# Computing Isosurface on a chosen attribute

**URL:** https://discourse.vtk.org/t/computing-isosurface-on-a-chosen-attribute/7283
**Category:** Support
**Created:** [November 26, 2021, 11:37am UTC](https://discourse.vtk.org/t/computing-isosurface-on-a-chosen-attribute/7283 "2021-11-26T11:37:45Z")
**Posts on this page:** 2
**Page:** 1

<div class="post-metadata">

### Author: ![Martin3d](https://discourse.vtk.org/user_avatar/discourse.vtk.org/martin3d/32/4303_2.png) [@Martin3d](https://discourse.vtk.org/u/Martin3d)
#### Post date: [November 26, 2021, 11:37am UTC](https://discourse.vtk.org/t/computing-isosurface-on-a-chosen-attribute/7283/1 "2021-11-26T11:37:45Z")

</div>

Hi,

I work with an unstructured grid containing multiple attributes. I wish to **process the isosurface of one of these attributes** , which has been prototyped with Paraview successfully. I thrive to do the same with VTK based on the [vtkFlyingEdge3D example](https://kitware.github.io/vtk-examples/site/Cxx/Modelling/MarchingCubes/).

 ![image](https://discourse.vtk.org/uploads/default/original/2X/f/f4f331fab30bdf62494a52a68c8bbfd43605f6f1.jpeg)

The main point is that I want to express the contour levels in terms of **this attribute value range**. However, when using **vtkVoxelModeler and vtkImageData** as preprocessing for vtkFlyingEdges3D, I can only express contour in terms of values in range [0;1] (i.e. [background; foreground] values of vtkVoxelModeller).

**How should I adapt my pipeline to process contours of this attribute?**

Here is a summary of my program inspired by vtkFlyingEdges3D

```auto
String propertyName = "temp";
double[] levels = {800, 1000, 1200, 1400, 1600, 1800, 2000};
vtkUnstructuredGrid ugrid = ... // load exodus file
ugrid.GetPointData().SetActiveScalars(propertyName);

// Convert to poly data
vtkGeometryFilter toPoly = new vtkGeometryFilter();
toPoly.SetInputData(uGrid);

// Converts to a voxel representation
vtkVoxelModeller voxelModeller = new vtkVoxelModeller();
voxelModeller.SetSampleDimensions(10, 10, 10);
voxelModeller.SetScalarTypeToDouble();
voxelModeller.SetModelBounds(toPoly.GetOutput().GetBounds());
voxelModeller.SetInputConnection(toPoly.GetOutputPort());

// Copy voxel to image data structure
vtkImageData volume = new vtkImageData();
volume.DeepCopy(voxelModeller.GetOutput());

// Apply contour surface to voxels
vtkFlyingEdges3D isoSurface = new vtkFlyingEdges3D();
isoSurface.SetInputData(volume);
isoSurface.SetNumberOfContours(levels.length);
for (int i = 0; i < levels.length; i++) {
  isoSurface.SetValue(i, levels[i]); 
}
isoSurface.Update();

System.out.println("Number of contour : " + isoSurface.GetNumberOfContours());
System.out.println("Number of cells : " + isoSurface.GetOutput().GetNumberOfCells());

```

I got an empty output

```auto
Number of contour : 7
Number of cells : 0 // <<<<<<<< PROBLEM

```

If I however uses **levels in [0;1]**

```auto
double[] levels = {0.08, 0.10, 0.12, 0.14, 0.16, 0.18, 0.20};

```

I got something

```auto
Number of contour : 7
Number of cells : 5488

```

---

<div class="post-metadata">

### Author: ![Hilcovm](https://discourse.vtk.org/letter_avatar_proxy/v4/letter/h/48db29/32.png) [@Hilcovm](https://discourse.vtk.org/u/Hilcovm)
#### Post date: [December 1, 2021, 7:21am UTC](https://discourse.vtk.org/t/computing-isosurface-on-a-chosen-attribute/7283/2 "2021-12-01T07:21:14Z")

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Hi Martin,

It seems your data is already in a 3D cells format. This means you could use something like the vtkContourFilter directly on the grid ([https://vtk.org/doc/nightly/html/classvtkContourFilter.html#details](https://vtk.org/doc/nightly/html/classvtkContourFilter.html#details)) or vtkMarchingContourFilter.

So, you would have something like:

```auto
String propertyName = "temp";
double[] levels = {800, 1000, 1200, 1400, 1600, 1800, 2000};
vtkUnstructuredGrid ugrid = ... // load exodus file
ugrid.GetPointData().SetActiveScalars(propertyName);

vtkNew<vtkContourFilter> isoSurface;
isoSurface->SetInputData(ugrid);
for (int i = 0; i < levels.length; i++) {
  isoSurface.SetValue(i, levels[i]); 
}
isoSurface.Update();

```

Cheers,  
Hilco