# ImageData/PolyData XML writer: how save a lookup table associated to a cell data array
**URL:** https://discourse.vtk.org/t/imagedata-polydata-xml-writer-how-save-a-lookup-table-associated-to-a-cell-data-array/6302
**Category:** Support
**Created:** [August 6, 2021, 1:37am UTC](https://discourse.vtk.org/t/imagedata-polydata-xml-writer-how-save-a-lookup-table-associated-to-a-cell-data-array/6302 "2021-08-06T01:37:15Z")
**Posts on this page:** 4
**Page:** 1
### Author:  [@bva99](https://discourse.vtk.org/u/bva99)
#### Post date: [August 6, 2021, 1:37am UTC](https://discourse.vtk.org/t/imagedata-polydata-xml-writer-how-save-a-lookup-table-associated-to-a-cell-data-array/6302/1 "2021-08-06T01:37:15Z")
I associated a simple lookup table to the cellarray of an image data.
After generating a `vtkPolyData` from the `vtkImageData` using `vtkImageDataGeometryFilter`, it is possible to write the resulting output either to a legacy vtk file using `vtkPolyDataWriter` or to an XML file using `vtkXMLPolyDataWriter`.
The legacy vtk file saves the associated lookup table, while XML file does not. Thus, when loading the former in ParaView, it shows the coloring as intended, unlike the latter.
Is there a way to write a “composite” XML file that associates a .vtp file with a .vtt file directly? Or maybe even a .vti file with a .vtt file… **Edit** : I checked the doxygen more carefully. .vtt files are for `vtkTable`, not lookup tables. Nvm. Only post I’ve found is an [archived post](https://public.kitware.com/pipermail/vtkusers/2017-September/099715.html) of someone who managed to use a .json file for the lookup table, which I don’t know how to use.
The python script below is a MWE that generates the .vtk and .vtp files. A png image of the output is shown here after the script (the code with the pngwriter is not in the script, for brevity).
```python
import numpy as np
from vtk.util.numpy_support import numpy_to_vtk
import vtk
def main():
colors = vtk.vtkNamedColors()
# create color lookup table
lut = vtk.vtkLookupTable()
lut.SetNumberOfTableValues(2)
yellow = colors.GetColor3d("light_goldenrod")
blue = colors.GetColor3d("royal_blue")
lut.SetTableValue(0, yellow[0], yellow[1], yellow[2], 0.8)
lut.SetTableValue(1, blue[0], blue[1], blue[2], 1.0)
# Create an image
myMesh = vtk.vtkImageData()
myMesh.SetOrigin(0., 0., 0.)
myMesh.SetSpacing(0.01, 0.01, 0.)
nx = 10
ny = 5
myMesh.SetExtent(0, nx, 0, ny, 0, 0)
# solid or void
scalarsnp = np.ones(nx*ny, dtype=np.int32) # all solids
scalarsnp[[nx-1, nx*ny-1]] = 0 # two voids
scalarsvtk = numpy_to_vtk(scalarsnp, deep=1, array_type=vtk.VTK_LONG)
scalarsvtk.SetName("colorsArray")
scalarsvtk.SetLookupTable(lut) # associate lookup table to array
# allocate the scalars to the vtkImageData object
myMesh.GetCellData().SetScalars(scalarsvtk)
# create a filter object
myMeshPolyData = vtk.vtkImageDataGeometryFilter()
myMeshPolyData.SetInputData(myMesh)
# write polygonal dataset to legacy vtk
# this saves the associated lookup table
vtkWriter = vtk.vtkPolyDataWriter()
vtkWriter.SetInputConnection(myMeshPolyData.GetOutputPort())
vtkWriter.SetFileTypeToASCII()
vtkWriter.SetFileName("./mesh.vtk")
vtkWriter.SetHeader("Simple colored ImageData")
vtkWriter.Write()
# write polygonal dataset to XML vtp file
# this does not
vtpWriter = vtk.vtkXMLPolyDataWriter()
vtpWriter.SetInputConnection(myMeshPolyData.GetOutputPort())
vtpWriter.SetDataModeToAscii()
vtpWriter.SetFileName("./mesh.vtp")
vtpWriter.Write()
if __name__ == ' __main__':
main()
```
---
### Author:  [@bva99](https://discourse.vtk.org/u/bva99)
#### Post date: [August 6, 2021, 1:44pm UTC](https://discourse.vtk.org/t/imagedata-polydata-xml-writer-how-save-a-lookup-table-associated-to-a-cell-data-array/6302/2 "2021-08-06T13:44:17Z")
Following the suggestions [in this post](https://discourse.vtk.org/t/any-way-to-insert-comments-in-a-file-created-by-vtkxmlimagedatawriter/161), using the [/Utilities/SaveSceneToFieldData example](https://kitware.github.io/vtk-examples/site/Cxx/Utilities/SaveSceneToFieldData/), I’ve managed to write the lookuptable by getting it via `lut.GetTable()` and associating it to the `vtkImageData`'s `vtkFieldData`. The code is simply:
```auto
lut.GetTable().SetName("colorsArray")
myMesh.GetFieldData().AddArray(lut.GetTable())
```
But I still cannot make, for instance, ParaView, use this fielddata as the lookuptable to map the cell scalars.
Interestingly, [an issue on this was opened in 2004](https://vtk.org/Bug/view.php?id=484), but never solved.
---
### Author:  [@Charles\_Gueunet](https://discourse.vtk.org/u/Charles_Gueunet)
#### Post date: [August 6, 2021, 2:26pm UTC](https://discourse.vtk.org/t/imagedata-polydata-xml-writer-how-save-a-lookup-table-associated-to-a-cell-data-array/6302/3 "2021-08-06T14:26:46Z")
The XML Reader and Writer are meant to save data sets while the lookup table is a matters of representation. If you want to save both the data sets and the representation, you may want to export the scene. This can be done using a `vtkExporter`. Take the implementation of this class that suits your need and use `SetRenderWindow` to give it the scene to save, it should take the light, camera position and LUT into account if the format accept it.
---
### Author:  [@bva99](https://discourse.vtk.org/u/bva99)
#### Post date: [August 6, 2021, 2:55pm UTC](https://discourse.vtk.org/t/imagedata-polydata-xml-writer-how-save-a-lookup-table-associated-to-a-cell-data-array/6302/4 "2021-08-06T14:55:39Z")
Thank you for the suggestion. I wanted to avoid using `vtkExporter` because of the added information being stored, since it is much further down the visualization pipeline.
Instead, I’ve settled for using the field data method. The user has to read the .vti XML file and manually set the field data array as the lookup table for the cell data array.
Suppose the data is written as a .vti file, for simplicity, and continuing the example written in the OP:
```auto
# write polygonal dataset to XML vtp file
# this does not
# vtpWriter = vtk.vtkXMLPolyDataWriter()
# vtpWriter.SetInputConnection(myMeshPolyData.GetOutputPort())
# vtpWriter.SetDataModeToAscii()
# vtpWriter.SetFileName("./mesh.vtp")
# vtpWriter.Write()
lut.GetTable().SetName("colorsArray")
myMesh.GetFieldData().AddArray(lut.GetTable())
vtiWriter = vtk.vtkXMLImageDataWriter()
vtiWriter.SetInputData(myMesh)
vtiWriter.SetFileName("./mesh.vti")
vtiWriter.Write()
vtiReader = vtk.vtkXMLImageDataReader
vtiReader.SetFileName("./mesh.vti")
vtiReader.Update()
myMesh2 = vtiReader.GetOutput()
vtiReader.FastDelete() # is this safe?
lut2 = vtk.vtkLookupTable()
lut2.SetTable(myMesh2.GetFieldData().GetArray(0))
myMesh2.GetCellData().GetArray(0).SetLookupTable(lut2)
```
It would be great if this could be done implicitly, as in the legacy .vtk files.