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?
采纳的回答
Sean de Wolski
2011-6-3
[r c] = find(bwperim(I==8));
3 个评论
Mohammad Golam Kibria
2011-6-5
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).
更多回答（4 个）
Walter Roberson
2011-5-31
If you have the image processing toolbox, you can use
bwboundary(I==8)
3 个评论
Mohammad Golam Kibria
2011-6-1
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)
Ivan van der Kroon
2011-5-31
[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 个评论
Mohammad Golam Kibria
2011-5-31
Matt Fig
2011-6-1
What would be your expected result in this case?
Walter Roberson
2011-6-1
Mohammad's expected result would exclude any of the 8's that are eight-connected only to other 8's: he wants only the 8's that are boundary pixels.
Matt Fig
2011-6-3
I was wondering if Mohammad wanted 4 or 8 connected 8s excluded... I.e., in his example above, are there two 8s excluded or 1?
Walter Roberson
2011-6-3
Hmmm, hard to say for sure, as his original example did not happen to include any 8's that were 4 connected to other 8's.
Mohammad Golam Kibria
2011-6-5
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
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 ?
Andrei Bobrov
2011-6-2
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 个评论
Mohammad Golam Kibria
2011-6-5
Andrei Bobrov
2011-6-6
small correction in answer
Andrei Bobrov
2011-6-6
more correction
Teja Muppirala
2011-6-6
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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