how to select region with longest length on segmented image ?

I have segmented the image like the following: and within the boundaries I want to select the boundary with the longest length from the boundaries extracted from the image like the image below: help me please.

% Initialization Steps.

clc; % Clear the command window.

close all; % Close all figures (except those of imtool.)

clear; % Erase all existing variables. Or clearvars if you want.

workspace; % Make sure the workspace panel is showing.

format long g;

format compact;

fontSize = 15;

markerSize = 4;

%[filename, pathname] = uigetfile({'*.*'},'File Selector');

%if ~isequal(filename,0)

%fullname = strcat(pathname,filename);

I=imread('MH2.jpeg');

%else

% msgbox('Ban chua chon anh?')

%end

Id = size(I);

if length(Id) == 3

B0 = rgb2gray(I);

else

B0 = I;

end

B0 = imadjust(B0);

k1 = fspecial('gaussian', [5 5], 2.5);

Ic = imfilter(B0,k1);

figure(1),subplot(3,3,1), imshow(I);

title('anh goc');

figure(1), subplot(3,3,2), imshow(B0);

title('anh tang tuong phan');

figure(1), subplot(3,3,3), imshow(Ic);

title('anh loc gaussian');

%loc trung vi

I_m = medfilt2(B0,[3 3]);

figure(1),subplot(3,3,4), imshow(Ic);

title('anh loc trung vi');

%anh nhi phan

thresholdValue = 60;

bw = Ic > thresholdValue;

bw = bwareafilt(bw, 1);

figure(1),subplot(3,3,5), imshow(bw);

title('anh nhi phan');

%dong hinh thai

se = strel("disk",10);

C1 = imclose(bw, se);

figure(1),subplot(3,3,6), imshow(C1);

title('dong hinh thai');

%khai thac ranh gioi

[B,L,N] = bwboundaries(C1);

figure(2);

%subplot(3,3,7);

imshow(C1); hold on;

title('khai thac ranh gioi');

for k=1:length(B)

boundary = B{k};

if(k > N)

plot(boundary(:,2), boundary(:,1), 'g','LineWidth',1.5);

else

plot(boundary(:,2), boundary(:,1), 'r','LineWidth',1.5);

end

end

%

Answer 1

[yanqi liu]

% Initialization Steps.
clc;    % Clear the command window.
close all;  % Close all figures (except those of imtool.)
clear;  % Erase all existing variables. Or clearvars if you want.
workspace;  % Make sure the workspace panel is showing.
format long g;
format compact;
fontSize = 15;
markerSize = 4;

%[filename, pathname] = uigetfile({'*.*'},'File Selector');
%if ~isequal(filename,0)
%fullname = strcat(pathname,filename);
I=imread('https://ww2.mathworks.cn/matlabcentral/answers/uploaded_files/935629/MH2.jpeg');
%else
% msgbox('Ban chua chon anh?')
%end
Id = size(I);
if length(Id) == 3
    B0 = rgb2gray(I);
else
    B0 = I;
end
B0 = imadjust(B0);
k1 = fspecial('gaussian', [5 5], 2.5);
Ic = imfilter(B0,k1);
figure(1),subplot(3,3,1), imshow(I);
title('anh goc');
figure(1), subplot(3,3,2), imshow(B0);
title('anh tang tuong phan');
figure(1), subplot(3,3,3), imshow(Ic);
title('anh loc gaussian');
%loc trung vi
I_m = medfilt2(B0,[3 3]);
figure(1),subplot(3,3,4), imshow(Ic);
title('anh loc trung vi');
%anh nhi phan
thresholdValue = 60;
bw = Ic > thresholdValue;
bw = bwareafilt(bw, 1);
figure(1),subplot(3,3,5), imshow(bw);
title('anh nhi phan');
%dong hinh thai
se = strel("disk",10);
C1 = imclose(bw, se);
figure(1),subplot(3,3,6), imshow(C1);
title('dong hinh thai');

%khai thac ranh gioi
[B,L,N] = bwboundaries(C1);
figure(2);
%subplot(3,3,7);
imshow(C1); hold on;
title('khai thac ranh gioi');
len = [];
for k=1:length(B)
    boundary = B{k};
    % comput length
    len(k) = (max(boundary(:,2))-min(boundary(:,2))).^2+(max(boundary(:,1))-min(boundary(:,1))).^2;
    if(k > N)
        plot(boundary(:,2), boundary(:,1), 'g','LineWidth',1.5);
    else
        plot(boundary(:,2), boundary(:,1), 'r','LineWidth',1.5);
    end
end
% find
[~,ind] = max(len);
boundary = B{ind};
plot(boundary(:,2), boundary(:,1), 'm--','LineWidth',2);

[Thanh Vu]

Thank you very very much yanqi liu. I am very grateful for your help for me.

Can you help me one more time? after getting the longest length boundary from extracted boundaries like you helped me , i want to extract the NFL layer on the OCT image of the retina of the eye like the following one:

I'm new to using matlab, so there's a lot I don't know, please help me

[yanqi liu]

yes，sir，let us check the follow method

% Initialization Steps.
clc;    % Clear the command window.
close all;  % Close all figures (except those of imtool.)
clear;  % Erase all existing variables. Or clearvars if you want.
workspace;  % Make sure the workspace panel is showing.
format long g;
format compact;
fontSize = 15;
markerSize = 4;

%[filename, pathname] = uigetfile({'*.*'},'File Selector');
%if ~isequal(filename,0)
%fullname = strcat(pathname,filename);
I=imread('https://ww2.mathworks.cn/matlabcentral/answers/uploaded_files/935629/MH2.jpeg');
%else
% msgbox('Ban chua chon anh?')
%end
Id = size(I);
if length(Id) == 3
    B0 = rgb2gray(I);
else
    B0 = I;
end
B0 = imadjust(B0);
k1 = fspecial('gaussian', [5 5], 2.5);
Ic = imfilter(B0,k1);
figure(1),subplot(3,3,1), imshow(I);
title('anh goc');
figure(1), subplot(3,3,2), imshow(B0);
title('anh tang tuong phan');
figure(1), subplot(3,3,3), imshow(Ic);
title('anh loc gaussian');
%loc trung vi
I_m = medfilt2(B0,[3 3]);
figure(1),subplot(3,3,4), imshow(Ic);
title('anh loc trung vi');
%anh nhi phan
thresholdValue = 60;
bw = Ic > thresholdValue;
bw = bwareafilt(bw, 1);
figure(1),subplot(3,3,5), imshow(bw);
title('anh nhi phan');
%dong hinh thai
se = strel("disk",10);
C1 = imclose(bw, se);
figure(1),subplot(3,3,6), imshow(C1);
title('dong hinh thai');

%khai thac ranh gioi
b1 = bwperim(C1);
b1(:,[1:2 end-2:end]) = 0;
b1 = logical(b1);
[B,L,N] = bwboundaries(b1);
figure(2);
%subplot(3,3,7);
imshow(C1); hold on;
title('khai thac ranh gioi');
len = [];
for k=1:length(B)
    boundary = B{k};
    % comput length
    len(k) = (max(boundary(:,2))-min(boundary(:,2))).^2+(max(boundary(:,1))-min(boundary(:,1))).^2;
    if(k > N)
        plot(boundary(:,2), boundary(:,1), 'g','LineWidth',1.5);
    else
        plot(boundary(:,2), boundary(:,1), 'r','LineWidth',1.5);
    end
end

% find
[~,ind] = max(len);
boundary = B{ind};
figure;
%subplot(3,3,7);
imshow(C1); hold on;
plot(boundary(:,2), boundary(:,1), 'm--','LineWidth',2);

[Thanh Vu]

I'm really grateful to you yanqi liu. I still have one step to do is to extract the depth feature to classify the image by SVM method like the algorithm in figure below. But I don't understand what the edge pixels mentioned in the algorithm mean. so can you help me write this algorithm on matlab? I know I asked you too much, but I really don't understand the matlab code that I had to submit the report for the other day. help me please :((((((

I would appreciate it if you could help me. I'm really sorry if my requests bother you