[vtkusers] VTKboolean smoothes the results! Any other libraries for boolean operation on polydata?

[Cory Quammen]

What I think you are seeing with the rough surface is surface normals
generated from an image volume. I am assuming a lot here, so please correct
me if I am wrong. It looks like you have a geometry of a bone extracted
from a CT scan. The extraction algorithm was probably a variant of marching
cubes or a similar algorithm. The surface normals estimated from the
surface extraction algorithm were probably computed from the intensity
gradient of the CT scan. This is a reasonable approach, but noise in the CT
scan will affect the gradient calculation and may perturb the surface
normals in the geometry a little. When this happens, you can get a rough
appearing surface from the surface lighting calculation. Google "bump
mapping" for how this can be used for a nice graphics effect - I think
roughly the same thing is happening with your original data.

When you use vtkPolyDataNormals, the normals are computed from the surface
data, which was smooth to begin with. Hence, the normals are not perturbed,
so you don't get a bumpy appearance.

In any case, the measurements you want to take are not dependent on the
surface normals. As long as the points on the surface are not smooth, which
they should not be, your measurement procedure should be fine.

To convince yourself the geometry is the same, you can display both the
original "bumpy" surface and the smoothed surface at the same time with
different colors. You should see just one surface or the other, probably
the one you added to the vtkRenderer last. If the geometry is different,
you will see some of one surface color and some of the other depending on
which geometry is closer to the camera. I suspect you will see just one
surface.

HTH,
Cory

On Mon, Jul 17, 2017 at 11:22 AM, Majid Mohammad sadeghi <
majid.msadeghi at yahoo.com> wrote:

> Dear Cory,
>
> As I applied vtkPolyDataNormals to the polydata before the boolean
> operation I got the smoothed object, so you were right. But I dont
> understand exactly what is happening here? Since the geometry dimentions
> are very important for my application, Can I have the noisy version? Is
> there any way I do not get it smoothed?
>
> Thanks,
>
>
>
>
> On Monday, July 17, 2017, 6:48:58 PM GMT+3, Cory Quammen <
> cory.quammen at kitware.com> wrote:
>
>
> There shouldn't be any geometry smoothing going on in this filter. Can you
> verify that the geometry has changed in the smoothed version?
>
> This is speculation, but one possibility is that the surface normals in
> your original surface come from a gradient computation in the volume from
> which the surface was extracted and noise in the volume result in the bumpy
> appearance. You can check this by applying the vtkPolyDataNormals filter to
> your original surface and looking at the result.
>
> HTH,
> Cory
>
> On Mon, Jul 17, 2017 at 10:21 AM, Majid Mohammad sadeghi <
> majid.msadeghi at yahoo.com> wrote:
>
> Dear Cory,
>
> Thanks for the reply, I set the actors property to  SetInterpolationToFlat
> (); as you said, but the result is the same I put a picture before and
> after boolean operation here. Any suggestion? Are you familier with any
> other library? Current boolean operation takes about 150 seconds on my
> system, do you think this is normal?
>
> [image: Inline image]
>
>
>
>
> On Monday, July 17, 2017, 6:00:50 PM GMT+3, Cory Quammen <
> cory.quammen at kitware.com> wrote:
>
>
> What do you mean by smooth? The lighting is smooth? I think the filter
> may be adding surface normals, in which case any actor for the data
> will turn on Gouraud shading (or maybe Phong) by default, which will
> make your surface look smooth. You can turn off smooth lighting as
> shown in this example:
>
> http://www.vtk.org/Wiki/VTK/ Examples/Cxx/Visualization/ FlatShading
> <http://www.vtk.org/Wiki/VTK/Examples/Cxx/Visualization/FlatShading>
>
> On Mon, Jul 17, 2017 at 9:49 AM, Majid Mohammad sadeghi via vtkusers
> <vtkusers at vtk.org> wrote:
> > Hi everyone,
> >
> > I am using vtkBooleanOperationPolyDataFil ter, and it works fine except
> after
> > the operation it smoothes the polydata, which I dont want. Is there a
> way to
> > stop that? Or is there another library for boolean operations on
> polydata?
> >
> > Thanks.
> >
> >
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> --
> Cory Quammen
> Staff R&D Engineer
> Kitware, Inc.
>
>
>
>
> --
> Cory Quammen
> Staff R&D Engineer
> Kitware, Inc.
>



-- 
Cory Quammen
Staff R&D Engineer
Kitware, Inc.
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