# Extract maximum "i" surface from vtkStructuredGrid?

**URL:** https://discourse.vtk.org/t/extract-maximum-i-surface-from-vtkstructuredgrid/12401
**Category:** Support
**Created:** [October 2, 2023, 8:13pm UTC](https://discourse.vtk.org/t/extract-maximum-i-surface-from-vtkstructuredgrid/12401 "2023-10-02T20:13:56Z")
**Posts on this page:** 2
**Page:** 1

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### Author: ![rexthor](https://discourse.vtk.org/user_avatar/discourse.vtk.org/rexthor/32/6481_2.png) [@rexthor](https://discourse.vtk.org/u/rexthor)
#### Post date: [October 2, 2023, 8:13pm UTC](https://discourse.vtk.org/t/extract-maximum-i-surface-from-vtkstructuredgrid/12401/1 "2023-10-02T20:13:56Z")

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I have a vtkStructuredGrid, read in from a CFD datafile.  
I’d like to be able to extract the surface where _i_ is maximum, that is to say the surface where the first index is maximum, over all of the other two indices.

Is there a way to do that? (The surface is like a tube of toothpaste that has been twisted up - I can’t just dot the surface normal with (1, 0, 0) for instance).

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<div class="post-metadata">

### Author: ![rexthor](https://discourse.vtk.org/user_avatar/discourse.vtk.org/rexthor/32/6481_2.png) [@rexthor](https://discourse.vtk.org/u/rexthor)
#### Post date: [October 2, 2023, 8:20pm UTC](https://discourse.vtk.org/t/extract-maximum-i-surface-from-vtkstructuredgrid/12401/2 "2023-10-02T20:20:08Z")

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I just found [vtkStructuredGridGeometryFilter](https://vtk.org/doc/nightly/html/classvtkStructuredGridGeometryFilter.html), which looks like it might work for me.