Finding peak with two nulls around in a 1d plot

How can we find the presence of a peak (global maxima) with two nulls (or crossing lower threshold) on each side of the peak in a 1d plot.

And is there a way to find the width of the peak at a particular threshold level

[KALYAN ACHARJYA]

Have you try here

https://in.mathworks.com/help/signal/ref/findpeaks.html

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Answer 1

[Sai Pavan]

Hello Vinay,

I see that you want to find the presence of a peak with two nulls on each side of the peak in a 1d plot and find the width of the peak at a particular threshold level.

Please refer to the below workflow to find the presence of a peak with two nulls (or crossing a lower threshold) on each side of the peak in a 1D plot, and to find the width of the peak at a particular threshold level:

• Identify the peaks using the 'findpeaks' function.
• For each detected peak, find the points where the signal crosses the lower threshold level on both sides of the peak.
• Measure the width of the peak at the threshold level by calculating the distance between the two crossing points.

Please refer to the below code that illustrates the above mentioned workflow:

% Sample data
x = linspace(0, 10, 1000);
y = exp(-(x-5).^2) + 0.1*randn(size(x)); % Gaussian peak with some noise

plot(x, y);
hold on
threshold = 0.1; % Set to consider as null. Adjust the threshold variable to the level at which you want to measure the peak widths.

[peakValues, peakLocs] = findpeaks(y, x, 'MinPeakHeight', threshold); % Find the peaks
plot(peakLocs, peakValues, 'r*', 'MarkerSize', 10);

% For each peak, find the width at the threshold level
for i = 1:length(peakLocs)
    peakX = peakLocs(i);
    peakY = peakValues(i);
    
    % Find where the signal crosses the threshold on the left side of the peak
    leftSide = y(1:find(x==peakX));
    leftCross = find(leftSide < threshold, 1, 'last');
    if isempty(leftCross)
        leftCross = 1; % If it never crosses, use the start of the data
    end
    
    % Find where the signal crosses the threshold on the right side of the peak
    rightSide = y(find(x==peakX):end);
    rightCross = find(rightSide < threshold, 1, 'first');
    if isempty(rightCross)
        rightCross = length(rightSide); % If it never crosses, use the end of the data
    else
        rightCross = rightCross + find(x==peakX) - 1; % Adjust index
    end
    
    % Calculate the width at the threshold
    peakWidth = x(rightCross) - x(leftCross);
    % Plot the width
    plot(x([leftCross rightCross]), [threshold threshold], 'k-', 'LineWidth', 2);
    % Display the width
    disp(['Peak ' num2str(i) ' width at threshold ' num2str(threshold) ': ' num2str(peakWidth)]);
end

hold off

Please refer to the below documentation to learn more about 'findpeaks' function: https://www.mathworks.com/help/signal/ref/findpeaks.html

Hope it helps!