[vtkusers] Mean Curvature

[Mathieu Malaterre]

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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