# Extract a user-defined surface from 3D data

**URL:** https://discourse.paraview.org/t/extract-a-user-defined-surface-from-3d-data/2858
**Category:** ParaView Support
**Created:** [October 27, 2019, 6:34pm UTC](https://discourse.paraview.org/t/extract-a-user-defined-surface-from-3d-data/2858 "2019-10-27T18:34:51Z")
**Posts on this page:** 2
**Page:** 1

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### Author: ![henry2004y](https://discourse.paraview.org/user_avatar/discourse.paraview.org/henry2004y/32/2296_2.png) [@henry2004y](https://discourse.paraview.org/u/henry2004y)
#### Post date: [October 27, 2019, 6:34pm UTC](https://discourse.paraview.org/t/extract-a-user-defined-surface-from-3d-data/2858/1 "2019-10-27T18:34:51Z")

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Hi there,

I have a 3D unstructured VTK file from fluid simulation, and I want to extract a physical boundary (e.g. shock front) from the data. The boundary can be defined with certain kind of math expressions, for example, then density jump/gradient larger than a threshold. However, it may be more complicated than this. For example, I may need a boundary defined by the topology of the magnetic field, i.e. the location of the end points of the fieldline that connects through a given cell. What is the proper way of doing this? Can I extract the information from that surface and do more analysis later on?

Thanks!

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### Author: ![mwestphal](https://discourse.paraview.org/user_avatar/discourse.paraview.org/mwestphal/32/17_2.png) [@mwestphal](https://discourse.paraview.org/u/mwestphal)
#### Post date: [October 28, 2019, 7:43am UTC](https://discourse.paraview.org/t/extract-a-user-defined-surface-from-3d-data/2858/2 "2019-10-28T07:43:12Z")

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Can you compute the boundary considering each cell or points individually ?  
If yes, then use PythonCalculator.  
If not, because you need to do more global computation over the whole mesh, then ProgrammableFilter.