gf_exp4
nxx = gf_exp4(ncols - x - 1);
nyy = gf_exp4(ncols - y - 1);
nzz = gf_exp4(ncols - z - 1);
coeff[MUL_R_X] = gf_exp4(255 - (ncols - x - 1));
a = gf_exp4(x + 255 - y);
b = gf_exp4(255 - (ncols - x - 1));