Conversion of (129x7 complex double) to an Image

I have a variable s (129x7 complex double). I want to save it as an image. I have tried many ways but I can't save it.

for k=1:size(cm,2)
    s = spectrogram(cm(:,k));
    save(['Spectro_Subj' num2str(h) '_channel' num2str(i) '_window' num2str(k) '.mat'],'s')
end

[DGM]

Typical image formats (BMP,PNG,JPG) support unsigned integer (or logical) datatypes. Some (TIFF) can be wrangled into supporting floating-point numeric data, but you shouldn't expect anything to be able to read the file. No image format that I know of supports complex-valued numeric data. You may be able to store the components of the data in separate images (real and imag), or perhaps it may suffice to reduce it to magnitude only.

Long story short, if you have arbitrarily-scaled complex data and you want to save it as a typical image format, you'll have to decide on a means of preserving scale information, and you'll have to decide which components of the complex data you want to keep or how you want to encode them into a single real-valued integer array (perhaps magnitude/phase can be encoded as intensity/hue?).

[Walter Roberson]

Tiff supports ComplexIEEEFP. However MATLAB's interface to libTiff does not support that.

Answer 1

[Image Analyst]

Try exportgraphics

for k=1:size(cm,2)
    spectrogram(cm(:,k));
    baseFileName = sprintf('Spectro_Subject %d_channel %d_window %d.png', h, i, k);
    fullFileName = fullfile(pwd, baseFileName);
    fprintf('For k = %d, saving "%s".\n', k, fullFileName);
    exportgraphics(gca, fullFileName);
end

[DGM]

The plot won't be created if spectrogram() is called with output arguments.

[Muhammad]

thank you so much for the answer, but yeah the "The plot won't be created if spectrogram() is called with output arguments".

[Image Analyst]

Thanks @DGM, I didn't know that. I just assumed his code for producing the image worked. You might want to specifically mention that in your answer.

And I couldn't try it with "cm" because @Muhammad forgot to attach his data in a text or .mat file with the paperclip icon, probably because he has not seen this yet: TUTORIAL: How to ask a question (on Answers) and get a fast answer

I was just addressing the saving problem and I wanted to mention that exportgraphics is generally what Mathworkers tell me is the most recommended function for saving figures and axes. Of course you have to have a figure or axes showing in the first place before you can save it.

[DGM]

I don't think I've ever used spectrogram() before this, so it was all new to me too. @Muhammad, you might want to check spectrogram() to make sure there aren't other parameters you want to specify (sampling rate, etc). There are a lot of options that may be relevant.

[Muhammad]

@DGM@Image Analyst thank you so much for the precious dicussion, I finally got it.

s = spectrogram(cm(:,k));

spectrogram(cm(:,k),'yaxis')

saveas(gcf,['Spectro_Subj' num2str(h) '_channel' num2str(i) '_window' num2str(k)

'.png']);

[Image Analyst]

I think my way was easier to follow. Create the file name with sprintf() and fullfile(), then call exportgraphics() instead of saveas().

If one of the Answers solves your original question, then could you please click the "Accept this answer" link to award the answerer with "reputation points" for their efforts in helping you? They'd appreciate it. Thanks in advance. 🙂 Note: you can only accept one answer (so pick the best one) but you can click the "Vote" icon for as many Answers as you want. Voting for an answer will also award reputation points.

[Muhammad]

@Image Analyst thank you so much i will. In the mean time as you can see i segment the signal into 3.5s frame windows, i need to have time (0-3.5s) on x axis and Frequency on Y axis. Currently i am just getting samples and normalized frequency. please have a look into the below attachement.

[DGM]

Note the only reason why I don't use exportgraphics() isn't because saveas() is better. I just run an older version, so I can't verify my example with something I can't run.

Answer 2

[Walter Roberson]

you could use the tiff gateway and write the imaginary component as unassociated alpha. There is an example at

https://www.mathworks.com/help/matlab/ref/tiff.html#mw_a2efd938-557e-41ad-925b-64f84a51f07b

The difference is that you would specify 2 samples per pixel instead of 4, and you would use IEEEFP sample format.

There is a file exchange contribution for writing floating-point tiff that is worth studying.

So... you would be able to use an image file, and use floating point samples, and write two channels.

What you should not expect is for any other program to be able to produce a readable image from the data

Answer 3

[DGM]

Are you sure you don't want to just save a graphical representation (i.e. a plot)?

% some fake test signal
fs = 1000;
t = 0:1/fs:2-1/fs;
y = chirp(t,100,1,200,'quadratic');

% plot the spectrogram
spectrogram(y,100,80,100,fs,'yaxis')
colormap(parula(256))

% save the figure
saveas(gcf,'myplot.png')

If so, there are many different ways it can be represented.

[Muhammad]

thanks alot that is what i was looking for.