Fastest calculation method to: Count elements in a matrix, in the neighborhood of some element, having some value

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Christopher
Christopher 2013-11-24
编辑： Matt J 2013-11-24
For an element (i,j) in a matrix I want to calculate the number of neighboring elements which have the same value as that in (i,j). I currently have made the straightforward code:
for i=2:ynum+brd2-1
for j=2:xnum+brd2-1
if x(i,j)==x(i-1,j+1)
e(i,j)=e(i,j)+1;
end;
if x(i,j)==x(i,j+1)
e(i,j)=e(i,j)+1;
end;
if x(i,j)==x(i+1,j+1)
e(i,j)=e(i,j)+1;
end;
if x(i,j)==x(i-1,j)
e(i,j)=e(i,j)+1;
end;
if x(i,j)==x(i+1,j)
e(i,j)=e(i,j)+1;
end;
if x(i,j)==x(i-1,j-1)
e(i,j)=e(i,j)+1;
end;
if x(i,j)==x(i,j-1)
e(i,j)=e(i,j)+1;
end;
if x(i,j)==x(i+1,j-1)
e(i,j)=e(i,j)+1;
end;
e(i,j)=8-e(i,j);
end
end
which works just fine and is designed for a 9 point stencil (I look at the 8 nearest neighbors).
The problem is that it is slow (or probably much slower than another method) and I want to do the same thing with a 37 point stencil that looks like this:
000
00000
0000000
000X000
0000000
00000
000
where the x is (i,j), instead of
000
0X0
000
I assume I should use some kind of countif or sum(sum())methods, but I am new to matlab and do not know what the fastest operations are.
Is it fastest to count over a rectangle around the circle and then subtract the 3 points near the vertices?
Thanks
  2 个评论
Matt J
Matt J 2013-11-24
编辑：Matt J 2013-11-24
Are there any special restrictions that on the matrix data that you're working with? I would guess, for example, that the x(i,j) values are all integers. Otherwise, you would be comparing x(i,j) with its neighbors using a tolerance for floating point differences.

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回答（1 个）

Matt J
Matt J 2013-11-24
编辑：Matt J 2013-11-24
I would expect this to be faster. It's for a 3x3 stencil, but it can easily be generalized.
stencil=zeros(3);
stencil(5)=1;
e=zeros(size(x));
for i=[1:4,6:8]
stencil(i)=-1;
e(2:end-1,2:end-1)=e(2:end-1,2:end-1) + ~conv2(x,stencil,'valid');
stencil(i)=0;
end

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