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

[ochampao]

Hi David and thanks for the detailed reply.

Your reply makes clear the difference between "streaming" data vs. loading
the whole volume in memory. However, with these in mind, I still can't
explain the behaviour I observe in the minimal example that I have posted
above (see [1], same code repeated below as well). Namely, if we assume that
reader->Update() loads the whole volume in memory, then why calling
resliceImageViewer->SetInputData(data) takes longer to complete if Update()
is called at positions (1) or (2)?

When stepping through the code to see what causes this delay (when Update()
is called at position (1) or (2) ) I have noticed that execution spends most
of its time at the following line:

  in->GetScalarRange(range);
  
of the function vtkResliceImageViewer::SetInputData(vtkImageData *in) (line
450 in vtkResliceImageViewer.cxx).

I guess, since the image is not yet loaded (when Update() is called at (3) )
calculating the scalar range on an "empty" image takes no time. It is as if
in case (3) we are "cheating" and skipping the GetScalarRange() calculation.
Doesn't GetScalarRange() get executed at some point even for case (3), and
if yes, why does it take less time?

Thanks,
P.

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[1]
http://vtk.1045678.n5.nabble.com/vtkResliceImageViewer-SetInputData-slow-when-executed-after-vtkDICOMImageReader-Update-td5746409.html#a5746426

======================= 

#include "vtkDICOMImageReader.h" 
#include "vtkImageData.h" 
#include "vtkInteractorStyleImage.h" 
#include "vtkRenderer.h" 
#include "vtkRenderWindow.h" 
#include "vtkRenderWindowInteractor.h" 
#include "vtkResliceImageViewer.h" 
#include "vtkSmartPointer.h" 

int main(int argc, char** argv) 
{ 
    vtkNew<vtkDICOMImageReader> reader; 
    vtkNew<vtkResliceImageViewer> resliceImageViewer; 
    vtkNew<vtkRenderWindowInteractor> renderWindowInteractor; 
    vtkNew<vtkInteractorStyleImage> style; 
    vtkNew<vtkRenderWindow> renderWindow; 
    vtkNew<vtkRenderer> renderer; 
    vtkSmartPointer<vtkImageData> data; 

    renderWindow->AddRenderer(renderer); 
    renderer->SetBackground(0.0, 0.0, 0.0); 
    renderWindowInteractor->SetInteractorStyle(style); 

    reader->SetDirectoryName("path/to/dicom/series"); 
    //reader->Update();  // --> (1) SetInputData() takes ~18 seconds 
    data = reader->GetOutput(); 
    //reader->Update();  // --> (2) SetInputData() takes ~18 seconds 
        
    resliceImageViewer->SetInputData(data); 
    reader->Update();  // --> (3) SetInputData() takes less than 1 seconds 
    resliceImageViewer->SetRenderWindow(renderWindow); 
    resliceImageViewer->SetupInteractor(renderWindowInteractor); 
    resliceImageViewer->SetResliceModeToAxisAligned(); 
    resliceImageViewer->SetSlice(50); 
    resliceImageViewer->SetColorLevel(-27); 
    resliceImageViewer->SetColorWindow(1358); 
    
    renderWindowInteractor->Start(); 

    return 0; 
} 

======================= 



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