islocalmax('flat selection') selecting 0 value points

I am trying to select these 'flat' peaks from a set of data and as I need the range of index values I am using islocalmax(data,'FlatSelection','all') and for most of my data it does a good job as shown in the picture above but others end up looking like this.

Was wondering if there is a way to fix this? I have tried altering things like minprominence and such.

Answer 1

[Star Strider]

In the lower plot image, if you are referring to the collection of orange asterisks before the start of the variable section (for want of a better way to describe it), the ‘spurious’ data could represent noise. In addition to 'MinProminence', setting 'ProminenceWindow' could be an option. Another option would be various approaches to filtering to elinminate the high-frequency noise, if that is the problem. (Start that approach by calculating the one-sided Fourier transform to see if there is band-limited high-frequency noise that could be filtered. Then design a frequency-selective lowpass filter using that information.)

It would help to have the data to experiment with. Without it, I can only speculate as to what the problem is, assuming that I even understand your question correctly.

[Simeon]

Ok thanks

[Star Strider]

My pleasure!

EDIT — (25 Jul 2024 at 17:28)

Thinking about it further, filtering may not produce the desired results, because it could affect the areas you want to detect. The solution to that problem could be to use thresholding, and only apply islocalmax to the areas above and below a specific threshold. This approach calculates the threshold using a logical vector, and then logically excludes it from the islocalmax result.

The code —

x = linspace(0, 50, 250);
y = [x(x<=10)*0 sign(sin(x(x>10 & x<40))*2*pi*10) x(x>=40)*0] + randn(size(x))*0.005;

figure
plot(x, y)
title('Original Data')

Lv1 = islocalmax(y, 'FlatSelection','all');

figure
plot(x, y)
hold on
plot(x(Lv1), y(Lv1),'.r')
hold off
title('Original Data With identified Regions')

Lvt = (y<0.05) & (y>-0.05);                                     % Set Threshold & Return Appropriate Logical Vector
nnz(Lvt)                                                        % Check Threshold

ans = 100

Lv1= islocalmax(y, 'FlatSelection','all');
Lvlm = Lv1 & ~Lvt;                                              % Logically Combine Vectors To Get Result Logical Vector

figure
plot(x, y, 'DisplayName','Original Data')
hold on
plot(x(Lvlm), y(Lvlm),'.r', 'DisplayName','Desired Result')
plot(x(Lvt), y(Lvt), '.g', 'DisplayName','Excluded Region')
hold off
legend('Location','N')
ylim([min(ylim) max(ylim)*1.25])
title('Original Data With Selectied Regions Showing Excuded Region')

Lvt = (y<0.05) & (y>-0.05);                                     % Set Threshold & Return Appropriate Logical Vector
nnz(Lvt)                                                        % Check Threshold

ans = 100

Lv1= islocalmax(y, 'FlatSelection','all');
Lvlm = Lv1 & ~Lvt;                                              % Logically Combine Vectors To Get Result Logical Vector

figure
plot(x, y, 'DisplayName','Original Data')
hold on
plot(x(Lvlm), y(Lvlm),'.r', 'DisplayName','Desired Result')
% plot(x(Lvt), y(Lvt), '.g', 'DisplayName','Excluded Region')
hold off
legend('Location','N')
ylim([min(ylim) max(ylim)*1.25])
title(["Original Data With Selectied Regions" "Omitting Specific Plot Of Excuded Region"])

.