parse the four input points {ip[0..3]}: */
        int i;
        for (i = 0; i < 4; i++)
          { ip[i].c.c[0] = 1.0;
            ip[i].c.c[1] = argparser_get_next_double(pp, -BIG, +BIG);
            ip[i].c.c[2] = argparser_get_next_double(pp, -BIG, +BIG);
          }
        /* Parse the four output points {op[0..3]}: */
        for (i = 0; i < 4; i++)
          { op[i].c.c[0] = 1.0;
            op[i].c.c[1] = argparser_get_next_double(pp, -BIG, +BIG);
            op[i].c.c[2] = argparser_get_next_double(pp, -BIG, +BIG);
          }
        hr2_pmap_t IM = hr2_pmap_from_four_points(&(ip[0]), &(ip[1]), &(ip[2]), &(ip[3]));
        hr2_pmap_t OM = hr2_pmap_from_four_points(&(op[0]), &(op[1]), &(op[2]), &(op[3]));
        r3x3_mul(&(IM.inv), &(OM.dir), &(o->M.dir));
        r3x3_mul(&(OM.inv), &(IM.dir), &(o->M.inv));
      }
    else 
      { r3x3_ident(&(o->M.dir));  r3x3_ident(&(o->M.inv)); }

/* ---------------------------------------------------------------------- */