How to advect first-image pixels to second-image time with PIV velocity?

Brian Mapes

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Sep 10, 2022, 2:29:08 PM9/10/22

to openpiv-users

The 2D PIV velocity field looks good from my data using the first tutorial notebook. Hooray!

But what I really want is an Eulerian field of the Lagrangian tendency: isolating the development of convection cells, wave modulations, etc. That is, I want to subtract from image2 an image composed of the image1's pixels, shifted according to the PIV-derived velocity (or really, displacement) field.

I can see ways how I could proceed, using the output coarse V field to shift and write sliced-out patches of values from image1 into a full-resolution dummy array. There would be gaps to fill where the flow is divergent, and overlaps to resolve somehow where it is convergent.

If I used buffer zones or "overlap" then I could avoids gaps and only have to resolve overlaps. As a programmer, I could do something from scratch.

winsize = 32 # pixels, interrogation window size in frame A
searchsize = 38 # pixels, search in image B

overlap = 12 # pixels, 50% overlap

But am I reinventing the mousetrap here? Does someone already have an elegant code? Or the right words to use to improve my google-fu about finding more?

Brian Mapes

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Sep 10, 2022, 3:05:31 PM9/10/22

to openpiv-users

For definiteness, here is the movie I am trying to decompose into advection + evolution + waves:

and here is the OpenPIV tutorial used naively: quite successful in the vectors matching my eyes:

Alex Liberzon

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Sep 10, 2022, 4:06:04 PM9/10/22

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Just read the source code of windef - it's doing precisely this, but in the opposite direction - from coarse to fine

https://openpiv.readthedocs.io/en/latest/src/windef.html

https://github.com/OpenPIV/openpiv-python/blob/master/openpiv/windef.py

if need more inputs, make your repo fork, and mention me in the issue

Alex Liberzon

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Sep 10, 2022, 4:28:51 PM9/10/22

to Brian Mapes, openpiv-users

Just a note - you can use MP4 directly, see some examples on openpiv-python-examples

https://github.com/OpenPIV/openpiv-python-examples/blob/main/notebooks

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Brian Mapes

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Sep 10, 2022, 4:37:14 PM9/10/22

to Alex Liberzon, openpiv-users

Oh nifty, !

https://github.com/OpenPIV/openpiv-python-examples/blob/main/notebooks/analyse_movie.ipynb