[vtkusers] vtkResliceImageViewer::SetInputData() slow when executed after vtkDICOMImageReader::Update()

[ochampao]

Hello vtkusers! 

I am experimenting with a simple dicom viewer similar to the FourPaneViewer 
example (see [1] for link). But I am observing a behaviour which I can't 
explain. In particular, when I place the
vtkResliceImageViewer::SetInputData() 
calls before calling the vtkDICOMImageReader::Update(), the dataset is 
loaded extremely fast (approximately 2 seconds). However when 
vtkResliceImageViewer::SetInputData() calls are placed after 
vtkDICOMImageReader::Update(), then loading the dataset requires 
significantly more time to load (approximately 22 seconds). See minimal 
code example below. 

Why is the code slower when executed after the Update() call and how can 
this be avoided? 

Is it wrong to place the SetInputData() call after Update()? If yes, how can
the vtkImageData object be passed to SetInputData() so that it is still
fast?

For the slower case, when stepping through the code with the debugger, I can 
see that the call to Update() is still fast but the debugger spends most of 
its time at SetInputData(). 

Eventually, what I'd like to do is group the code that reads the dicom into 
a separate function that returns a vtkImageData reference to the pipeline. 
But from this experiment it seems to work very slow. 

Thanks, 
Panos. 

======= 

Here is a minimal example of my code to demonstrate the two situation 
(commented is the slow case): 

void addImage(const char* filename) 
{ 
    vtkSmartPointer< vtkDICOMImageReader > reader = 
        vtkSmartPointer< vtkDICOMImageReader >::New(); 
    reader->SetDirectoryName(filename); 

    vtkSmartPointer<vtkImageData> data = 
        vtkSmartPointer<vtkImageData>::New(); 

    data = reader->GetOutput(); 
    
    //--------------------------------------- 
    // SetInputData() works very fast if here 
    resliceImageViewer1->SetInputData(data); 
    resliceImageViewer2->SetInputData(data); 
    resliceImageViewer3->SetInputData(data); 
    // --------------------------------------- 

    reader->Update(); 
    
    //--------------------------------------- 
    // SetInputData() works very slow if here 
    // 
    //resliceImageViewer1->SetInputData(data); 
    //resliceImageViewer2->SetInputData(data); 
    //resliceImageViewer2->SetInputData(data); 
    //--------------------------------------- 

    /* 
        code to setup widgets (orientation, slice#, window/level color) 
    */ 

    resliceImageViewer1->Render(); 
    resliceImageViewer2->Render(); 
    resliceImageViewer3->Render(); 
} 

======== 
References: 

[1]: 
https://github.com/Kitware/VTK/tree/master/Examples/GUI/Qt/FourPaneViewer



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