# Change color of poly inside an imperfect surface.

**URL:** https://discourse.vtk.org/t/change-color-of-poly-inside-an-imperfect-surface/1719
**Category:** Support
**Created:** [September 7, 2019, 1:43pm UTC](https://discourse.vtk.org/t/change-color-of-poly-inside-an-imperfect-surface/1719 "2019-09-07T13:43:35Z")
**Posts on this page:** 2
**Page:** 1

<div class="post-metadata">

### Author: ![bruce.bogtrotter](https://discourse.vtk.org/letter_avatar_proxy/v4/letter/b/c2a13f/32.png) [@bruce.bogtrotter](https://discourse.vtk.org/u/bruce.bogtrotter)
#### Post date: [September 7, 2019, 1:43pm UTC](https://discourse.vtk.org/t/change-color-of-poly-inside-an-imperfect-surface/1719/1 "2019-09-07T13:43:36Z")

</div>

I’m trying to find the best way of changing color of polydata that has penetrated the surface of an (unperfectly closed) object.

for example, if I have a foam football (with holes in it/imperefectly closed surface), and i were to push a pencil half way into the foam ball. I would like the pencil outside of the ball to be blue, and the pencil the inside of the ball to be red.

I have tried doing this with: [https://vtk.org/Wiki/VTK/Examples/Cxx/PolyData/PointInsideObject](https://vtk.org/Wiki/VTK/Examples/Cxx/PolyData/PointInsideObject).

However I do not have a closed surface, and the results are not brilliant.

I am trying to do it with a distance filter, however i do not know exactly how, or how to change the scalars to just red and blue .

The best way I can think fo doing it is somehow using the suface normals of the object, to detect whether a point is inside an object.

My question is, is there a good, prefered way of doing things like/similar this that is relativelty simple?  
I’m quite stuck on this at the momemnt, so any help is very much appreciated.

---

<div class="post-metadata">

### Author: ![bruce.bogtrotter](https://discourse.vtk.org/letter_avatar_proxy/v4/letter/b/c2a13f/32.png) [@bruce.bogtrotter](https://discourse.vtk.org/u/bruce.bogtrotter)
#### Post date: [September 8, 2019, 10:47am UTC](https://discourse.vtk.org/t/change-color-of-poly-inside-an-imperfect-surface/1719/2 "2019-09-08T10:47:08Z")

</div>

vtkSmartPointer sphereSource =  
vtkSmartPointer::New();  
sphereSource-\>SetCenter(0.0, 0.0, 0.0);  
sphereSource-\>SetRadius(1.0f);  
sphereSource-\>Update();  
vtkSmartPointer sphereMapper =  
vtkSmartPointer::New();  
sphereMapper-\>SetInputConnection( sphereSource-\>GetOutputPort() );  
sphereMapper-\>ScalarVisibilityOff();  
vtkSmartPointer sphereActor =  
vtkSmartPointer::New();  
sphereActor-\>SetMapper( sphereMapper );  
sphereActor-\>GetProperty()-\>SetOpacity(.3);  
sphereActor-\>GetProperty()-\>SetColor(1,1,1);

vtkSmartPointer implicitPolyDataDistance =  
vtkSmartPointer::New();  
implicitPolyDataDistance-\>SetInput(sphereSource-\>GetOutput());

// Setup a grid  
vtkSmartPointer points =  
vtkSmartPointer::New();  
float step = 0.1;  
for(float x = - 2.0; x \<= 2.0; x += step)  
{  
for(float y = - 2.0; y \<= 2.0; y += step)  
{  
for(float z = - 2.0; z \<= 2.0; z += step)  
{  
points-\>InsertNextPoint(x,y,z);  
}  
}  
}

// Add distances to each point  
vtkSmartPointer signedDistances =  
vtkSmartPointer::New();  
signedDistances-\>SetNumberOfComponents(1);  
signedDistances-\>SetName(“SignedDistances”);

unsigned char red[3] = { 255, 0, 0 };  
unsigned char blue[3] = { 0, 0, 255 };

vtkSmartPointer colors =  
vtkSmartPointer::New();  
colors-\>SetNumberOfComponents(3);  
colors-\>SetName(“Colors”);

vtkSmartPointer points2 =  
vtkSmartPointer::New();

cout \<\< "get number of points " \<\< points-\>GetNumberOfPoints() \<\< endl;  
// Evaluate the signed distance function at all of the grid points  
for(vtkIdType pointId = 0; pointId \< points-\>GetNumberOfPoints(); ++pointId)  
{  
double p[3];  
points-\>GetPoint(pointId, p);

```
float signedDistance = implicitPolyDataDistance->EvaluateFunction(p);
//signedDistances->InsertNextValue(signedDistance);
//cout << "signed distance " << signedDistance << endl; 
if (signedDistance < 0)
{

	cout << "point number" << pointId << " " << signedDistance << endl;
	colors->InsertNextTypedTuple(red);

}
else
{
	colors->InsertNextTypedTuple(blue);
}

```

}

vtkSmartPointer polyData = vtkSmartPointer::New();  
polyData-\>SetPoints(points);  
polyData-\>GetPointData()-\>SetScalars(colors);

vtkSmartPointer vertexGlyphFilter =  
vtkSmartPointer::New();  
vertexGlyphFilter-\>SetInputData(polyData);  
vertexGlyphFilter-\>Update();

vtkSmartPointer signedDistanceMapper =  
vtkSmartPointer::New();  
signedDistanceMapper-\>SetInputConnection(vertexGlyphFilter-\>GetOutputPort());  
signedDistanceMapper-\>ScalarVisibilityOn();

vtkSmartPointer signedDistanceActor =  
vtkSmartPointer::New();  
signedDistanceActor-\>SetMapper( signedDistanceMapper );

vtkSmartPointer renderer =  
vtkSmartPointer::New();  
renderer-\>AddViewProp(sphereActor);  
renderer-\>AddViewProp(signedDistanceActor);

vtkSmartPointer renderWindow =  
vtkSmartPointer::New();  
renderWindow-\>AddRenderer( renderer );

vtkSmartPointer renWinInteractor =  
vtkSmartPointer::New();  
renWinInteractor-\>SetRenderWindow( renderWindow );

renderWindow-\>Render();  
renWinInteractor-\>Start();