Findpeaks creates empty arrays in for loop

Hello,

I'm defining a for loop to use on a laser stripe image and does the following:

1. takes a row
2. filters the row ( I take only the bigest and lowest peak)
3. gets the gradient
4. use findpeaks to get the position and value of the gradient max and min
5. create a line between these two points
6. get the 0 crossing

Right now im able to get the 0 crossing of just one line. Im stuck in the "for" loop as the findpeaks function is creating empty arrays I dont know why!

The aim is to get the 0 crossing of all the image rows, then plot all.

Please any help?

img_peaks = NaN(rows, 1);

xp = [];
yp = [];
Xn=[];
Yn=[];

for i=1:rows
    
    img_puntos(i,:)=img(i,:);
    %filtrado
    filtrado=sgolayfilt(double(img_puntos(i,:)),3,33 );
    for k=1:length(filtrado)
        if filtrado(k)<0
       filtrado(k)=0;
        end
    end
    %derivada
    derivada=gradient(filtrado);
    %maximo y minimo
    [yp,xp]=findpeaks(derivada); % HERE IS THE PROBLEM!!!
    [Xn,Yn]=findpeaks(-derivada);% HERE IS THE PROBLEM!!!
     %coeficientes de la recta
%     coeficientes = polyfit([Xp, Xn], [Yp, -Yn], 1);
%     m = coeficientes (1);
%     n = coeficientes (2);
%     % 0 crossing
%     zerocross= -m/n;
    img_peaks(i) = subpixel;
end

[dpb]

Not the problem with the data, but in MATLAB replace

for k=1:length(filtrado)
  if filtrado(k)<0
    filtrado(k)=0;
  end
end

with

filtrado(filtrado<0)=0;

The problem in your code is that you didn't write code to handle the possibility a given row in the image may, indeed, not have a discernible peak.

We have no way to know anything about whether there's some way to be able to help findpeaks locate minimal excursions or not without the data to observe; generally it is pretty adept--but, it's designed for typical time series one obtains from things like measurements of acceleration or the like; depending upon what your image looks like, it might not be the best tool.

Or, there just may not be any discernible peaks in the data you have...