# legacy file format: data type and version
**URL:** https://discourse.vtk.org/t/legacy-file-format-data-type-and-version/16299
**Category:** Development
**Tags:** file-formats
**Created:** [February 22, 2026, 9:33am UTC](https://discourse.vtk.org/t/legacy-file-format-data-type-and-version/16299 "2026-02-22T09:33:11Z")
**Posts on this page:** 7
**Page:** 1
### Author:  [@MarDiehl](https://discourse.vtk.org/u/MarDiehl)
#### Post date: [February 22, 2026, 9:33am UTC](https://discourse.vtk.org/t/legacy-file-format-data-type-and-version/16299/1 "2026-02-22T09:33:11Z")
VTK 9.6.0 has a weird change behavior when it comes to the legacy file format: The data type for `OFFSETS` and `CONNECTIVITY` of `POLYDATA` is `vtktypeint64` after creation (using the Python bindings) but changes to `long` when saving+reading to VTKHDF. When writing without the detour via VTKHDF, the file has a mixture of C-types (`double`) and VTK-types (`vtktypeint64`), with the 9.6.0 change this is more consistent.
My understanding is that the VTK-specific types such as `vtktypeint64` are fixed while `long` etc. can differ between C libraries/operating systems (see, e.g., [https://stackoverflow.com/questions/63938192](https://stackoverflow.com/questions/63938192)), so VTK-types seem to be the better choice.
[Simple Legacy Formats - VTK documentation](https://docs.vtk.org/en/latest/vtk_file_formats/vtk_legacy_file_format.html) does not mention `vtktypeint64` as a valid `dataType`. But it also states that the current version is 3.0 while VTK 9.5.2 and 9.6.0 write version 5.1. The link in [VTK Legacy File Format Specification](https://discourse.vtk.org/t/vtk-legacy-file-format-specification/13591) does not work anymore.
My questions would be
- Which datatypes are supported?
- Is there a preferred variant, e.g. `vtktypeint64` or `long`?
- Can I contribute to the documentation?
---
### Author:  [@mwestphal](https://discourse.vtk.org/u/mwestphal)
#### Post date: [February 23, 2026, 8:42am UTC](https://discourse.vtk.org/t/legacy-file-format-data-type-and-version/16299/2 "2026-02-23T08:42:42Z")
Can you share steps to reproduce that highlight the regression ?
@Louis_Gombert @lgivord
---
### Author:  [@spyridon97](https://discourse.vtk.org/u/spyridon97)
#### Post date: [February 23, 2026, 4:12pm UTC](https://discourse.vtk.org/t/legacy-file-format-data-type-and-version/16299/3 "2026-02-23T16:12:20Z")
@MarDiehl a reproducer would be really appriciated.
---
### Author:  [@MarDiehl](https://discourse.vtk.org/u/MarDiehl)
#### Post date: [February 25, 2026, 6:11pm UTC](https://discourse.vtk.org/t/legacy-file-format-data-type-and-version/16299/4 "2026-02-25T18:11:58Z")
here is a reproducer. It’s from a larger code base and a few things can be stripped down but I think it’s easy to follow.
```python
#! /usr/bin/env python3
import os
import multiprocessing as mp
import logging
import contextlib
from pathlib import Path
from typing import Optional, Union, Literal, Sequence
import numpy as np
# needed for visualization but might not be available everywhere
# https://gitlab.kitware.com/vtk/vtk/-/issues/19687
with contextlib.suppress(ImportError):
import vtkmodules.vtkRenderingOpenGL2 # noqa
from vtkmodules.vtkCommonCore import (
vtkVersion,
vtkPoints,
vtkStringArray,
vtkLookupTable,
)
if has_vtkhdf := (np.lib.NumpyVersion(vtkVersion.GetVTKVersion()) >= '9.4.0'):
from vtkmodules.vtkIOHDF import vtkHDFReader, vtkHDFWriter
from vtkmodules.vtkCommonDataModel import (
vtkDataSet,
vtkCellArray,
vtkImageData,
vtkRectilinearGrid,
vtkUnstructuredGrid,
vtkPointData,
vtkCellData,
vtkPolyData,
VTK_LAGRANGE_TRIANGLE,
VTK_LAGRANGE_QUADRILATERAL,
VTK_LAGRANGE_TETRAHEDRON,
VTK_LAGRANGE_HEXAHEDRON,
)
from vtkmodules.vtkIOLegacy import (
vtkGenericDataObjectReader,
vtkDataSetWriter,
)
from vtkmodules.vtkIOXML import (
vtkXMLWriter,
vtkXMLImageDataReader,
vtkXMLImageDataWriter,
vtkXMLRectilinearGridReader,
vtkXMLRectilinearGridWriter,
vtkXMLUnstructuredGridReader,
vtkXMLUnstructuredGridWriter,
vtkXMLPolyDataReader,
vtkXMLPolyDataWriter,
)
from vtkmodules.vtkRenderingCore import (
vtkDataSetMapper,
vtkActor,
vtkRenderer,
vtkRenderWindow,
vtkRenderWindowInteractor,
)
from vtkmodules.vtkRenderingAnnotation import (
vtkScalarBarActor,
)
from vtkmodules.util.numpy_support import (
numpy_to_vtk,
numpy_to_vtkIdTypeArray,
vtk_to_numpy,
)
class VTK:
"""
Spatial visualization (and potentially manipulation).
High-level interface to VTK.
"""
def __init__ (self,
vtk_data: vtkDataSet):
"""
New spatial visualization.
Parameters
----------
vtk_data : subclass of vtkDataSet
Description of geometry and topology, optionally with attached data.
Valid types are vtkImageData, vtkUnstructuredGrid,
vtkPolyData, and vtkRectilinearGrid.
"""
self.vtk_data: vtkDataSet = vtk_data
@staticmethod
def from_poly_data(points: np.ndarray) -> 'VTK':
"""
Create VTK of type polyData.
This is the common type for point-wise data.
Parameters
----------
points : numpy.ndarray, shape (:,3)
Spatial position of the points.
Returns
-------
new : damask.VTK
VTK-based geometry without nodal or cell data.
"""
N = points.shape[0]
vtk_points = vtkPoints()
vtk_points.SetData(numpy_to_vtk(np.ascontiguousarray(points)))
vtk_cells = vtkCellArray()
vtk_cells.SetNumberOfCells(N)
vtk_cells.SetCells(N,numpy_to_vtkIdTypeArray(np.stack((np.ones (N,dtype=np.int64),
np.arange(N,dtype=np.int64)),axis=1).ravel(),deep=True))
vtk_data = vtkPolyData()
vtk_data.SetPoints(vtk_points)
vtk_data.SetVerts(vtk_cells)
return VTK(vtk_data)
@staticmethod
def load(fname: Union[str, Path],
dataset_type: Literal[None, 'ImageData', 'UnstructuredGrid', 'PolyData', 'RectilinearGrid'] = None) -> 'VTK':
"""
Load from VTK file.
Parameters
----------
fname : str or pathlib.Path
Filename to read.
Valid extensions are .vti, .vtu, .vtp, .vtr, vtkhdf, and .vtk.
dataset_type : {'ImageData', 'UnstructuredGrid', 'PolyData', 'RectilinearGrid'}, optional
Name of the vtkDataSet subclass when opening a .vtk file.
Returns
-------
loaded : damask.VTK
VTK-based geometry from file.
Notes
-----
Loading VTKHDF files requires VTK 9.4 or later and presently supports
PolyData, UnstructuredGrid, and ImageData. Loading ImageData is untested
because VTK does not yet provide the functionality to write ImageData
into VTKHDF format.
"""
if not Path(fname).expanduser().is_file(): # vtk has a strange error handling
raise FileNotFoundError(f'file "{fname}" not found')
if (ext := Path(fname).suffix) == '.vtk' or dataset_type is not None:
reader_legacy = vtkGenericDataObjectReader()
reader_legacy.SetFileName(str(Path(fname).expanduser()))
reader_legacy.Update()
if dataset_type is None:
raise TypeError('missing dataset type for legacy VTK file')
dtl = dataset_type.lower()
vtk_data = (
reader_legacy.GetStructuredPointsOutput() if dtl.endswith(('imagedata', 'image_data')) else
reader_legacy.GetUnstructuredGridOutput() if dtl.endswith(('unstructuredgrid', 'unstructured_grid')) else
reader_legacy.GetPolyDataOutput() if dtl.endswith(('polydata', 'poly_data')) else
reader_legacy.GetRectilinearGridOutput() if dtl.endswith(('rectilineargrid', 'rectilinear_grid')) else
None
)
if vtk_data is None:
raise TypeError(f'unsupported VTK dataset type "{dataset_type}"')
else:
reader = (
vtkXMLImageDataReader() if ext == '.vti' else
vtkXMLUnstructuredGridReader() if ext == '.vtu' else
vtkXMLPolyDataReader() if ext == '.vtp' else
vtkXMLRectilinearGridReader() if ext == '.vtr' else
vtkHDFReader() if (ext == '.vtkhdf') else
None
)
if reader is None:
raise TypeError(f'unsupported VTK file extension "{ext}"')
reader.SetFileName(str(Path(fname).expanduser()))
reader.Update()
vtk_data = reader.GetOutputAsDataSet()
return VTK(vtk_data)
def as_ASCII(self) -> str:
"""ASCII representation of the VTK data."""
writer = vtkDataSetWriter()
writer.WriteToOutputStringOn()
writer.SetInputData(self.vtk_data)
writer.Write()
return writer.GetOutputString()
def save_VTKHDF(self,
fname: Union[str, Path]):
"""
Save as VTKHDF file.
Parameters
----------
fname : str or pathlib.Path
Filename to write.
Notes
-----
Saving as VTKHDF file requires VTK 9.4 or later and only supports
PolyData and UnstructuredGrid.
"""
if not isinstance(self.vtk_data, (vtkPolyData, vtkUnstructuredGrid)):
raise TypeError(f'unsupported vtk_data type "{type(self.vtk_data)}"')
writer = vtkHDFWriter()
ext = Path(fname).suffix
writer.SetFileName(str(Path(fname).expanduser())+('.vtkhdf' if '.vtkhdf' != ext else ''))
writer.SetInputData(self.vtk_data)
writer.Write()
rng = np.random.default_rng(20191102)
points = rng.random((100,3))
v = VTK.from_poly_data(points)
v.save_VTKHDF('polyData')
vtkhdf = VTK.load('polyData.vtkhdf')
print(v.as_ASCII())
print(vtkhdf.as_ASCII())
assert (v.as_ASCII() == vtkhdf.as_ASCII())
```
---
### Author:  [@mwestphal](https://discourse.vtk.org/u/mwestphal)
#### Post date: [February 26, 2026, 8:39am UTC](https://discourse.vtk.org/t/legacy-file-format-data-type-and-version/16299/5 "2026-02-26T08:39:17Z")
Could you just share the .vtk file that exhibit the behavior when save into VTKHDF ?
---
### Author:  [@MarDiehl](https://discourse.vtk.org/u/MarDiehl)
#### Post date: [February 27, 2026, 9:55am UTC](https://discourse.vtk.org/t/legacy-file-format-data-type-and-version/16299/6 "2026-02-27T09:55:03Z")
attached are the two files (writing `v.as_ASCII()` and `vtkhdf.as_ASCII()`) to file).
`diff polyData_detour.vtk polyData_orig.vtk` gives:
```auto
41c41
< OFFSETS long
---
> OFFSETS vtktypeint64
54c54
< CONNECTIVITY long
---
> CONNECTIVITY vtktypeint64
```
[polyData\_detour.vtk](https://discourse.vtk.org/uploads/short-url/kpV6OGd0dRc4u2G3tyePvJNHFx5.vtk) (4.9 KB)
[polyData\_orig.vtk](https://discourse.vtk.org/uploads/short-url/fHXeuF4EIBnIxhZcEskMM8Mczag.vtk) (4.9 KB)
---
### Author:  [@mwestphal](https://discourse.vtk.org/u/mwestphal)
#### Post date: [March 2, 2026, 9:27am UTC](https://discourse.vtk.org/t/legacy-file-format-data-type-and-version/16299/7 "2026-03-02T09:27:59Z")
Confirmed. the vtkHDFReader seems to have an impact here.
@Louis_Gombert @lgivord wdyt ?