Sampling time of matlab figure data

I have some real time experiment data saved in matlab figures. While saving the data in simulink scope, the sampling time was set as the inherited sampling time of '-1'. Now, I have those scope data saved as only matlab figures (.fig files). Is it possible to change the sampling time of the saved data to 0.1? Without changing the scope settings and running the real time experiment again? This is required to reduce the number of data points from 200001 to 2001.

[Aniket]

Hi @SM, can you please clarify if you want to extract data from saved figure and resample it to 0.1 OR do you want to save scope data to MATLAB fig with sample rate of 0.1 ?

[SM]

Yes, I want to extract data from saved figure and resample it to 0.1.

Answer 1

[Mathieu NOE]

hello

to retrieve data from a figure , you can do it by two ways :

• for recent releases :

openfig('data points.fig');
set(gcf, 'Visible','off')
Ax = gca;                                                   % Another Option
Lines =  findobj(Ax, 'Type','line');                        % Return Handles To All 'Line' Objects
x =  Lines.XData(:);                                        % Independent Variable (Force Column Vector With '(:)')
y =  Lines.YData(:);                                        % Dependent Variable  (Force Column Vector With '(:)')
Figure_Table = table(x,y, 'VariableNames',{'X values','Y values'})

• for older releases

data = extract_data_from_figures('data points.fig');  % uses function extract_data_from_figures below 
VN = data.names; % variable names 
data = data.Y;

x = data(:,1);
y = data(:,2);
 
figure(1)
plot(x,y);

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

function data = extract_data_from_figures(filename)
%%
% 
% Input  : File name <filename.fig> 
%          with multiple plots in a single file
% Output : struct data
%        : data.names  contains names of the display object Yvalues
%        : data.Y      contains the actual plot values withthe first column
%                      containing the x-values
%
% Written by Chetanya Puri, 2019
% Last Modified: Nov 6, 2019
%

fig = openfig(filename); % Open figure and assign it to fig object
dataObjs = findobj(fig,'-property','YData'); % Find all graphic objects with YData, in our case line values
xval        = dataObjs(1).XData; % Find the X-axis value
Ymat = [xval(:)]; % Create a matrix with first column of x values
for i=1:length(dataObjs)
    legend_name{i,1} = dataObjs(i).DisplayName;
    yval        = dataObjs(i).YData;
    Ymat = [Ymat yval(:)]; % Keep appending column vectors
end
close(fig); % close the figure
data.names = ['X';legend_name]; 
data.Y = Ymat;
end

once you have data in your workspace you can use decimate or resample to change the rate as you like

decimate - Decimation — decrease sample rate by integer factor - MATLAB - MathWorks France

resample - Resample uniform or nonuniform data to new fixed rate - MATLAB - MathWorks France

those functions requires the Signal Processing Tbx

If you don't have it , you can make a crude downsampling by taking every k samples of the original data (would normally require that you first low pass your data to avoid aliasing)

Answer 2

[Aniket]

Hi @SM,

You can extract the data from figure and then resample it. To extract the data, open the figure and use ‘findobj’ function to locate the line objects within figure. Then the data ('XData' and 'YData') can be extracted from these line objects. Find a sample code below:

fig = openfig('test_figure.fig'); 

lineObjs = findobj(fig, 'type', 'line'); 
xdata = get(lineObjs, 'XData'); 
ydata = get(lineObjs, 'YData'); 

Refer this answer for detailed explanation on extracting data from a figure:

https://www.mathworks.com/matlabcentral/answers/100687-how-do-i-extract-data-from-matlab-figures

To resample it, first calculate the original sampling time and use ‘resample’ function to get new 'YData'. You can also create a new time vector (x values) if you want to plot the new figure.

original_sampling_time = mean(diff(xdata)); 
new_sampling_time = 0.1; 
[p, q] = rat(original_sampling_time/ new_sampling_time);  % Calculate resampling factors 
  
% Resample the yData 
new_yData = resample(ydata, p, q); 
  
% Resample the yData 
new_yData = resample(ydata, p, q); 
new_num_points = length(new_yData); 
new_time_vector = (0:new_num_points-1) * new_sampling_time; 
  
figure; 
plot(new_time_vector, new_yData); 
xlabel('Time (s)'); 
ylabel('Amplitude'); 
title('Resampled Data'); 

savefig('resampled_figure.fig');

Refer to MATLAB documentation on ‘resample’ function for further details:

https://www.mathworks.com/help/signal/ref/resample.html

Hope this answers your query!