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Better skeletonization of an image to found branchpoints

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tsaiwu on 4 Jan 2017
Commented: Massimo Zanetti on 9 Jan 2017
I have an image of an grid and want to find the 12 branchpoints of it. But the skeleton is really "rough" so I have to many branchpoints. I tried different "clean up" methods but I can't clean it up enough. Can someone help me with my problem, please.
Grid = imread('grid.jpg');
gray_grid = rgb2gray(Grid);
binary_grid = im2bw(gray_grid);
inverse_grid = imcomplement(binary_grid);
grid_skel = bwmorph(inverse_grid,'skel',inf);
%%clean skel
skel_1 = bwmorph(grid_skel,'spur',inf);
skel_2 = bwmorph(skel_1,'clean',inf);
%%find branchpoints
branchpoints = bwmorph(skel_2,'branchpoints');
[rowsBP, columnsBP] = find(branchpoints)
%%show branchpoints
hold on;
for k = 1 : length(rowsBP);
plot(columnsBP(k), rowsBP(k), 'r+', 'MarkerSize', 8, 'LineWidth', 1);


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Answers (1)

Massimo Zanetti
Massimo Zanetti on 5 Jan 2017
Dilate your image before skeletonize it. Try this:
%read image
G = imread('grid.jpg');
G = im2bw(rgb2gray(G));
%dilate image complement
I = imdilate(imcomplement(G),strel('disk',15));
figure; imshow(I)
%skeletonize, and remove spur branches
S = bwmorph(I,'skel',inf);
S2 = bwmorph(S,'spur',inf);
figure; imshow(S2);
%find branchpoints, dilate them to see them in image
B = bwmorph(S2,'branchpoints');
B2 = imdilate(B,strel('disk',10));
figure; imshow(B2);


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Massimo Zanetti
Massimo Zanetti on 6 Jan 2017
I have dilated the branchpoints (image B2) only to enhance their visualization.. In matrix B you have only the right pixels on.. to get the coordinates (i,j index) just call [I,J]=find(B).
There is no need to run additional code after the piece of code I wrote.
tsaiwu on 6 Jan 2017
If i ran find[B] it gives me to many branchpoints backs, like I said before. I want to find only the intersections point of the grid, for example in the first grid 12 points.
And do you have an idea to get the intersection points of the second grid? thanks
Massimo Zanetti
Massimo Zanetti on 9 Jan 2017
Ok, I got it. So, capture the centroids of the cirlces using regionprops. Like this:
I = imdilate(imcomplement(G),strel('disk',15));
S = bwmorph(I,'skel',inf);
S2 = bwmorph(S,'spur',inf);
B = bwmorph(S2,'branchpoints');
B2 = imdilate(B,strel('disk',10));
s = regionprops(B2,'centroid');
centroids = cat(1, s.Centroid);
imshow(G); hold on;
hold off;

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