# Set block color of a MultiBlockDataSet
**URL:** https://discourse.vtk.org/t/set-block-color-of-a-multiblockdataset/16301
**Category:** Support
**Created:** [February 23, 2026, 3:26pm UTC](https://discourse.vtk.org/t/set-block-color-of-a-multiblockdataset/16301 "2026-02-23T15:26:55Z")
**Posts on this page:** 8
**Page:** 1
### Author:  [@BotellaA](https://discourse.vtk.org/u/BotellaA)
#### Post date: [February 23, 2026, 3:26pm UTC](https://discourse.vtk.org/t/set-block-color-of-a-multiblockdataset/16301/1 "2026-02-23T15:26:55Z")
I have a MultiBlockDataSet imported from a .vtm file. I am trying to set up specific colors for every block. My first attempt is to set the same color for every blocks but the color never changes.
Here is my Python code:
```python
import vtkmodules.vtkRenderingOpenGL2
from vtkmodules.vtkIOXML import vtkXMLMultiBlockDataReader
from vtkmodules.vtkFiltersGeometry import vtkGeometryFilter
from vtkmodules.vtkRenderingCore import (
vtkActor,
vtkCompositePolyDataMapper,
vtkCompositeDataDisplayAttributes,
vtkRenderer,
vtkRenderWindow,
vtkRenderWindowInteractor,
)
from vtkmodules.vtkInteractionStyle import vtkInteractorStyleTrackballCamera
from vtkmodules.vtkCommonDataModel import vtkDataObjectTreeIterator
VTM_PATH = "viewable.vtm"
renderer = vtkRenderer()
renderer.SetBackground(0.2, 0.2, 0.2)
render_window = vtkRenderWindow()
render_window.AddRenderer(renderer)
render_window.SetSize(1200, 700)
reader = vtkXMLMultiBlockDataReader()
reader.SetFileName(VTM_PATH)
reader.Update()
geo_filter = vtkGeometryFilter()
geo_filter.SetInputConnection(reader.GetOutputPort())
mapper = vtkCompositePolyDataMapper()
mapper.SetInputConnection(geo_filter.GetOutputPort())
attributes = vtkCompositeDataDisplayAttributes()
mapper.SetCompositeDataDisplayAttributes(attributes)
actor = vtkActor()
actor.SetMapper(mapper)
renderer.AddActor(actor)
original_block_iterator = vtkDataObjectTreeIterator(
data_set=reader.GetOutput(), visit_only_leaves=True, traverse_sub_tree=True)
original_block_iterator.InitTraversal()
while not original_block_iterator.IsDoneWithTraversal():
original_block = original_block_iterator.GetCurrentDataObject()
attributes.SetBlockColor(original_block, [0,1,1])
original_block_iterator.GoToNextItem()
mapper.Modified()
mapper.Update()
renderer.ResetCamera()
interactor = vtkRenderWindowInteractor()
interactor.SetRenderWindow(render_window)
style = vtkInteractorStyleTrackballCamera()
interactor.SetInteractorStyle(style)
render_window.Render()
interactor.Start()
```
Here is the result:
Maybe I am not using the MultiBlockDataSet correctly.
Thanks for the help
---
### Author:  [@jaswantp](https://discourse.vtk.org/u/jaswantp)
#### Post date: [February 23, 2026, 3:34pm UTC](https://discourse.vtk.org/t/set-block-color-of-a-multiblockdataset/16301/2 "2026-02-23T15:34:18Z")
In your pipeline, the output of `geo_filter` is passed to the mapper, so pass that output to the block iterator.
Here’s code. I cleaned it up a bit. Removed unneded calls to Modified() and Update(). See new code with the comment `# NEW`
```py
import vtkmodules.vtkRenderingOpenGL2
from vtkmodules.vtkIOXML import vtkXMLMultiBlockDataReader
from vtkmodules.vtkFiltersGeometry import vtkGeometryFilter
from vtkmodules.vtkRenderingCore import (
vtkActor,
vtkCompositePolyDataMapper,
vtkCompositeDataDisplayAttributes,
vtkRenderer,
vtkRenderWindow,
vtkRenderWindowInteractor,
)
from vtkmodules.vtkInteractionStyle import vtkInteractorStyleTrackballCamera
from vtkmodules.vtkCommonDataModel import vtkDataObjectTreeIterator
VTM_PATH = "viewable.vtm"
renderer = vtkRenderer()
renderer.SetBackground(0.2, 0.2, 0.2)
render_window = vtkRenderWindow()
render_window.AddRenderer(renderer)
render_window.SetSize(1200, 700)
reader = vtkXMLMultiBlockDataReader()
reader.SetFileName(VTM_PATH)
geo_filter = vtkGeometryFilter()
geo_filter.SetInputConnection(reader.GetOutputPort())
geo_filter.Update()
# NEW
geometry_output = geo_filter.GetOutput()
mapper = vtkCompositePolyDataMapper()
mapper.SetInputConnection(geometry_output) # NEW
attributes = vtkCompositeDataDisplayAttributes()
mapper.SetCompositeDataDisplayAttributes(attributes)
actor = vtkActor()
actor.SetMapper(mapper)
renderer.AddActor(actor)
original_block_iterator = vtkDataObjectTreeIterator(
data_set=geometry_output, # NEW
visit_only_leaves=True, traverse_sub_tree=True)
original_block_iterator.InitTraversal()
while not original_block_iterator.IsDoneWithTraversal():
original_block = original_block_iterator.GetCurrentDataObject()
attributes.SetBlockColor(original_block, [0,1,1])
original_block_iterator.GoToNextItem()
renderer.ResetCamera()
interactor = vtkRenderWindowInteractor()
interactor.SetRenderWindow(render_window)
style = vtkInteractorStyleTrackballCamera()
interactor.SetInteractorStyle(style)
render_window.Render()
interactor.Start()
```
---
### Author:  [@BotellaA](https://discourse.vtk.org/u/BotellaA)
#### Post date: [February 23, 2026, 3:48pm UTC](https://discourse.vtk.org/t/set-block-color-of-a-multiblockdataset/16301/3 "2026-02-23T15:48:00Z")
Thanks so much for the fast answer!!
Indeed, I need to use the output of the geometry filter to have to correct dataset for the traverse.
However, it does not change the resulting image.
I also need to leave the mapper.SetInputConnection(geo\_filter.GetOutputPort()) since the mapper want an input connection.
---
### Author:  [@jaswantp](https://discourse.vtk.org/u/jaswantp)
#### Post date: [February 23, 2026, 3:49pm UTC](https://discourse.vtk.org/t/set-block-color-of-a-multiblockdataset/16301/4 "2026-02-23T15:49:25Z")
> [@jaswantp](#):
>
> ```auto
> mapper.SetInputConnection(geometry_output) # NEW
>
> ```
sorry, this should be `mapper.SetInputData(geometry_output)`
---
### Author:  [@BotellaA](https://discourse.vtk.org/u/BotellaA)
#### Post date: [February 23, 2026, 3:51pm UTC](https://discourse.vtk.org/t/set-block-color-of-a-multiblockdataset/16301/5 "2026-02-23T15:51:39Z")
I also tried that but I got the same error:
```auto
vtkCompositeDataPipeline (0x31ffd870): Input port 0 of algorithm vtkCompositePolyDataMapper (0x32012040) has 0 connections but is not optional.
```
---
### Author:  [@jaswantp](https://discourse.vtk.org/u/jaswantp)
#### Post date: [February 23, 2026, 4:01pm UTC](https://discourse.vtk.org/t/set-block-color-of-a-multiblockdataset/16301/6 "2026-02-23T16:01:36Z")
okay. it complains because `GetOutput` returns a `vtkPolyData` but in your case we want a `vtkCompositeDataSet`. Replace `GetOutput` with `GetOutputDataObject`, and `SetInputData` with `SetInputDataObject`.
Full code that should work (tested on my end)
```py
import vtkmodules.vtkRenderingOpenGL2
from vtkmodules.vtkIOXML import vtkXMLMultiBlockDataReader
from vtkmodules.vtkFiltersGeometry import vtkGeometryFilter
from vtkmodules.vtkRenderingCore import (
vtkActor,
vtkCompositePolyDataMapper,
vtkCompositeDataDisplayAttributes,
vtkRenderer,
vtkRenderWindow,
vtkRenderWindowInteractor,
)
from vtkmodules.vtkInteractionStyle import vtkInteractorStyleTrackballCamera
from vtkmodules.vtkCommonDataModel import vtkDataObjectTreeIterator
VTM_PATH = "viewable.vtm"
renderer = vtkRenderer()
renderer.SetBackground(0.2, 0.2, 0.2)
render_window = vtkRenderWindow()
render_window.AddRenderer(renderer)
render_window.SetSize(1200, 700)
reader = vtkXMLMultiBlockDataReader()
reader.SetFileName(VTM_PATH)
geo_filter = vtkGeometryFilter()
geo_filter.SetInputConnection(reader.GetOutputPort())
geo_filter.Update()
# NEW
geometry_output = geo_filter.GetOutputDataObject(0)
mapper = vtkCompositePolyDataMapper()
mapper.SetInputDataObject(geometry_output) # NEW
attributes = vtkCompositeDataDisplayAttributes()
mapper.SetCompositeDataDisplayAttributes(attributes)
actor = vtkActor()
actor.SetMapper(mapper)
renderer.AddActor(actor)
original_block_iterator = vtkDataObjectTreeIterator(
data_set=geometry_output, # NEW
visit_only_leaves=True, traverse_sub_tree=True)
original_block_iterator.InitTraversal()
while not original_block_iterator.IsDoneWithTraversal():
original_block = original_block_iterator.GetCurrentDataObject()
attributes.SetBlockColor(original_block, [0,1,1])
original_block_iterator.GoToNextItem()
renderer.ResetCamera()
interactor = vtkRenderWindowInteractor()
interactor.SetRenderWindow(render_window)
style = vtkInteractorStyleTrackballCamera()
interactor.SetInteractorStyle(style)
render_window.Render()
interactor.Start()
```
---
### Author:  [@BotellaA](https://discourse.vtk.org/u/BotellaA)
#### Post date: [February 23, 2026, 5:09pm UTC](https://discourse.vtk.org/t/set-block-color-of-a-multiblockdataset/16301/7 "2026-02-23T17:09:49Z")
Thanks it is working.
Can you explain why I need to get the data out from the filter and not directly connect the algorithm connection output? Looks counter intuitive with the classic vtk pipeline.
---
### Author:  [@jaswantp](https://discourse.vtk.org/u/jaswantp)
#### Post date: [February 24, 2026, 3:00am UTC](https://discourse.vtk.org/t/set-block-color-of-a-multiblockdataset/16301/8 "2026-02-24T03:00:47Z")
the `vtkCompositeDataDisplayAttributes` needs the data object.
You can still make a pipeline connection. Just ensure that the data objects passed in `attributes.SetBlockColor` are valid datasets that are rendered by the mapper.