why my binariation image appears like this ?

hi evreyone please someone can tel me why my image after adaptive thres operation appears like that and how can i remove this white boundary ??

the code:

function [o] = adaptiveThres(a,W,noShow)
W=16;
%adaptiveThres(a,W);
[w,h] = size(a);
o = zeros(w,h);
%seperate it to W block
%step to w with step length W
for i=1:W:w
for j=1:W:h
mean_thres = 0;
%white is ridge -> large

if i+W-1 <= w & j+W-1 <= h
   	mean_thres = mean2(a(i:i+W-1,j:j+W-1));
   	%threshold value is choosed
      mean_thres = 0.8*mean_thres;
      %before binarization
      %ridges are black, small intensity value -> 1 (white ridge)
      %the background and valleys are white, large intensity value -> 0(black)
      o(i:i+W-1,j:j+W-1) = a(i:i+W-1,j:j+W-1) < mean_thres;
end;
end;
end;
if nargin == 2
imshow(o);
imwrite(o,'C:\Users\home\Documents\PFE\method\BINimg.jpg');
end

thanks for helping

[Walter Roberson]

%seperate it to W block

That should tell you right there why you are having the problem. You are dividing the image up into W blocks by W blocks, and processing each one separately. The result is going to be blocky. You could reduce the size of the white border by increasing W.