[vtkusers] Black renderwindow with vtkGPUVolumeRayCastMapper/MaximumIntensityProjection

[Sophonet]

Hi list,

in an earlier version of my software using VTK7.x and the OpenGL 
backend, MIP rendering was working using vtkGPUVolumeRayCastMapper and 
MaximumIntensityProjection.

However, in VTK 8.x (e.g. VTK 8.1.0), using the OpenGL2 backend, the 
renderwindow stays black/empty - VTK does not show an error message, and 
the IsRenderSupported (see below) does not seem to fail.

Any hints what is wrong with the code below? By the way: I have added 
the volumeProperty (and the piecewise function) after following a 
current VTK test program 
(https://github.com/Kitware/VTK/blob/master/Rendering/Volume/Testing/Cxx/TestGPURayCastFourComponentsMIP.cxx).

vtkimagedata is a scalar medical image (unsigned short), so nothing 
special.

Thanks,

          Sophonet

    vtkNew<vtkGPUVolumeRayCastMapper> mapper;
    mapper->SetInputData(vtkimagedata);
    mapper->SetBlendModeToMaximumIntensity();

    vtkNew<vtkVolume> volume;
    volume->SetMapper(mapper);
    // ... connect to renderer etc.

    //NOTE: The following lines from VTK's 
TestGPURayCastFourComponentsMIP.cxx did not solve the problem
    //vtkNew<vtkVolumeProperty> volumeProperty;
    //volumeProperty->IndependentComponentsOn();
    //volumeProperty->SetInterpolationType(VTK_LINEAR_INTERPOLATION);
    //volumeProperty->SetShade(1);

    //vtkNew<vtkPiecewiseFunction> f;
    //f->AddPoint(0, 0.0);
    //f->AddPoint(255, 1.0);
    //volumeProperty->SetScalarOpacity(f);

    //int valid = 
mapper->IsRenderSupported(vtkrenderer_->GetRenderWindow(), 
volumeProperty);

    // NOTE: No exception was thrown on my system
    //if (valid == 0)
    //	throw std::runtime_error("MIP rendering is not supported");
    //volume->SetProperty(volumeProperty);