[vtkusers] Set the opacity or thresholding of a VTKVolume

[David Brayford]

I am having trouble setting the threshold value for a CT volume and
only see the outside shape and not the internal geometry.

Is the code below the method I need to use to be able to see the
internal structures and as the outer casing is dense what values do I
need to increase to get better penetrating power?

	volumeActor = vtkVolume::New();
	Renderer->AddVolume(volumeActor);

	vtkVolumeProperty *prop = vtkVolumeProperty::New();
	vtkPiecewiseFunction *opacity = vtkPiecewiseFunction::New();
	opacity->AddPoint(84, 0);
	opacity->AddPoint(151, .3);
	opacity->AddPoint(255, 1);
	prop->SetScalarOpacity( opacity );

	// connect up the volume to the property and the mapper
	volumeActor->SetProperty( prop );