[vtkusers] Outer surface

[David.Pont at ForestResearch.co.nz]

Salut Perrine,
  This had me thinking too... not easy... is it actually one surface
(topologically) with holes? or given segmentation, probably one or more
chunks like that?

If so, my 2c thought was:
   get Center for the dataset (somewhere inside the skull)
  for each cell, calculate dot product of cell normal with vector from
dataset Centre to cell Centre
  if <= 0 discard the cell

This would remove 'inward' facing cells. May want to try values 'around' 0
(if so normalise the vector) to control behaviour at the 'edges'. Could
manually specify dataset 'Centre'...

   Dave P



                                                                                                                     
                      "John Biddiscombe"                                                                             
                      <jbiddiscombe at skippingm        To:       "Perrine PAUL" <perrine.paul at chu-rennes.fr>,          
                      ouse.co.uk>                     <vtkusers at vtk.org>                                             
                      Sent by:                       cc:                                                             
                      vtkusers-admin at vtk.org         Subject:  Re: [vtkusers] Outer surface                          
                                                                                                                     
                                                                                                                     
                      31/01/2004 02:47                                                                               
                                                                                                                     
                                                                                                                     




NB. This assumes closed surfaces. If the surfaces aren't closed, it's not
obvious what's in and wha'ts out.

----- Original Message -----
From: "John Biddiscombe" <john.biddiscombe at mirada-solutions.com>
To: "Perrine PAUL" <perrine.paul at chu-rennes.fr>; <vtkusers at vtk.org>
Sent: Friday, January 30, 2004 12:17 PM
Subject: RE: [vtkusers] Outer surface


For each cell in polydata
  Find cell normal
  Fire ray from point (cell centre + epsilon*normal) to {x,y,+infinity}
  Count number of itersections of ray along path
  if number odd. Cell interior - kill it
  if number even (or zero), cell on surface. Keep it
End loop
(For best results, do three rays, in +x, +y, +z directions to minimize
chance of ray passinng through crack between edges of polygons, use
tolerance carefully).

Use vtkCellLocator + IntersectWith line + some custom checks to get all
intersections instead of just first one.

JB



> -----Original Message-----
> From: vtkusers-admin at vtk.org [mailto:vtkusers-admin at vtk.org]
> On Behalf Of Perrine PAUL
> Sent: 29 January 2004 16:36
> To: vtkusers at vtk.org
> Subject: [vtkusers] Outer surface
>
>
> Hello everyone,
> I have a  skull surface (in VTK format) segmented from a CT
> scan. The problem is: since the skull has a thickness, my
> surface is composed of 2 connected surfaces, I call them the
> outer and the inner surfaces. I would like to build a new
> polydata with only the outer surface. Do you have any idea
> about how do that? Cheers, Perrine
>
> --
> Perrine PAUL - PHD Student
> +33 2 23 23 49 91
> Laboratoire IDM, http://idm.univ-rennes1.fr
> Université de Rennes 1, Faculté de médecine
>
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