How to do a Algoritm of globals' thresholding segmentation?

Hi:

I must do a practice of a globals' thresholding segmentation for one leason at university, and I'm a little bit lost.

This is the code if have done, but it has a bug...Could some one help me?

im=imread('C:\Users\Fran\Desktop\Universidad\Ing. Industrial\5º AÑO\1º Cuatrimestres\Vision por computador\Imagenes\texto.jpg');
U=graythresh(im);
US=im2double(im);
UI=im2bw(US);
U0=0.5;
[nfilas, ncolumnas]=size(UI);
tic
while U<U0
    G1M=0;G1=0;
    G2M=0;G2=0;
    for i=1:nfilas
        for j=1:ncolumnas
            U1=im(i,j);
            if U1>=U
                    G1=G1+U1;
                    G1M=G1M+1;
                    im(i,j)=1;
             end
             if U1<U
                    %G2A=G2;
                    G2=G2+U1;
                    G2M=G2M+1;
                    im(i,j)=0;
             end
        end
        M1=G1/G1M;
        M2=G2/G2M;
        U=0.5*(M1+M2);

      end
     figure, imshow(im); 
  end
  toc;

Thanks a lot

[Matt J]

PLease provide information about the bug, i.e., how you know there is one. If there are error messages, copy/paste them in full.

Answer 1

[Image Analyst]

You don't need that for loop. Simply do

binaryImage = im>U; % Binarize.
G1M = sum(im(binaryImage)); % Sum gray levels above threshold (for some reason???)
G2M = sum(im(~binaryImage)); % Sum gray levels below threshold (for some reason???)

Not sure what G1M and G2M were computed for.

[Image Analyst]

Francisco. Nice try but not very efficient. See my code attached below.

Answer 2

[Francisco]

the exercice is:

Other way to chose the threshold "Automaticly", is doing the next steps:

1. Chose one initial threshold U, it's recommended to be betwen maximun and minimum intensity levels of the image.
2. Do two groups, G1 pixel with a value of intensity >=U,and G2 pixel with value intensity <U.
3. Calculate average intensity m1 and m2 for pixels of G1 and G2.
4. Calculate new threshold: U=0.5(m1+m2),
5. Repeat steps 2 and 4, until the diference in U in diferents iterations are nether than U0=0.5