[vtkusers] MarchingCubes missunderstandig GenerateValues and Contours

[Cory Quammen]

Great!

On Sun, Dec 6, 2015 at 10:44 AM, Imre Goretzki <goretzki.imre at gmail.com>
wrote:

> 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>
> 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.
>
>
>


-- 
Cory Quammen
R&D Engineer
Kitware, Inc.
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