# vtkHDF: Add support for polyhedron cells

**URL:** https://discourse.vtk.org/t/vtkhdf-add-support-for-polyhedron-cells/13089
**Category:** Development
**Tags:** file-formats
**Created:** [January 17, 2024, 4:32pm UTC](https://discourse.vtk.org/t/vtkhdf-add-support-for-polyhedron-cells/13089 "2024-01-17T16:32:10Z")
**Posts on this page:** 1
**Showing post:** 33

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### Author: ![olesenm](https://discourse.vtk.org/user_avatar/discourse.vtk.org/olesenm/32/10443_2.png) [@olesenm](https://discourse.vtk.org/u/olesenm)
#### Post date: [November 7, 2025, 7:45am UTC](https://discourse.vtk.org/t/vtkhdf-add-support-for-polyhedron-cells/13089/33 "2025-11-07T07:45:36Z")

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Very late reply here.

I very much like the idea of negative indices to indicate a flipped face orientation. In fact, we use it frequently enough in our own code.

The only adjustment you need is to define it as `-idx-1`, (or as `-(idx+1) `if overflow isn’t an issue). This means that a flipped face 0 would be -1, a flipped face 1 is -2, etc.

Another convention we use is that a flipped face always retains its point 0 and only the rest of the list is reversed. This makes life much easier.

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