[vtkusers] vtkContourFilter issue

[Bill Lorensen]

Why dpn't we change vtkMath::Random's implementation so that it uses
vtkMinimalStandardRandomSequence.

On Wed, Dec 9, 2009 at 1:31 PM, Francois Bertel
<francois.bertel at kitware.com> wrote:
> Don't use drand48() but DON'T USE vtkMath::Random() ANYMORE. Use a
> vtkMinimalStandardRandomSequence object.
>
>
> Even if you use vtkMath::Random() , the following line is not portable
> across compilers because
> the random number generator uses a static variable and the evaluation
> order of the argument of a method/function is compiler dependent:
>
> points->InsertNextPoint(drand48(), drand48(), drand48());
>
> Imaging that drand48() or vtkMath::Random() generates the following
> sequence of three numbers: 0.1 0.2 0.3 (randomly chosen by myself :-)
> )
>
> If the compiler evaluates the last argument first, the line will be
> equivalent to
> points->InsertNextPoint(0.3,0.2,0.1);
>
> Another compiler can decide to evaluate the first argument first, the
> line will be equivalent to
> points->InsertNextPoint(0.1,0.2,0.3);
>
> Notably, gcc and Visual Studio uses different order.
>
> It can also differ from the same compiler if the optimization flags
> are activated or not.
>
> One way to ensure the order of evaluation is to force a sequence of calls:
> double x=vtkMath::Random(); // 0.1
> double y=vtkMath::Random(); // 0.2
> double z=vtkMath::Random(); // 0.3
>
> points->InsertNextPoint(x,y,z);
>
> But I just said to not use vtkMath::Random() ANYMORE. The replacement
> for vtkMath::Random() and others is the class
> Common/vtkMinimalStandardRandomSequence:
>
> This class as no nasty side-effect: GetValue() just get the current
> value of the random sequence, it does not go the next random value.
> Next() just move the the next number in the random sequence but does
> not return the new value. (Command-query separation principle).
>
> Therefore the way the class is written force you to break down the
> calls so you magically avoid the compiler specific behavior:
>
> vtkMinimalStandardRandomSequence *seq=vtkMinimalStandardRandomSequence::New();
> seq->SetSeed(1);  // initialize the sequence
> double x=seq->GetValue(); // or seq->GetRangeValue(-1.0,1.0); in your case
> seq->Next();
> double y=seq->GetValue();
> seq->Next();
> double z=seq->GetValue();
> points->InsertNextPoint(x,y,z);
> ...
> ...
>
> seq->Delete();
>
> The other great thing with sequence objects is you can have multiple
> independent random sequences. Some classes use random numbers
> internally for diverse reasons (it can be sampling/jittering, cheery
> picking). With vtkMath::Random() using a static seed, your random
> number generation could interfere with the random number generation of
>  an other component. (and all the other drawbacks of static storage
> you can image, like issues with multithreading)
>
>
> On Wed, Dec 9, 2009 at 1:11 PM, David Doria <daviddoria+vtk at gmail.com> wrote:
>> On Wed, Dec 9, 2009 at 12:38 PM, Bill Lorensen <bill.lorensen at gmail.com> wrote:
>>> David,
>>>
>>> 1) Please do not use drand48. It's not portable. Use
>>> vtkMath::Random(0.0,1.0) for equivalent.
>>> 2) drand48 generates numbers between 0,1. You cannot extract an
>>> isosurface of 0. Try vtkMath::Random(-1.0,1.0)
>>>
>>> Bill
>>
>> Bill,
>>
>> 1) Good point. Will do.
>>
>> 2) You are correct, that was an artifact of simplifying my code into a
>> demo. In the demo, when this was changed, it worked as expected. In my
>> real code, the values were indeed on both sides of zero.
>>
>> I have isolated the problem: I was setting up the ImageData in a
>> separate class that derives from vtkImageAlgorithm (aside: why is it
>> not called vtkImageDataAlgorithm?)
>>
>> When I cut and paste the contents of the RequestData function from
>> that class into the function we have been discussing, it works fine.
>>
>> I checked the input pointer in RequestData and it has the correct
>> number of points, so the input of the pipeline has been setup
>> correctly. The output of the voxelizeFilter seems to be setup properly
>> back in the calling function. It seems to me that these two should
>> behave identically, but one produces valid output while the other
>> produces empty output.
>>
>> Here is the VoxelizePolyData class:
>> http://www.rpi.edu/~doriad/VTK_List/vtkVoxelizePolyData/
>>
>> Does anyone see why these two things would be any different:
>>
>> A)
>>  vtkSmartPointer<vtkVoxelizePolyData> voxelizeFilter =
>> vtkSmartPointer<vtkVoxelizePolyData>::New();
>>  voxelizeFilter->SetInput(input);
>>  voxelizeFilter->Update();
>>  vtkImageData* grid = voxelizeFilter->GetOutput();
>>
>> B)
>>
>>  //get the bounds of the input PolyData
>>    double bounds[6];
>>    input->GetBounds(bounds);
>>  //xmin, xmax, ymin, ymax, zmin, zmax
>>    double xmin = bounds[0];
>>    double xmax = bounds[1];
>>    double ymin = bounds[2];
>>    double ymax = bounds[3];
>>    double zmin = bounds[4];
>>    double zmax = bounds[5];
>>
>>  //create a grid the size of the point set
>>    vtkSmartPointer<vtkImageData> grid = vtkSmartPointer<vtkImageData>::New();
>>
>>  //set the bottom left corner of the grid to the bottom left corner
>> of the data bounding box
>>    grid->SetOrigin(xmin, ymin, zmin);
>>
>>  //create a grid of voxels of size (NumberOfCellsX x NumberOfCellsY x
>> NumberOfCellsZ)
>>  //there are (NumberOfCellsX+1 x NumberOfCellsY+1 x NumberOfCellsZ+1)
>> points to make this size voxel grid
>>    grid->SetExtent(0, 10, 0, 10, 0, 10);
>>
>>  //set the size of each element/cell/voxel so that the grid spans the
>> entire input point set volume
>>  //grid->SetSpacing((xmax-xmin)/static_cast<double>(this->NumberOfCellsX),
>> (ymax-ymin)/static_cast<double>(this->NumberOfCellsY),
>> (zmax-zmin)/static_cast<double>(this->NumberOfCellsZ));
>>    grid->SetSpacing((xmax-xmin)/static_cast<double>(10),
>> (ymax-ymin)/static_cast<double>(10),
>> (zmax-zmin)/static_cast<double>(10));
>>
>> Have I done something wrong in setting up the pipeline connections of
>> the voxelizePolyData filter?
>>
>> Thanks,
>>
>> David
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>
>
>
> --
> François Bertel, PhD  | Kitware Inc. Suite 204
> 1 (518) 371 3971 x113 | 28 Corporate Drive
>                      | Clifton Park NY 12065, USA
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