[vtkusers] vtkContourFilter issue

[Francois Bertel]

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