Computing findpeaks along 3-Dimensional matrix

Question

Vivek 2020-10-19

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https://ww2.mathworks.cn/matlabcentral/answers/619323-computing-findpeaks-along-3-dimensional-matrix

评论： Star Strider 2020-10-20

I have a 3-dimensional matrix, after some code manipulation on a cell array. The matrix is of size 101x151x476.

The first number represents the y space, the second number is the x space, and the third number is the number of time steps. So essentially, the matrix contains magnitudes along a y-x plane, for varying timesteps.

I am trying to compute the peaks at each (y,x) across the timesteps. I am doing this using findpeaks. This is what I have so far:

for yIndex= 1:1:101
    for xIndex= 1:1:151 
        
        [peaks{y,x}] = findpeaks(a3Dmatrix(yIndex,xIndex,:)); 
    end
end 

However, the cell "peaks" is incorrect, as it is not of the size 101x151 (y by x). Any help with this?

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Answer 1

Star Strider 2020-10-20

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I am guessing here, since I do not have your data. Since findpeaks is going to return vectors for the peaks, locations (and othher outputs if you want them), I would do something like this:

for yIndex= 1:1:101
    for xIndex= 1:1:151 
        [peaks{yIndex,xIndex,:},locs{yIndex,xIndex,:}] = findpeaks(a3Dmatrix(yIndex,xIndex,:)); 
    end
end 

I cannot test that because I do not have your data, so I am posting it as UNTESTED CODE.

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Star Strider 2020-10-20

在 MATLAB Online 中打开

I do not understand what line threw that error. Cell arrays can certainly contain empty vectors (denoted as {[]}), and it is certainly possible that there are no identifiable peaks in your signal (constant, or that the maximum is at one or the other end of the vector, so not a ‘peak’ as findpeaks defines it). That would result in findpeaks returning an empty output for that particular set of coordinates.

Perhaps a better option is:

for yIndex= 1:1:101
    for xIndex= 1:1:151 
        [pks,lcs] = findpeaks(a3Dmatrix(yIndex,xIndex,:));
        peaks{yIndex,xIndex} = pks;
        locs{yIndex,xIndex} = lcs;
    end
end 

That might make it easier to understand what the problem is, since you could look at the size function output of size(pks).

If that indicates an empty vector, then:

[max,idx] = maxk(a3Dmatrix(yIndex,xIndex,:), 2);

could suggest what the problem is.

You could put that in an if block to trap the empty vector, for example:

for yIndex= 1:1:101
    for xIndex= 1:1:151 
        [pks,lcs] = findpeaks(a3Dmatrix(yIndex,xIndex,:));
        if (numel(pks) < 1))
            [pks,lcs] = maxk(a3Dmatrix(yIndex,xIndex,:), 2);
        end
        peaks{yIndex,xIndex} = pks;
        locs{yIndex,xIndex} = lcs;
    end
end 