how to converts a gray scale image (All pixels from 0 to 255) to a black and white image (All pixels either 0 or 1)?

this my given code, and I want to complete it by one of clustering methods :

please read the code, and give me the complementary code.

thanx in advance..

% read image matrix from file
Img=imread('C:\Users\sony\Desktop\fp.png');
% convert to gray scale (remove color info)
Img = rgb2gray(Img);
% show the fingerprint image in a window
figure(1); imshow(Img);
% convert image data to double precision format
ImgD=double(Img);
% convert matrix data to single column vector format
% namely arrange all numbers in the matrix as a single column vector
grades=ImgD(:);
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% THIS IS YOUR BEGINNING OF HOMEWORK OR EXAM OR PROJECT ...
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% DO YOUR CLUSTERING HERE !
% For illustrative purposes, I compute the median, and use it to decide pass
% or fail. But you should do clustering and decide about the threshold !
t = median(grades);
% This selection will not result a good B&W image at the end.
% Your method should result a better B&W image !
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% END OF YOUR HOMEWORK OR EXAM OR PROJECT ...
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% generate a new image matrix in memory
newImg = zeros(size(Img));
for row=1:size(Img,1)
    for col=1:size(Img,2)
        if (ImgD(row,col) < t)
            newImg(row,col) = 0; % below threshold fails
        else
            newImg(row,col) = 1; % above threshold passes
        end
    end
end
figure(2); imshow(newImg);

[Jos (10584)]

So, what did you try yourself so far for this

%%%%%%%%%%%%
% HOMEWORK %
%%%%%%%%%%%%
project?

[Ibrahim Alkhalifah]

I try to use kmeans function:

t=kmeans (Img(:),3)

but the image was bad!!

[Jos (10584)]

by the way, the two for-loops in the end can (and should) be replaced by the one-liner:

newImg = double(ImgD >= t) ;

[Ibrahim Alkhalifah]

this error is appeared!! :

Error using  >= 
Matrix dimensions must agree.