Help with Python Plugin Filter using VTKPythonAlgorithmBase approach

Development
1

I am trying to use the VTKPythonAlgorithmBase approach to create a custom filter plugin that will send the filter output back to the ParaView pipeline.

I’ve adopted the “Half V Filter” example in the chapter 12.4 of the ParaView guide for my particular dataset:

from vtkmodules.vtkCommonDataModel import vtkDataSet
from vtkmodules.util.vtkAlgorithm import VTKPythonAlgorithmBase
from vtkmodules.numpy_interface import dataset_adapter as dsa
from paraview.util.vtkAlgorithm import smproxy, smproperty, smdomain

@smproxy.filter(label="Half ID Filter")
@smproperty.input(name="Input")
class HalfIDFilter(VTKPythonAlgorithmBase):
    def __init__(self):
        VTKPythonAlgorithmBase.__init__(self)
    def RequestData(self, request, inInfo, outInfo):
        input0 = dsa.WrapDataObject(vtkDataSet.GetData(inInfo[0]))
        data = input0.CellData["ID"]/2.0
        output = dsa.WrapDataObject(vtkDataSet.GetData(outInfo))
        output.CellData.append(data, "ID_half")
        return 1

The above filter is applied to a data set that has been loaded into the pipeline. The data set contains cell data named “ID”. I’m hoping to apply this filter and get a new cell data entry named “ID_half”.

Currently, I am able to load the filter and apply it. However, nothing is outputted into the pipeline.

Thank you,
Victor

2

That certainly looks reasonable to me. It’s not obvious, but if you add ‘print’ statements to your filters, you can see them in the Output window.
In my filter, I was unable to use vtkDataSet.GetData(outInfo), I had to use a more specific type like vtkPolyData, but the example clearly uses vtkDataSet, and I’m not using numpy.

I suggest adding print(output) to see if it’s doing what you want.

HTH,

Aron

3

Hi Aron,
When printing the output in the custom filter above, I see

<vtkmodules.numpy_interface.dataset_adapter.PolyData object at ...>

However, when I do the same in the following programmable filter (which accomplishes what I’m hoping for in the above custom filter):

input0 = inputs[0]
data = input0.CellData["ID"]/2.0
output.CellData.append(data, "ID_half")
print(output)

I get

<vtkmodules.numpy_interface.dataset_adapter.UnstructuredGrid object at ...>

I notice that in the custom filter, output is a PolyData object, while in the programmable filter, output is an UnstructuredGrid object. I wonder if this is the reason the output is not sent back to the ParaView data pipeline.

Is there a way to make the outInfo variable in RequestData(...) to be the same type of object as the input, i.e., an UnstructuredGrid object?

Thanks,
Victor

4

Hi Victor,
If you go back to the ParaView Guide 12.4, you’ll see some of the examples specify the output type:

VTKPythonAlgorithmBase.init(self, nInputPorts=1, nOutputPorts=1, outputType=“vtkUnstructuredGrid” )

Hopefully that will fix your problem.

Regards,

Aron

5

Hi Aron,
Making the object type consistent did not solve the problem. I still cannot access the output data in the ParaView data pipeline. I have rewritten the custom filter as follows:

from vtkmodules.vtkCommonDataModel import vtkDataSet, vtkUnstructuredGrid
from vtkmodules.util.vtkAlgorithm import VTKPythonAlgorithmBase
from vtkmodules.numpy_interface import dataset_adapter as dsa
from paraview.util.vtkAlgorithm import smproxy, smproperty, smdomain

@smproxy.filter(label="Half ID Filter")
@smproperty.input(name="Input")
class HalfIDFilter(VTKPythonAlgorithmBase):
    def __init__(self):
        VTKPythonAlgorithmBase.__init__(self, 
                                        nInputPorts=1, 
                                        nOutputPorts=1, 
                                        outputType="vtkUnstructuredGrid")
    def RequestData(self, request, inInfo, outInfo):
        input0 = dsa.WrapDataObject(vtkUnstructuredGrid.GetData(inInfo[0]))
        data = input0.CellData["ID"]/2.0
        output = dsa.WrapDataObject(vtkUnstructuredGrid.GetData(outInfo))
        output.CellData.append(data, "ID_half")
        print("output object = ", output)
        print("output keys = ", output.CellData.keys())
        print("output data = ", output.CellData["ID_half"])
        return 1

Note that all the vtkDataSet calls have now been switched to vtkUnstructuredGrid. The output in the output window when applying the filter is

('output object = ', <vtkmodules.numpy_interface.dataset_adapter.UnstructuredGrid object at ... >)
('output keys = ', ['ID_half'])
('output data = ', VTKArray([], shape=(0L, 534L), dtype=float64)

As seen from the output, the object type is UnstructuredGrid, the "ID_half" key has been added, but the data in the output object is empty.

Thanks,
Victor

6

I’m pretty sure you can fix this problem by adding this just after output is defined:
output.ShallowCopy(input0)

I’m not sure why the example from the ParaView guide doesn’t include this, too.

Regards,

Aron

7

Hi Aron,
The shallow copy did the job of exposing the output of the filter to the ParaView data pipeline. I ended up performing the shallow copy before the append call as follows:

output = dsa.WrapDataObject(vtkUnstructuredGrid.GetData(outInfo))
output.ShallowCopy(vtkUnstructuredGrid.GetData(inInfo[0]))
output.CellData.append(data, "ID_half")

However, wouldn’t the shallow copy end up doubling my memory footprint? Is there a way to only expose the newly created data set.

Thanks,
Victor

8

No, it won’t increase your memory footprint - that is the purpose of ShallowCopy (instead of DeepCopy). The output will use references to all the input data structures, except for the one ID_half array you have added to the output data.

9

Got it. Thank you for all your help Aron!