[vtkusers] radii of vtkCylinder and vtkCylinderSource are unequal?

[Maarten Beek]

Very good suggestion, David!

But how would I code an optional step for which one does need a filter class (i.e. smoothing, normals)?
I am browsing through the vtk code, because I would assume this has been done before, but I have been unlucky so far.

Maarten



________________________________
 From: David Gobbi <david.gobbi at gmail.com>
To: Maarten Beek <beekmaarten at yahoo.com> 
Cc: "vtkusers at vtk.org" <vtkusers at vtk.org> 
Sent: Thursday, December 15, 2011 9:51:17 PM
Subject: Re: [vtkusers] radii of vtkCylinder and vtkCylinderSource are unequal?
 
Hi Maarten,

Instead of vtkTransformPolyData, a better method might be to use the
TransformPoints method of vtkTransform:

vtkPoints *newpoints = vtkPoints::New();
transform->TransformPoints(oldpoints, newpoints);

- David


On Thu, Dec 15, 2011 at 11:40 AM, Maarten Beek <beekmaarten at yahoo.com> wrote:
> Hi David, others,
>
> I used the vtkButterflySubdivisionFilter class, which generated a much
> better result. However it also introduced an error (?), but it proved it was
> an accuracy issue and not my mistake.
>
> Therefore, I created my own source which is working well.
> I now want to implement an optional transformation to put the model anywhere
> I want.
>
> So far I coded it like:
>
> vtkPolyData* output = vtkPolyData::SafeDownCast(
> outInfo->Get(vtkDataObject::DATA_OBJECT));
>
> < snip >
>
> output->SetPoints( pts );
> output->SetPolys( tris );
>
> if ( this->Transform )
> {
>     vtkSmartPointer<vtkTransformPolyDataFilter> transformer =
> vtkTransformPolyDataFilter::New();
>     transformer->SetInput( output );
>     transformer->SetTransform( this->Transform );
>     transformer->Update();
>
>     transformer->SetInput( 0 ); // to disconnect input ?
>     output = transformer->GetOutput();
> }
>
> In other words I try to pass the data from input to output to avoid
> temporarily copies of the data. (ITK has the DisconnectPipeline function for
> this). The code above however is not working.
>
> The following code does work, however it uses this temporarily copy I try to
> avoid:
>
> if ( this->Transform )
> {
>     vtkSmartPointer<vtkPolyData> temp = vtkPolyData::New();
>     temp->ShallowCopy( output );
>
>     vtkSmartPointer<vtkTransformPolyDataFilter> transformer =
> vtkTransformPolyDataFilter::New();
>     transformer->SetInput( temp);
>     transformer->SetTransform( this->Transform );
>     transformer->Update();
>
>     output->ShallowCopy( transformer->GetOutput() ); // Why is this shallow
> copy required?
> }
>
> Any suggestions?
>
> Maarten
>
> ________________________________
> From: David Gobbi <david.gobbi at gmail.com>
> To: Maarten Beek <beekmaarten at yahoo.com>
> Cc: "vtkusers at vtk.org" <vtkusers at vtk.org>
> Sent: Wednesday, December 7, 2011 5:03:12 PM
>
> Subject: Re: [vtkusers] radii of vtkCylinder and vtkCylinderSource are
> unequal?
>
> Hi Maarten,
>
> VTK has lots of subdivision filters in the "Graphics" directory.  I would
> not advise
> trying to change the way VTK does clipping.
>
> Writing your own source is a good idea.  After you have done it once, it is
> easy
> to do over and over again for whatever shape you need.  I have my own set of
> special-purpose polydata sources and haven't used the ones that come with
> VTK in ages.
>
>  - David
>
>
> On Wed, Dec 7, 2011 at 2:33 PM, Maarten Beek <beekmaarten at yahoo.com> wrote:
>
> Thanks David,
>
> Good to know it is an approximating issue, and not me.
>
> However, I don't really know where the plane is. Although I could calculate
> it, but then I could position the vertices and triangulate between them
> myself (no need for the vtkClipPolyData class...).
>
> I guess improving the clipping would involve vtkGenericCell::Clip().
>
> Also, I made a simple application in which I can change the length of the
> cone relative to the radius of the cylinder; I don't see the clipping
> improve when I do this... so I am not yet convinced regarding your rule.
>
> Does VTK have a class that triangulates the triangles in a vtkPolyData? The
> cone resolution just makes the triangles narrower or wider (they keep going
> from bottom to top).
>
> I think that creating my own ConeCylinderSource class is easiest...
>
> Maarten
>
> ________________________________
> From: David Gobbi <david.gobbi at gmail.com>
> To: Maarten Beek <beekmaarten at yahoo.com>
> Cc: "vtkusers at vtk.org" <vtkusers at vtk.org>
> Sent: Wednesday, December 7, 2011 3:26:42 PM
> Subject: Re: [vtkusers] radii of vtkCylinder and vtkCylinderSource are
> unequal?
>
> Refining the cone resolution will help, but if you can, you should use
> a plane to clip the cone, instead of using a cylinder.
>
> VTK does clipping by evaluating the implicit function at each end of
> each line segment, and then it does a linear interpolation along the
> line segment to find the point where it does the clip.  This works
> perfectly when the function is a plane function, but it is only
> approximate if the function is a curved surface.
>
> The rule is: the lengths of the polygon edges must be much less then
> the radius of curvature of the implicit function.  A plane has an
> infinite radius of curvature, so clipping with a plane is always
> ideal.
>
> - David
>
>
> On Wed, Dec 7, 2011 at 12:46 PM, Maarten Beek <beekmaarten at yahoo.com> wrote:
>> Hi all,
>>
>> See code below.
>> What I am trying to do is to connect a piece of cone to a cylinder. To do
>> this I cut the top of the cone using an implicit cylinder with the same
>> radius as real cylinder.
>> However, the radius of the cut cone top is not equal to the radius of the
>> real cylinder.
>> What am I doing wrong?
>>
>> Does this have to do with how the clip values in the points are
>> interpolated? And should I refine the triangulation of the cone to get a
>> better result?
>>
>> Thanks - Maarten
>>
>> vtkSmartPointer<vtkConeSource> startcone = vtkConeSource::New();
>>   startcone->SetResolution( 30 );
>>   startcone->SetHeight( this->EntryLength );
>>   startcone->SetRadius( this->EntryRadius );
>>   startcone->SetCenter( 0.0, -0.5*this->Length + 0.5*this->EntryLength,
>> 0.0
>> );
>>   startcone->SetDirection( 0.0, 1.0, 0.0 );
>>   startcone->CappingOff();
>>
>>   vtkSmartPointer<vtkCylinderSource> cylinder = vtkCylinderSource::New();
>>   cylinder->SetResolution( 30 );
>>   cylinder->SetRadius( this->CylinderRadius );
>>   cylinder->SetHeight( this->Length );
>>   cylinder->SetCenter( 0.0, 0.0, 0.0 );
>>   cylinder->CappingOff();
>>
>>   vtkSmartPointer<vtkCylinder> cyl = vtkCylinder::New();
>>   cyl->SetRadius( this->CylinderRadius );
>>   cyl->SetCenter( 0.0, 0.5*this->Length, 0.0 );
>>
>>   vtkSmartPointer<vtkClipPolyData> clipper1 = vtkClipPolyData::New();
>>   clipper1->SetClipFunction( cyl );
>>   clipper1->SetInputConnection( startcone->GetOutputPort() );
>>
>>   vtkSmartPointer<vtkAppendPolyData> append = vtkAppendPolyData::New();
>>   append->AddInput( clipper1->GetOutput() );
>>   append->AddInput( cylinder->GetOutput() );
>
>
>
>
>
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