I tried to calculate Distance Transform using City Block filter but failed to get required result. please help

I am calculating Distance Transform of binary image using City Block filter,but its not working for me. There must be something wrong i am doing. Please guide me. Here is the code, i am not using the built in functions.

clc;
clear all;
close all;
M =imread('Picture1.png');
cb = [2 1 2;1 0 1;2 1 2];
% M = rand(20,20)*100<1;
[W H D] = size(M);
C = zeros(12,12);
for i=2:W
  for j=2:H
      C(i,j) = M(i-1,j-1);
  end
end
[r c] = size(C);
for i=1:r
  for j=1:c
      if(C(i,j) == 1)
          C(i,j) = 0+50;
      else
          C(i,j) = C(i,j)+50;
      end
  end
end
for i=2:r - 1
  for j=2:c - 1
      lr_tl = C(i-1,j-1) + 2;
      lr_l  = C(i-1,j) + 1;
      lr_bl = C(i-1,j+1) + 2;
      lr_c  = C(i,j) + 0;
      lr_tc = C(i,j-1) + 1;        
      A = [lr_tl lr_l lr_bl lr_c lr_tc];        
      C(i,j) = min(A);       
  end
end
for i=r-1:-1:2
  for j=c - 1:-1:2
      rl_tr = C(i+1,j-1) + 2;
      rl_r  = C(i+1,j) + 1;
      rl_br = C(i+1,j+1) + 2;
      rl_c  = C(i,j) + 0;
      rl_bc = C(i,j+1) + 1;        
      A = [rl_tr rl_r rl_br rl_c rl_bc];        
      C(i,j) = min(A);        
  end
end
[k t] = size(C);
for i=1:k
  for j=1:t
       O(i,j) = abs(C(i,j)-50);
  end
end
% figure();
% imshow(M);
figure();
imshow(C);
figure();
imshow(O);

I am using the following image as input

And this output is required , which i am fail to get it. The output image should be

Please correct me where i am doing wrong process.

thanks

[Walter Roberson]

What are those "50" ?

[Muhammad Sohail]

as in Distance transform you just increase the intensities of pixels with any value as told by our teacher,but still i dont understand what would is the correct way to do that.