Edge Detection in a 2D Matrix

Hi there,

I have a 2D matrix which contains contains 0s and 1s as shown in the left image below. I would like to get the outer boundary of this shape. By using the edge function, it gives a nice estimate of the boundary, but some of the points lie on the original shape. What I did was to use the "edge" function directly with only my 2D matrix as input. Are there any ways to get the outer boundary only and without any points intersecting the original shape? Thank you.

[Dyuman Joshi]

What is the output you want to obtain?

Also, can you attach your data? Use the paperclip button to do so.

[Wayne]

Hi @Dyuman Joshi thanks for the reply - i have attached the data. I would like to have a boundary of a single pixel which surrounds the original figure. The "edge" function does that to some extent, but there are still some overlapped regions.

Answer 1

[DGM]

Consider the example:

% a logical image
mask = imread('tinyblobs.png');
imshow(mask)

% generate the perimeter image
paddedmask = padarray(mask,[1 1],'replicate','both');
perim = bwperim(~paddedmask);
perim = perim(2:end-1,2:end-1);
imshow(perim)

% show that the new pixels do not overlap the original blobs
% and that there is no gap between the two
imshow(imfuse(mask,perim))

[Wayne]

That's great, thank you!

Answer 2

[Matt J]

% a logical image
mask = imread('tinyblobs.png');

outerboundary = imdilate(mask,strel('disk',1))&~mask;

imshow(mask+2*outerboundary,[])

[Wayne]

Thank you, appreciate the response!

Answer 3

[Bruno Luong]

Do you want this

load(websave('data.mat','https://www.mathworks.com/matlabcentral/answers/uploaded_files/1446792/data.mat'))

d=conv2(data,[0 1 0; 1 1 1; 0 1 0],'same')>0;
b=d-data;

figure
imagesc(data);
axis equal

figure
imagesc(b)
axis equal

[Wayne]

Yes this was what I wanted, thanks for your help!