[vtkusers] vtkSmartVolumeMapper opacity rendering changing drastically with data scaling

[Richard Whitehead]

Please can anyone explain the following problem, or is this a bug?  This is
with VTK 6.0.

Rendering a volume in 3D using vtkSmartVolumeMapper with opacity set, if
the data's spacing is approximately 1 then I get the picture I expect; if
much smaller then the image is nearly transparent; if much larger than 1
then the image is opaque.  It appears that the data scaling is affecting
the opacity calculation; is opacity in units per cubic spacing or something
(I was assuming that 1.0 meant opaque and I can't see any documentation
that tells me its units)?

Note that I have had to fix a couple of bugs (raised in Mantis) in order to
get multi-page TIFF reading to work but I don't see that this can be
relevant.

Many thanks for your assistance,  Richard



const std::string FILENAME = "xxx.tif"; // a 16-bit multi-page TIFF

double F = 1000.0;  // for good image; set to 1, ghostly; set to 1000000
opaque
double XY_RESOLUTION = F * 0.004;
double Z_RESOLUTION = F * 0.1;


int main(int argc, char* argv[])
{
vtkSmartPointer<vtkTIFFReader> reader =
vtkSmartPointer<vtkTIFFReader>::New();
reader->SetFileName(FILENAME.c_str());
reader->SetFileDimensionality(3);

reader->SetDataSpacing(XY_RESOLUTION, XY_RESOLUTION, Z_RESOLUTION);
reader->SpacingSpecifiedFlagOn(); // to make SetDataSpacing work!!!

reader->SetOrientationType( ORIENTATION_BOTLEFT );  // otherwise it comes
out upside down
reader->Update();

vtkSmartPointer<vtkRenderWindow> renWin =
 vtkSmartPointer<vtkRenderWindow>::New();
vtkSmartPointer<vtkRenderer>ren1 =     vtkSmartPointer<vtkRenderer>::New();

renWin->AddRenderer(ren1);

vtkSmartPointer<vtkRenderWindowInteractor>iren =
vtkSmartPointer<vtkRenderWindowInteractor>::New();
iren->SetRenderWindow(renWin);

renWin->Render(); // make sure we have an OpenGL context.

vtkSmartPointer<vtkSmartVolumeMapper> volumeMapper =
vtkSmartPointer<vtkSmartVolumeMapper>::New();
volumeMapper->SetBlendModeToComposite();
volumeMapper->SetInputConnection(reader->GetOutputPort());
vtkSmartPointer<vtkVolumeProperty> volumeProperty =
vtkSmartPointer<vtkVolumeProperty>::New();
volumeProperty->ShadeOff();
volumeProperty->SetInterpolationType(VTK_LINEAR_INTERPOLATION);

vtkSmartPointer<vtkPiecewiseFunction> compositeOpacity =
vtkSmartPointer<vtkPiecewiseFunction>::New();
compositeOpacity->AddPoint(0.0,     0.0);
compositeOpacity->AddPoint(12000.0, 0.001);
compositeOpacity->AddPoint(65535.0, 1.0);
volumeProperty->SetScalarOpacity(compositeOpacity);

vtkSmartPointer<vtkColorTransferFunction> color =
vtkSmartPointer<vtkColorTransferFunction>::New();
color->AddRGBPoint(0.0, 0.0,0.0,0.0);
color->AddRGBPoint(65535.0, 1.0,1.0,1.0);
volumeProperty->SetColor(color);
 vtkSmartPointer<vtkVolume> volume =     vtkSmartPointer<vtkVolume>::New();
volume->SetMapper(volumeMapper);
volume->SetProperty(volumeProperty);
ren1->AddViewProp(volume);
ren1->ResetCamera();

renWin->SetSize(500,500);
renWin->SetPosition(0,0);

ren1->SetBackground(0.25, 0.25, 0.5);
ren1->GetActiveCamera()->Elevation( 30 );

renWin->Render();

iren->Start();

return EXIT_SUCCESS;
}
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