Outer and inner contour of the image from the centroid

Hello everybody,

I have following image:

 I would like to find the Outer and inner contour of the image from the centroid, for example as here:

I would appreciate any help!

Thanks in advance!

Answer 1

[Image Analyst]

Use bwconvhull() and then bwboundaries(). The outer is the boundaries of the convex hull. To get the inner, subtract or XOR the original from the convex hull, then call bwboundaries().

[Ivan Shorokhov]

Dear Image Analyst,

I have tried to implement your answer, but it didn't work, could you show me please how to do it right.

Here is the result and code:

close all; clc; clear all;
original=imread('LA_shape.jpg');
original = rgb2gray(original);
figure; imshow(original);

outer = bwconvhull(original,'objects',4);
subplot(2,2,1);imshow(outer);

B = bwboundaries(outer,'noholes');
subplot(2,2,2); imshow(outer);

for k=1:length(B),
    boundary = B{k};
    hold on;
    plot(boundary(:,2), boundary(:,1));
end

inner = im2bw(original)- outer;

B2 = bwboundaries(inner,'noholes');
subplot(2,2,3);imshow(inner);

for l=1:length(B2),
    boundary1 = B2{l};
    hold on;
    plot(boundary1(:,2), boundary1(:,1),'r');
end
hold off;
subplot(2,2,4); imshow(original);
hold on; 

for l=1:length(B2),
    boundary1 = B2{l};
    plot(boundary1(:,2), boundary1(:,1),'r');
end

for k=1:length(B),
    boundary = B{k};
    plot(boundary(:,2), boundary(:,1));
end

[Image Analyst]

You need to take special care to handle the top of the U.