How can i make a function that return values from a plot?

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Georgiana Andreianu
回答： Ahmed Redissi 2021-4-14
Hello!
I need to write a function that tells me the value of spectral components from a plot (the y-axis correspondant of the frequency on the x-axis). Can i make a function that tells me the value straight from the plot? If so, than how can i do that. Otherwise, can i make a function that calculates the vallue directly?
The plot is a Discret Fourier Transform that i made using fft funtion.
  1 个评论
dpb
dpb 2021-4-14
How do you decide which spectral component you want? You have to have computed the frequency of the x-axis in order to have plotted versus frequency to have started with; do you want the nearest frequency bin, the integral of the peak associated with a frequency (the frequency input of a given signal may/probably won't match identically the centerline of a frequency bin in the FFT), or ... ???
Far too nebulous of a Q? to be able to answer.

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回答（1 个）

Ahmed Redissi
Ahmed Redissi 2021-4-14
If you only have a figure and not the data, you can follow this example to extract data from the figure containing the FFT:
clear
clc
close all
% Generate a signal
Fs = 1000; % Sampling frequency (Hz)
t = 0:1/Fs:0.3; % Time vector (s)
Nt = length(t);
f = (-Nt/2:Nt/2-1)*(Fs/Nt); % Frequency vector (Hz)
x = cos(2*pi*t*200); % Signal
% Calculate the FFT
y = fftshift(fft(x));
% Convert the magnitude to dB
ydb = 20*log10(abs(y));
% Plot the FFT magnitude
plot(f,ydb)
grid
xlabel('Frequency (Hz)')
ylabel('FFT Magnitude (dB)')
% Get the figure handle
h = findall(0,'Type','Figure');
% Get the FFT value from the figure
findex = 212; % Frequency index for 200 Hz
yvalue = getYValueFromFigure(h,findex);
function yvalue = getYValueFromFigure(h,index)
data = findall(h,'Type','Line');
ydata = data.YData;
yvalue = ydata(index);
end

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