FFT on plot data over time

Hi all,

I am importing a data set of .vtu files and used the “Plot data over time” filter to plot my data. I now want to apply a FFT using the “FFT of selection over time” and I get an error that says “No time steps in input data”. I am plotting time vs. velocity. Has anyone successfully used the FFT Filter on a set of data. Can’t find any sources on the web. Thanks in advance!

Are you using file series ? Please share an image of your pipeline browser.

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I’m guessing that you need to run the FFT Of Selection Over Time filter before adding the Plot Data Over Time. The FFT Of Selection Over Time filter will automatically pull data over time. So does the Plot Data Over Time, and these filters confuse each other.

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Yes I am, here is how the pipeline browser looks.image


When first applying the FFT filter, we get no plot visual or data shown on the left.

When applying the “Plot data over time” first, we get visuals and data to choose on the left.

When trying to apply “FFT filter” onto the “Plot data over time”, I get an error like so,

Thanks for the help in advance!

This might be easier if you could post some example data.

Like I said, running FFT Of Selection Over Time on the output of Plot Data Over Time just won’t work.

It’s weird that the selection box says the Process ID is -1. Try making the selection again and hitting the Copy Active Selection button.

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Hi Kenneth,

The process ID is no longer -1. Here’s screenshot after applying the FFT Filter. I checked the vtkValidPointMask (id=0) box to see the horizontal black line.

I feel we are a step closer. I was looking to apply it to the y-velocity (in red) from my previous replies.
Thank you!

Is your selection of the same type as the field data you are taking an FFT of? If you are taking an FFT of point data, is it a point selection? If you are taking an FFT of cell data, is it a cell selection?

Also, I believe the FFT only works on scalar data. If you are trying to take the FFT of vectors, you will have to use the Calculator filter to first extract whatever components or magnitude you want an FFT of.

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Hi Kenneth,

I applied the Calculator filter, set it to scalar velocity_y, then applied the FFT filter onto the Calculator Filter, and here are the results. I also trimmed my data from my previous replies from [0, 55] to [44, 55].

I was looking to apply the FFT filter to find the frequencies. It seems the x-axis is time.

The X axis is not time. It is just the index into the array. The FFT Of Selection Over Time filter does not compute the frequencies, so the plot just gives you the indices of the array holding the values computed by the FFT. The frequencies wouldn’t mean much anyway since the the vtu file series does not properly mark time anyway.

Since both FFT Of Selection Over Time and Plot Data Over Time are using array indices for the x axis, and both arrays are the same length, they look the same. But they do not have the same meaning.

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Hi Kenneth,

Yes, I understand now. The plot from my previous reply is just what I was going for. Thank you very much for your help.