[vtkusers] Mean Curvature

[Philip Batchelor]

But I am a bit surprised that it would explain that the mean curvatures 
are zero in the current vtkCurvatures? I dont know that it is calling
vtkExtractTensorComponents.cxx at all?! The original mean curvature 
computation wouldnt require it.

Ph

Mathieu Malaterre wrote:

>
> This was fixed in VTK CVS:
>
> http://vtk.org/Bug/bug.php?op=show&bugid=1922
>
> Mathieu
>
> Gorman, Gerard wrote:
>
>> It is a bug in VTK. It always gives you a determinant of zero when 
>> you extract tensor components. On inspection of file
>>
>> ./VTK/Graphics/vtkExtractTensorComponents.cxx
>>
>> you see:
>>   else //VTK_EXTRACT_EFFECTIVE_DETERMINANT
>>         {
>>         }
>>
>> ie. the determinant is simply not evaluated.
>>
>> The fix is
>>
>>  else //VTK_EXTRACT_EFFECTIVE_DETERMINANT
>>         {
>>          s = tensor[0]*(tensor[4]*tensor[8]-tensor[5]*tensor[7]) -
>>              tensor[1]*(tensor[3]*tensor[8]-tensor[5]*tensor[6]) +
>>              tensor[2]*(tensor[3]*tensor[7]-tensor[4]*tensor[6]);
>>         }
>>
>> Cheers
>> Gerard.
>>
>> -----Original Message-----
>> From: vtkusers-bounces+g.gorman=imperial.ac.uk at vtk.org on behalf of 
>> Philip Batchelor
>> Sent: Tue 7/5/2005 4:23 PM
>> To: Prathap Nair
>> Cc: vtkusers at vtk.org
>> Subject: Re: [vtkusers] Mean Curvature
>>
>> Hi
>>
>> I am not sure why you get this, would need to see the data I guess.
>> There are some 'constant mean curvature' surfaces, but if you found one
>> by luck, you'll make
>>  many researchers in maths very jealous...
>>
>> NB: the curvatures class are meant to be upgraded to have a more general
>> structure, and include tensor computations, which would give access to
>> principal curvatures directly.
>> In the meantime, you are welcome to try this temporary class (with
>> proviso that it is rather untested).
>>
>> Ph
>>
>>
>> Prathap Nair wrote:
>>
>>  > Hello,
>>  >  > I am using VTK 4.2 on Windows XP with VC++ 6.0 and am 
>> relatively new
>>  > to VTK. I am doing a project where I have to register two 2.5 D scans
>>  > (two surfaces) which are in wavefront obj format. I have had no
>>  > problems so far in terms of rendering or implementation of the
>>  > Iterative Closest Point Transform.
>>  >  > Now i need to implement a landmark detection algorithm that 
>> needs the
>>  > values of Gaussian and Mean curvatures at each point of the 
>> surface. I
>>  > have written the code using the VTK class vtkCurvatures and it
>>  > compiles and runs error free. I am able to get values for Gaussian
>>  > curvature, but for mean curvature i get the value zero at all points
>>  > of the scan. Snippet of my code is provided below, if anyone has 
>> faced
>>  > this problem before or knows what I am doing wrong, please please let
>>  > me know.
>>  >  > Thanks in advance.
>>  >  > Regards,
>>  >  > Prathap Nair
>>  > Dept of Electronic Engineering,
>>  > Queen Mary, University of London,
>>  > Mile End Road, London E1 4NS (UK).
>>  >  > /************************Code Snippet***************/
>>  >  > //face 3 is a polydata object containing the surface
>>  > this->face3->DeepCopy(face2->GetOutput());
>>  >  > //Get the Mean curvature and store in vtkDoubleArray "mvals"
>>  > this->curvm->SetInput(this->face3);                   //curvm is an
>>  > object of vtkCurvatures
>>  > this->curvm->SetCurvatureTypeToMean();
>>  > this->curvm->Update();
>>  > 
>> this->mvals->DeepCopy(curvm->GetOutput()->GetPointData()->GetScalars());
>>  > //Get the Gaussian curvature and store in vtkDoubleArray "gvals"
>>  > this->curvg->SetInput(this->face3);                    //curvg is an
>>  > object of vtkCurvatures
>>  > this->curvg->SetCurvatureTypeToGaussian();
>>  > this->curvg->Update();
>>  > 
>> this->gvals->DeepCopy(curvg->GetOutput()->GetPointData()->GetScalars());
>>  >
>>  >------------------------------------------------------------------------ 
>>
>>  >
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>>
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