[vtkusers] vtkCutter doesn't return cells that are totally inside the cutting vtkPlane

[Jim Peterson]

Bill,

Sorry to tag in, but are you saying the reason a point defined as 
(-1,0,0) is not included as on the plane origin (0,0,0) normal (0,0,1) 
is essentially because of the float imprecision, that the eventual 
representation of the point after some other math processing is slightly 
off the plane? so defining a clip with two planes origin1 
(0.,0.,0.00001) normal1 (0.,0.,-1.) and  origin2 (0.,0.,-0.00001)  
normal2(0.,0.,1.) might return the set of points "on" the plane? That 
would minimize the need to make sure your hexahedron extends to the 
edges of the original points.

Thanks,
Jim

Bill Lorensen wrote:
> Dan,
>
> I misunderstood your question. The cutting plane is infinitesimally
> thin so it would be difficult to extract those points via clipping.
> Also, both clipping and cutting introduce new points. I can think of 3
> ways to do this (there may be more):
>
> 1) Define a thin hexahedral cell that contains the plane. Then use
> vtkSelectEnclosedPoints.
> 2) Use vtkPlaneSource to define the plane. Then loop through all of
> the points and use the quad's EvaluatePosition to determine
> inside/outside. This will probably be slow.
> 3) Loop over all points and compute the distance to the plane using
> the plane equation. Retain those points that are within a tolerance of
> 0.
>
>
> Bill
> .
> On Thu, Sep 16, 2010 at 4:47 PM, Dan Asimov <dan.asimov at gmail.com> wrote:
>   
>> Hi Bill,
>>
>> Thanks for the reply!
>>
>> I already tried this but vtkClipPolydata will return all the points on one
>> side of the cutting plane. I need the points inside the cutting plane only.
>>
>> For ex: if I move the origin of the cutting plane to
>>
>> pPlane->SetOrigin(0,0,-0.1);
>>
>> then vtkClipPolydata will return five points (the two cutting points and the
>> three points in the x-y plane). However, vtkCutter will return only the two
>> cutting points.
>>
>> what do you think?
>>
>> Thanks!
>>
>>
>> On Thu, Sep 16, 2010 at 2:33 PM, Bill Lorensen <bill.lorensen at gmail.com>
>> wrote:
>>     
>>> You want vtkClipPolydata rather than vtkCutter.
>>>
>>> On Thu, Sep 16, 2010 at 11:29 AM, Dan Asimov <dan.asimov at gmail.com> wrote:
>>>       
>>>> Hi,
>>>>
>>>> I'm new to vtk. So, forgive me for my simple question!
>>>>
>>>> I'm trying to use vtkCutter on a vtkPolydata to get all the points
>>>> inside a
>>>> specific plane. However, it seems vtkCutter doesn't return any cell that
>>>> lies completely inside the cutting plane!
>>>>
>>>> Here is a very simple example in C++:
>>>>
>>>> //Adding some points (3 of them in the x-y plane)
>>>> vtkPoints *pPoints = vtkPoints::New();
>>>> pPoints->InsertNextPoint(1,0,-0.5);
>>>> pPoints->InsertNextPoint(0,1,0);
>>>> pPoints->InsertNextPoint(-1,0,0);
>>>> pPoints->InsertNextPoint(0,-1,0);
>>>>
>>>> vtkPolyData *pPolydata = vtkPolyData::New();
>>>> pPolydata->SetPoints(pPoints);
>>>>
>>>> vtkPolygon *pPolygon = vtkPolygon::New();
>>>> pPolygon->GetPointIds()->SetNumberOfIds(4);
>>>> for (int i=0;i<4;i++)
>>>> {
>>>>     pPolygon->GetPointIds()->SetId(i,i);
>>>> }
>>>> pPolydata->Allocate(1,1);
>>>>
>>>> pPolydata->InsertNextCell(pPolygon->GetCellType(),pPolygon->GetPointIds());
>>>>
>>>> // The cutting plane is the x-y plane
>>>> vtkPlane *pPlane = vtkPlane::New();
>>>> pPlane->SetOrigin(0,0,0);
>>>> pPlane->SetNormal(0,0,1);
>>>>
>>>> // Cut
>>>> vtkCutter *pCutter = vtkCutter::New();
>>>> pCutter->SetCutFunction(pPlane);
>>>> pCutter->SetInput(pPolydata);
>>>> pCutter->Update();
>>>> vtkPolyData *pCutterOutput = pCutter->GetOutput();
>>>>
>>>> vtkPoints *Pts = pCutterOutput->GetPoints();
>>>> int nPts = Pts->GetNumberOfPoints();
>>>>
>>>> now, the pCutterOutput contains only two points (0,1,0) & (0,-1,0). I
>>>> was
>>>> expecting it will contain also (-1,0,0) since it's in the same plane
>>>> (x-y
>>>> plane). Am I missing something??
>>>>
>>>> Thanks for your help!
>>>>
>>>>
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>>     
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