# VTKHDF point ownership

**URL:** https://discourse.vtk.org/t/vtkhdf-point-ownership/13913
**Category:** Support
**Tags:** file-formats
**Created:** [May 21, 2024, 9:54am UTC](https://discourse.vtk.org/t/vtkhdf-point-ownership/13913 "2024-05-21T09:54:12Z")
**Posts on this page:** 1
**Showing post:** 4

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### Author: ![jorgensd](https://discourse.vtk.org/user_avatar/discourse.vtk.org/jorgensd/32/8861_2.png) [@jorgensd](https://discourse.vtk.org/u/jorgensd)
#### Post date: [May 27, 2024, 9:02am UTC](https://discourse.vtk.org/t/vtkhdf-point-ownership/13913/4 "2024-05-27T09:02:39Z")

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If the points are duplicate over multiple processes, one has to adjust the global indices on all other processes, which makes it non-trivial (i.e. requires a global exscan) to write data in parallel, and would make the data more challenging to read in again at a later stage, as one would have to eliminate duplicate points.

Thus I would go for the non-duplicate version, i.e. I have M global nodes, which in the connectivity array is filled with the global indices. However, then it is unclear to me how to specify that a node is shared between multiple (let say two) processes, as the only information we provide in the VTKHDF file is

> We describe the split into partitions using HDF5 datasets `NumberOfConnectivityIds`, `NumberOfPoints` and `NumberOfCells`.

However, this doesn’t indicate the start and end of NumberOfPoints.  
Following is a minimal example of how a mesh is distributed in my code:

We have a 1x1 unit square consisting of two triangles, and in total four nodes.  
Each processes owns a single triangle, lets say  
Proc 0 owns the cell with vertices [0,0], [1,0], [0,1]  
Proc 1 owns the cell with vertices [1,0], [1,1],[0,1]

If we write the unique nodes to file, say in the following order  
[[0,0],[1,0],[0,1],[1,1]]  
we need to indicate that process 0 has to access the first three points, range [0,3) while process 1 wants to access the nodes in range [1, 4).

I am not sure how to specify this with VTKHDF.

To make the issue even clearer, one could add another process with a single cell  
[-1,0], [0,0], [0, 1], where we append [-1,0] at the end of the points array, meaning that process (2) would need point indices 0, 2 and 4 available (as that would be the connectivity array).

Would `NumberOfPoints` be three for each process?

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