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

Question

Mohammad Golam Kibria 2011-5-31

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Hi, I have a matrix as follows:

I =

   1     1     1     8     1     2
   1     8     8     8     2     1
   8     8     1     2     1     1
   1     8     2     1     1     1
   2     2     1     1     1     1
   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?

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Answer 1

Sean de Wolski 2011-6-3

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[r c] = find(bwperim(I==8));

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Walter Roberson 2011-6-5

That's because you keep changing the stated requirements :(

Ivan van der Kroon 2011-6-6

This method and Andrei's still gives the (1,4) and leaves the (3,3).

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Answer 2

Walter Roberson 2011-5-31

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If you have the image processing toolbox, you can use

bwboundary(I==8)

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Walter Roberson 2011-6-1

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.

Sean de Wolski 2011-6-2

Typo Walter: bwboundaries(I==8)

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Answer 3

Ivan van der Kroon 2011-5-31

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[rows,cols]=find(I==8)
rows =
     3
     2
     3
     4
     2
     1
     2
cols =
     2
     3
     3
     3
     4
     5
     5

Hope this helps.

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Mohammad Golam Kibria 2011-6-5

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

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 2011-6-5

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

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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Answer 4

Andrei Bobrov 2011-6-2

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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,:) = [];