bwboundaries: Explanation of a example

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Zynk
Zynk 2015-2-5
评论： Image Analyst 2021-9-7
This simple example is given in the Matlab Documentation:
I = imread('rice.png');
BW = im2bw(I, graythresh(I));
B = bwboundaries(BW,'noholes');
for k = 1:length(B)
boundary = B{k};
end
Even if I can see the result and why we need the loop, I don't fully get to understand what exactly is doing the "for" loop, I wouldn't know how to explain how it works. Can someone help me? Thank you

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Image Analyst
Image Analyst 2015-2-5
If you have 10 blobs, you would get 10 boundaries. It gets the k'th boundary with this line
boundary = B{k};
And then it gets the x and y coordinates. Think of it like this
for k = 1 : length(B)
thisBoundary = B{k}; % Get k'th boundary
x = thisBoundary(:, 2);
y = thisBoundary(:, 1);
plot(x, y, 'w', 'LineWidth', 2);
end
The 'w' means to plot in white color, and the line width is the thickness of the line it's using to plot the boundary in the overlay above the image.
  1 个评论
Zynk
Zynk 2015-2-5
Thanks, I understood why I needed the loop in order to plot,but not exactly how it was getting the boundary or why. Now it is clear.

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更多回答（2 个）

Anand
Anand 2015-2-5
I assume you are talking about Example 1 in the documentation.
I = imread('rice.png');
BW = im2bw(I, graythresh(I));
[B,L] = bwboundaries(BW,'noholes');
imshow(label2rgb(L, @jet, [.5 .5 .5]))
hold on
for k = 1:length(B)
boundary = B{k};
plot(boundary(:,2), boundary(:,1), 'w', 'LineWidth', 2)
end
The for loop here is used in order to plot a white outline displaying the boundaries on the image.
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Suresh D
Suresh D 2021-9-7
plot(boundary(:,2), boundary(:,1), 'w', 'LineWidth', 2)
Please explain this line
Image Analyst
Image Analyst 2021-9-7
boundary is an n-by-2 array of the row, column coordinates, in other words
[row1, col1;
row2, col2;
...
rown, coln];
which is
[y1, x1;
y2, x2;
...
yn, xn]
so boundary(:,2) is a column vector of all the x values and boundary(:,1) is a column vector of all the y values. So the plot command is essentially
plot(x, y, 'w', 'LineWidth', 2);
which will draw a white line between all the boundary points.

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Babu Sankhi
Babu Sankhi 2020-7-22
For my images I got the boundaries only for the very good contrast png images not for the less contrast images. Is there any way to get boundary even for the images with less contrast? In addition to that I want to find the distance between center and boundary(left/right) for several images. Is there any helpful codes/function so that I can find those distances more precisely?
Thank you
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Image Analyst
Image Analyst 2020-7-25
You can use bwdist(). Attach your original image in a new question if you still need help.
Babu Sankhi
Babu Sankhi 2020-7-26
Thank you analyst for your help. But bwdist() did not give any difinite value in my case so I am still stucked with that . I have attached my original microscopy image and the code I used to get the boudaries of desired region of image . After that my problem is to find the distance between right most points of innner(green) and outer(Red) boundaries . It means I have to find the distance between A and B in in resulting png.
Can you please share more ideas about it ?
thank you

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