draw 3 d surface of fuzzy relation

I want to draw 3D surface of fuzzy relation between error .change of error and control signal. How to do it ? should i take value for the input with step and run it. i wrote the fuzzy in m file i didn't use the toolbox functions

I write the membership function in mfile. but the resulted plot is flat. and workin at constant output

     x1=-3:.3:3%%% 1st input
     x2=-2:.2:2%%%2nd input
     for t=1:1:21
         % % % negative big
if x1(t)<=b
    NBU=1;
else if x1(t)>=b&&x1(t)<=d
        NBU=d-x1(t)/(d-b);
        else NBU=0;
    end
end

for i=1:1:21
    for j=1:1:21
 
 [y,yLeft(i),yRight(j),L,R]=EIASC(Xl,Xr,Wl,Wr,needSort);%%%inference engine

%%%%%%%%%%%%%%%%%%%%%%%output 
 cu(i,j)=(yLeft(i)+yRight(j))/2; %%%% fuzzy output 
    end 
end
    end

 surf(x1,x2,cu)