[vtkusers] Unstructured grid to structured grid?

[Chris Marsh]

Hi David,

To clarify: I want to take an unstructured grid (x,y) with face data (z)
and make it a 2D structured grid. I have connectivity information
(triangulation).

Looking at vtkPlaneSource: looks like it should work/make my life easier

However, even with the above linked demo with the slight modification, I
cannot make this work. This is most likely a PEBKAC as a result of my lack
of familiarity with VTK so I appreciate you taking the time to help. The
minimum working example is below, based upon this code
http://www.itk.org/Wiki/VTK/Examples/Cxx/PolyData/InterpolateMeshOnGrid

vtkSmartPointer<vtkPoints> points = vtkSmartPointer<vtkPoints>::New();
    vtkSmartPointer<vtkFloatArray>  data  =
vtkSmartPointer<vtkFloatArray>::New();
    data->SetName("elevation");

    unsigned int gridSize = 10;
    float maxHeight = 5;
    for ( unsigned int i = 0; i < 100; ++i )
    {
        double x = vtkMath::Random(0, gridSize);
        double y = vtkMath::Random(0, gridSize);
        double z = vtkMath::Random(0, maxHeight);
    points->InsertNextPoint ( x, y, 0);
    data->InsertNextValue(z);
    }

    // Add the grid points to a polydata object
    vtkSmartPointer<vtkPolyData> polydata =
vtkSmartPointer<vtkPolyData>::New();
    polydata->SetPoints ( points );
    polydata->GetPointData()->SetScalars(data);

    // Triangulate the grid points. If you do not have a mesh (points
    // only), the output will not be interpolated!
    vtkSmartPointer<vtkDelaunay2D> tri =
vtkSmartPointer<vtkDelaunay2D>::New();
    tri->SetInputData ( polydata );
    tri->Update();

    // Create a grid of points to interpolate over
    vtkSmartPointer<vtkPlaneSource> gridPoints =
vtkSmartPointer<vtkPlaneSource>::New();
    gridPoints->SetResolution(gridSize, gridSize);
    gridPoints->SetOrigin(0,  0, 0);

    // Perform the interpolation
    vtkSmartPointer<vtkProbeFilter> probeFilter =
 vtkSmartPointer<vtkProbeFilter>::New();
    probeFilter->SetSourceConnection(tri->GetOutputPort());
    probeFilter->SetInputData(gridPoints->GetOutput()); //
    // Interpolate 'Source' at these points
    probeFilter->Update();

    vtkSmartPointer<vtkDelaunay2D> gridDelaunay =
vtkSmartPointer<vtkDelaunay2D>::New();
    gridDelaunay->SetInputConnection ( probeFilter->GetOutputPort() );

    vtkSmartPointer<vtkXMLPolyDataWriter> gridWriter =
 vtkSmartPointer<vtkXMLPolyDataWriter>::New();
    gridWriter->SetFileName ( "gridSurface.vtp" );
    gridWriter->SetInputConnection ( gridDelaunay->GetOutputPort() );
    gridWriter->Write();




On Tue, Sep 16, 2014 at 12:07 PM, David Gobbi <david.gobbi at gmail.com> wrote:

> If you want to create a polydata that is a regular grid, then
> use vtkPlaneSource with SetResolution(gridSize, gridSize).
>
> If you want your regular grid to be a vtkImageData, then
> use vtkImageGridSource with SetDataExtent(), SetDataSpacing(),
> an SetDataOrigin() to create the needed dimensions.
>
> Note that if your point cloud is just points without any connectivity
> between the points (i.e. without any cells), then you cannot use
> vtkProbeFilter.
>
>
>
>
> On Tue, Sep 16, 2014 at 11:52 AM, Chris Marsh <chris.marsh at usask.ca>
> wrote:
> > Mmh thanks. I think I'll stick with the probe filter.
> >
> > I've been able to get  this example working
> > http://www.itk.org/Wiki/VTK/Examples/Cxx/PolyData/InterpolateMeshOnGrid
> > if I have a real-world pt cloud, I'm a bit unclear how I should make the
> > grid.
> >
> > If xll and yll are the lower x and y coordinates of an axis aligned
> bounding
> > box
> >
> > size_t gridSize = 100;
> >     for ( size_t x = 0; x < gridSize; x++ )
> >     {
> >         for ( size_t y = 0; y < gridSize; y++ )
> >         {
> >             gridPoints->InsertNextPoint ( x+xll, y+yll, 0);
> >         }
> >     }
> >
> > does not seem to produce any output.
> >
> > Any suggestions?
> >
> > On Mon, Sep 15, 2014 at 4:39 PM, David Gobbi <david.gobbi at gmail.com>
> wrote:
> >>
> >> Another filter that might work is vtkShepardMethod, which takes
> >> unstructured points (e.g. an arbitrary set of samples) as input and
> >> produces a vtkImageData as output.
> >>
> >> The difference between vtkProbeFilter and vtkShepardMethod is
> >> that the former performs the interpolation based on the connectivity
> >> of the points (i.e. based on the cells), while the latter ignores the
> >> connectivity and does the interpolation purely based on distance.
> >>
> >>  - David
> >>
> >> On Mon, Sep 15, 2014 at 4:23 PM, Chris Marsh <chris.marsh at usask.ca>
> wrote:
> >> > Thanks David, this looks perfect.
> >> >
> >> >
> >> > On Mon, Sep 15, 2014 at 4:10 PM, David Gobbi <david.gobbi at gmail.com>
> >> > wrote:
> >> >>
> >> >> Hi Chris,
> >> >>
> >> >> This sounds like a job for vtkProbeFilter.  The probe filter takes
> two
> >> >> inputs,
> >> >> the "input" and the "source".  The idea is that it resamples the
> >> >> scalars
> >> >> from
> >> >> the "source" onto the geometry of the "input".  So, in this case, the
> >> >> "source"
> >> >> is your unstructured grid, and the "input" is any vtkImageData that
> has
> >> >> the
> >> >> origin, spacing, and extent that you want to use for the regridding.
> >> >> The
> >> >> output of vtkProbeFilter will be an image that has the same geometry
> as
> >> >> the input, but that has pixel values that have been interpolated from
> >> >> your
> >> >> unstructured grid.  You should be able to find several examples on
> the
> >> >> wiki.
> >> >>
> >> >>  - David
> >> >>
> >> >>
> >> >> On Mon, Sep 15, 2014 at 3:59 PM, chrism <chris.marsh at usask.ca>
> wrote:
> >> >> > Is it possible to (easily) convert an unstructured grid to a
> >> >> > structured
> >> >> > grid
> >> >> > (Image data)?
> >> >> >
> >> >> > Is unstructured -> polydata -> image data  the only way? If so, how
> >> >> > example
> >> >> >
> >> >> > (
> http://www.vtk.org/Wiki/VTK/Examples/Cxx/PolyData/DataSetSurfaceFilter
> >> >> > ??)
> >> >> >
> >> >> > Ultimately, the unstructured grid represents a topography, and I am
> >> >> > looking
> >> >> > to 'rasterize' it for use in a GIS.
> >> >
> >> >
> >
> >
>
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