[vtkusers] PointInPolygon returns 1 for all points within bounds

[Bryn Lloyd]

You are right: vtkPolygon:: PointInPolygon is also for 3D polygons. 

 

But a surface mesh (obtained via marching cubes) does not correspond to a
polygon! Also the projection on the plane defined by the normal n (which you
provide to PointInPolygon) in general is not a (single) polygon. 

To test inside/outside you will need to 

-          project the surface mesh onto a plane, 

-          obtain the outer edges/points defining the silhouette boundary, 

-          put these points into the vtkPolygon as a list of ordered points
(oriented polygon ring).

 

 

 

 

From: Jothy [mailto:jothybasu at gmail.com] 
Sent: Monday, July 25, 2011 11:07 AM
To: Bryn Lloyd
Cc: VTK Mailing List
Subject: Re: [vtkusers] PointInPolygon returns 1 for all points within
bounds

 

Thanks for your reply Bryn!

vtkPolygon is meant to be used for 2D polygons, but the pointInPolygon uses
a ray-tracing to find the points inside the polygon so why it will work only
for 2D polygons. The arguments are [x,y,z].

I want to calculate the dose-volume histograms from these meshes.

I will make a new post with a more appropriate subject.

Thanks again

Jothy

On Mon, Jul 25, 2011 at 7:03 AM, Bryn Lloyd <lloyd at itis.ethz.ch> wrote:

Hi Jothy

 

Since you are using vtkDiscreteMarchingCubes, I assume your mesh is a 3d
surface mesh.

vtkPolygon is meant to used for 2D polygons (e.g. a triangle in the
xy-plane). Unless you project your 3D mesh to a 2d plane, and obtain the
silhouette, this will not work.

http://www.vtk.org/doc/nightly/html/classvtkPolyDataSilhouette.html

 

Also, it seems dangerous or even wrong to cast the points and polygons to a
double pointer like this. First, the points could also be floats. Second, I
doubt the polys in your mesh can be cast to <double*>.

 

 

Instead, you should probably use one of the cell locators, e.g. vtkOBBTree
or vtkCellLocator to test if your point is inside the mesh in 3D.

You could have a look at the code in "vtkSelectEnclosedPoints" to see how to
do this in a robust way. You can even use this filter directly, via the
function "vtkSelectEnclosedPoints::IsInsideSurface".

http://www.vtk.org/doc/nightly/html/classvtkSelectEnclosedPoints.html

 

Cheers

Bryn

 

 

 

From: vtkusers-bounces at vtk.org [mailto:vtkusers-bounces at vtk.org] On Behalf
Of Jothy
Sent: Friday, July 22, 2011 4:39 PM
To: VTK Mailing List
Subject: [vtkusers] PointInPolygon returns 1 for all points within bounds

 

Hi all,

I have a mesh generated from vtkDiscreteMarchingCubes and processed through
vtkDecimatePro and vtkWindowedSincPolyDataFilter. Now, I want to find the
points/voxels inside this mesh. I am using vtkPolygon::PointInPolygon. But,
the result is "1! for the all the points whichever falls with the bounds.

Here is the piece of code that I am using

(smoother is vtkWindowedSincPolyDataFilter)

 int npts=smoother->GetOutput()-> GetPoints()->GetNumberOfPoints();// test
point 
 //double n[3]={0.707, 0.707, 0};
 double n[3];

  vtkSmartPointer<vtkPolygon> polygon =
           vtkSmartPointer<vtkPolygon>::New();
 
polygon->ComputeNormal(mapper->GetInput()->GetPoints()->GetNumberOfPoints(),
 
static_cast<double*>(mapper->GetInput()->GetPoints()->GetData()->GetVoidPoin
ter(0)), n);               

   double bds[6];
   smoother->GetOutput()->GetBounds(bds);                  
    qDebug()<<
polygon->PointInPolygon(testIn,npts,static_cast<double*>(mapper->GetInput()-
>GetPolys()->GetData()->GetVoidPointer(0)),bds,n)<<"Result";

Are these two highlighted lines correct?

Many thanks

Jothy
                   

 

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