[vtkusers] MarchingCubes missunderstandig GenerateValues and Contours

[Imre Goretzki]

Thank you Cory,

I implemented the vtkThreshold today, now it is working fine.

Imre

Am 06.12.2015 um 16:22 schrieb Cory Quammen:
> On Sat, Dec 5, 2015 at 1:15 PM, Imre Goretzki <goretzki.imre at gmail.com 
> <mailto:goretzki.imre at gmail.com>> wrote:
>
>     Hey guys,
>
>     I have a question about the "GenerateValues" method of
>     MarchingCubes. My image has several points that have intensity
>     values that lie between 928 and 1600.
>
>     My understand of
>
>     int idxContour = 1;
>     double from = 920;
>     double to = 930;
>
>     ImageToVTKImageFilter con = ...
>
>     marchingCube->setInputData(con->GetOutput());
>     marchingCube->GenerateValues(idxContour, from, to);
>     marchingCube->Update();
>
>     is that the algorithm will take all points that have an intensity
>     value that lies between 920 and 930. These points are combined to
>     one contour. Via MATLAB and ITK's histogram I can validate the
>     amout of points that have a specific intensity value. 
>
>     If I plot the points that were generated by marching cubes, I do
>     not get all of them (I cannot see all of them), e.g.
>     the histogram says that there are about 200000 points that have an
>     intensity value 928. If I use 928 as the lower bound for the
>     GenerateValues method, there are just some of them plotted -
>     approximately.
>
>     Is my understanding of GenerateValues correct?
>
> Not exactly. vtkMarchingCubes will extract individual isocontours - 
> surfaces that represent the locations in a dataset with a particular 
> value. vtkMarchingCubes and other isocontour filters in VTK can 
> generate surfaces from more than one isovalue. GenerateValues() is a 
> convenience function to specify a range of isovalues.
>
> A vtkThreshold filter will extract all points in the dataset in a 
> value range and may be what you want.
>
>     Next question: (more than 1 contour)
>
>     If I want to generate more than 1 contour. How do I initialize the
>     marchingCube and, more interesting, how do I get the output of a
>     specific contour. I'm thinking about the following:
>
>
>     marchingCube->SetNumberOfContours(2);
>     marchingCube->GenerateValues(1, from_1, to_1);
>     marchingCube->GenerateValues(2, from_2, to_2);
>
>     vtkSmartPointer<vtkOutlineFilter> outline =
>     vtkSmartPointer<vtkOutlineFilter>::New();
>     outline->SetInputConnection(marchingCube->GetOutput(1));
>
>     vtkSmartPointer<vtkPolyDataMapper> outlineMapper =
>     vtkSmartPointer<vtkPolyDataMapper>::New();
>     outlineMapper->SetInputConnection(outline->GetOutputPort());
>
>     vtkSmartPointer<vtkPolyDataMapper> data1 =
>     vtkSmartPointer<vtkPolyDataMapper>::New();
>     data1->SetInputConnection(marchingCube->GetOutput(1));
>
>     vtkSmartPointer<vtkPolyDataMapper> data2 =
>     vtkSmartPointer<vtkPolyDataMapper>::New();
>     data2->SetInputConnection(marchingCube->GetOutput(2));
>
>     I know that the above lines are not working like this, but I
>     actually do not know how to do that. Right now I have 2
>     marchingCube instances that do the work for data1/data2 but that
>     should not do the trick.
>
>
> You can't set up two different isocontour surfaces and get two 
> different outputs this way. Using two separate vtkMarchingCubes 
> instances is the right way to go.
>
> HTH,
> Cory
>
>
>     I hope you can help me
>
>     Greetings
>     Imre
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>
> -- 
> Cory Quammen
> R&D Engineer
> Kitware, Inc.

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