gf_exp2
nx = gf_exp2(ncols - x - 1);
ny = gf_exp2(ncols - y - 1);
nx = gf_exp2(ncols - x - 1);
ny = gf_exp2(ncols - y - 1);
nz = gf_exp2(ncols - z - 1);
coeff[MUL_Q_X] = gf_exp2(255 - (ncols - x - 1));
a = gf_exp2(x + 255 - y);
b = gf_exp2(255 - (ncols - x - 1));