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

Question

Christopher 2013-11-24

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https://ww2.mathworks.cn/matlabcentral/answers/107335-fastest-calculation-method-to-count-elements-in-a-matrix-in-the-neighborhood-of-some-element-havi

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:

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

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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.

Image Analyst 2013-11-24

I guess I'm not understanding why you don' just use the other method which you say is faster. Care to explain?

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Answer 1

Matt J 2013-11-24

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链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/107335-fastest-calculation-method-to-count-elements-in-a-matrix-in-the-neighborhood-of-some-element-havi#answer_116180

编辑：Matt J 2013-11-24

在 MATLAB Online 中打开

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