How do I find the boundaries of a value in a matrix?

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Hi, I have a matrix as follows:
I =
1 1 1 1 8 1 2
1 1 8 8 8 2 1
1 8 8 1 2 1 1
1 1 8 2 1 1 1
2 2 2 1 1 1 1
2 2 2 1 1 1 1
I want a matrix which will be output of boundary value of 8 i.e. (1,5),(2,3),(2,4),(2.5),(3,2),(3.3),(4.3)
Is there any way to he find these location easily?

Accepted Answer

Sean de Wolski
Sean de Wolski on 3 Jun 2011
[r c] = find(bwperim(I==8));
  3 Comments
Walter Roberson
Walter Roberson on 5 Jun 2011
That's because you keep changing the stated requirements :(
Ivan van der Kroon
Ivan van der Kroon on 6 Jun 2011
This method and Andrei's still gives the (1,4) and leaves the (3,3).

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More Answers (4)

Walter Roberson
Walter Roberson on 31 May 2011
If you have the image processing toolbox, you can use
bwboundary(I==8)
  3 Comments
Walter Roberson
Walter Roberson on 1 Jun 2011
http://www.mathworks.com/help/toolbox/images/ref/bwboundaries.html
It appears you do not have the Image Processing Toolbox installed, or else you have a very old version. The Image Processing Toolbox is extra cost for all MATLAB editions except for the Student Edition.

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Ivan van der Kroon
Ivan van der Kroon on 31 May 2011
[rows,cols]=find(I==8)
rows =
3
2
3
4
2
1
2
cols =
2
3
3
3
4
5
5
Hope this helps.
  7 Comments
Mohammad Golam Kibria
Mohammad Golam Kibria on 5 Jun 2011
I =
1 1 8 8 8 1 2
1 1 8 8 8 2 1
1 8 8 8 8 1 1
1 1 8 2 1 1 1
2 2 2 1 1 1 1
2 2 2 1 1 1 1
output needs the following:
(1,3) (1,5) (2,3) (2,5)(3,2)(3,3)(3,4)(3,5) (4,3)
perhaps this will help for better understanding my output
Walter Roberson
Walter Roberson on 5 Jun 2011
So to confirm, any "8" that has only 8's as neighbors is to be excluded, and all other 8's are to be included?
If the entire matrix was 2x2 and was
88
88
then everything should be excluded?
and for
881
888
881
then the entire left side is to be excluded?
Just include the 8's for which at least one of the neighbors is a non-8 ?

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Andrei Bobrov
Andrei Bobrov on 2 Jun 2011
more variant
I1 = I == 8;
l1 = [ones(1,size(I1,2)); diff(I1)~=0];
l2 = [ones(size(I1,1),1) diff(I1,1,2)~=0];
lud = [l1(2:end,:);false(1,size(I1,2))];
llr = [l2(:,2:end) false(size(I1,1),1)];
[j i] = find(((lud+l1+llr+l2)&I1)');
out = [i j];
more more variant
I1 = I==8;
I2=ones(size(I1)+2);
I2(2:end-1,2:end-1) = I1;
I1(conv2(I2,ones(3),'valid')==9)=0;
[i j] = find(I1);
without conv2
I1 = I==8;
I2=ones(size(I1)+2);
I2(2:end-1,2:end-1) = I1;
[i j] = find(I1);
ij1 = [i j];
ij2 = ij1 + 1;
IJ = arrayfun(@(x)bsxfun(@plus,ij2(:,x),[-1 0 1]),1:size(ij2,2),'un',0);
ij1(arrayfun(...
@(x)sum(reshape(I2(IJ{1}(x,:),IJ{2}(x,:)),[],1)),1:size(ij2))==9,:) = [];
  3 Comments

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Teja Muppirala
Teja Muppirala on 6 Jun 2011
This will give all the 8's that are not entirely surrounded by other 8's. Assuming you have the Image Processing Toolbox.
[i,j] = find( (I==8) - imerode(I==8,ones(3)) )

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