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

[Bill Lorensen]

I'm confused. Do you have a set of contours or a mesh (polydata) that
was created from the contours?

If the later, then Cutter should work.

On Fri, Sep 17, 2010 at 11:10 AM, Dan Asimov <dan.asimov at gmail.com> wrote:
> Hi Bill,
>
> Thanks again for the detailed reply! Unfortunately, none can help me in my
> problem. I was sending a simplified version of my problem. My whole problem
> is that I have multiple 2D contours in parallel to each others in one
> vtkpolydata. I want to cut the contours with a plane. If the plane is
> exactly the plane of one of these 2D contours, the cut should return this
> exact 2D contour. Else, it should return the intersection points between the
> plane and the lines that connects the 2D contours. I was hoping vtkCutter
> will do the job and I won't need to divide the problem into two steps (one
> for the in-plane and one for the intersection).
>
>
> Also, I'm not expert, but I don't think it's the float imprecision problem
> here! When I debug the code inside the vtk classes, it all stops in
> vtkTriangle::Contour. The vtkpolydata points are evaluated with the Evalute
> function of the plane (this works nicely as expected, with scalar value = 0
> for the three points) then the points are passed to vtkTriangle::Contour.
>
> The first thing this function does is to build a case table with the
> following for loop
>
> static int CASE_MASK[3] = {1,2,4};
> // Build the case table
>   for ( i=0, index = 0; i < 3; i++)
>     {
>     if (cellScalars->GetComponent(i,0) >= value)
>       {
>       index |= CASE_MASK[i];
>       }
>     }
>
> Basically, if the index is 7 it ignore this cell at all. However, if the
> three points are evaluated to be on the plane (i.e. value=0), index will be
> essentially 7!! So, I believe this is done intentionally, maybe for reasons
> related to contouring the intersected points.
>
> what do you think?
>
> Thanks!
>
>
> On Fri, Sep 17, 2010 at 8:10 AM, Bill Lorensen <bill.lorensen at gmail.com>
> 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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