vtkMultivolume : Issue with setUserTransform() for a specific vtkVolume

Limz90 · June 2, 2025, 9:48am

Hello Guys,
I am using vtkMultiVolume to render 2 different vtkImageData objects but setting a vktTransform for a specific vtkvolume has no affect. Is it possible to apply transformations separately on the vtkVolumes added in the vtkMultiVolume object?
Thanks,

LucasGandel · June 4, 2025, 9:52am

Yes it should be supported. It’s handled in the mapper here and used in most of the vtkMultiVolume tests.
Tests call Rotate/Scale to modify the transform but using a UserTransform should work too because the mapper calls GetModelToWorldMatrix which takes it into account.

Limz90 · June 4, 2025, 1:59pm

Hi,
Thank you for your answer. I was using the vtkGPUVolumeRayCastMapper class which does not contain that function you mentionned. Well, I am compiling that vtk version and will test with vtkOpenGLGPUVolumeRayCastMapper. Thanks!

Sandor · June 5, 2025, 7:20am

Hi, I had the same issue and implemented a similar workaround found in Slicer, a dummy volume at port 0 of the multivolume actor:

github.com/Slicer/Slicer

Modules/Loadable/VolumeRendering/MRMLDM/vtkMRMLVolumeRenderingDisplayableManager.cxx

main


      
          }
          else if (displayNode->IsA("vtkMRMLMultiVolumeRenderingDisplayNode"))
          {
            int actorPortIndex = this->GetNextAvailableMultiVolumeActorPortIndex();
            if (actorPortIndex < 0)
            {
              vtkErrorWithObjectMacro(this->External, "AddDisplayNode: Cannot add Cannot add volume " << volumeNode->GetName() << "to multi-volume renderer");
              return;
            }
            PipelineMultiVolume* pipelineMulti = new PipelineMultiVolume(actorPortIndex);
            pipelineMulti->DisplayNode = displayNode;
            // Create a dummy volume for port zero if this is the first volume. Necessary because the first transform is ignored,
            // see https://gitlab.kitware.com/vtk/vtk/issues/17325
            //TODO: Remove this workaround when the issue is fixed in VTK
            double* multiVolumeBounds = this->MultiVolumeActor->GetBounds();
            if (multiVolumeBounds[0] > multiVolumeBounds[1]) // Prevent error that GetVolume throws if volume is null (TODO: need GetNumberOfVolumes)
            {
              this->MultiVolumeMapper->SetInputConnection(0, this->MultiVolumeDummyTrivialProducer->GetOutputPort());
          
              vtkNew<vtkPiecewiseFunction> dummyOpacity;
              dummyOpacity->AddPoint(0.0, 0.0);

Limz90 · June 5, 2025, 9:16am

Hi,
Thank you for you suggestion. I created a dummy volume and added at port 0 before adding my 2 volumes.

//set up the dummy volume
		vtkSmartPointer<vtkImageData> dummyImageData = vtkSmartPointer<vtkImageData>::New();
		dummyImageData->SetDimensions(1, 1, 1);
		dummyImageData->AllocateScalars(VTK_UNSIGNED_CHAR, 1);
		dummyImageData->GetPointData()->GetScalars()->FillComponent(0, 0);

		vtkSmartPointer<vtkTrivialProducer> dummyProducer = vtkSmartPointer<vtkTrivialProducer>::New();
		dummyProducer->SetOutput(dummyImageData);

		vtkAlgorithmOutput* dummyOutputPort = dummyProducer->GetOutputPort();
		multivolumicMapper->SetInputConnection(0, dummyOutputPort);
		//multivolumicMapper->SetInputDataObject(0, dummyImageData);

		pAttributes->_multivolumeActor->SetVolume(pAttributes->_dummyVolume, 0);
		pAttributes->_dummyVolume->SetMapper(multivolumicMapper);
		pAttributes->_dummyVolume->SetVisibility(false);
		pAttributes->_multivolumeActor->Update();

And now the transformation is correctly applied.
But i have another problem with the data: the data in the two real volume are completely black, same as the dummy volume data. I dont understand why.

Sandor · June 5, 2025, 11:29am

I think you can set volume props separately in the multivolume actor and the dummy needs to set to visible, just give it a fully transparent transfer function.

Limz90 · June 5, 2025, 2:42pm

Yes, it works now. I did separate the properties for the 2 volume actors, but i did not set a property for the dummy volume.
Thank you!