[vtkusers] vtkPolyDataToImageStencil

[Frank Gao]

David,

Thanks for your reply. Now I understand the problem. However, when the
spline is on YZ plane, vtkLinearExtrusionFilter just does not have any cell
data to create cap faces, so the capping option is actually ignored. Could
you elaborate what you said here " This would involve adding a contour
triangularization algorithm to the code"?

Thanks again,

Frank

-----Original Message-----
From: David Gobbi [mailto:dgobbi at atamai.com]
Sent: Wednesday, May 23, 2007 1:45 PM
To: gao at mdimaging.com
Cc: Vtkusers at Vtk.Org
Subject: Re: [vtkusers] vtkPolyDataToImageStencil

Hi Frank,

The algorithm in the class that I emailed to you
(vtkAtamaiPolyDataToImageStencil2) works by cutting the
polydata with the XY planes of the image slices to create
a series of polygons, and then it rasterizes each of these
polygons.    There is some logic that it uses close any
polygons that aren't closed, but there is some ambiguity
and you are never 100% safe unless your polydata is
closed.

The vtkPolyDataToImageStencil in VTK does not do
anything to try to close the polydata.  Instead, it rasterizes
the 3D surface directly, one X row at a time.  So if the
original contour is in the YZ plane, then extruding it
along X will give you tube that all of the raster lines will
pass through without ever intersecting with any polygons.
The result is no output volume.

You could try adding a "capping" option to
vtkLinearExtrusionFilter.  This would involve adding
a contour triangularization algorithm to the code.
(Delaunay won't work since the contour might not
be convex).

 - David


Frank Gao wrote:
> Hi David, All,
>
> It works well for me, but would you please shed some light on a problem I
am
> having with both old and new versions?
>
> What I am trying to do is to draw a closed profile and use it to cut a 3D
> image. I have a closed spline on a plane, extrude it in the normal
direction
> by vtkLinearExtrusionFilter with capping on. (Unfortunately, this
extrusion
> filter does not generate capping surfaces for this case.) Then I use
> vtkPolyDataToImageStencil to convert the extruded vtkPolyData into image
> stencil like a cookie cutter, and use vtkImageStencil to cut the dataset.
> What I expected here is vtkPolyDataToImageStencil can give me a volume, so
> that I can get the inside or outside part of my dataset by
vtkImageStencil.
>
> This works very well except when the spline is on YZ plane and the extrude
> direction is X axis, where I only get points on the surface instead of a
> volume. Does this have anything to do with the raterization algorithm in
> vtkPolyDataToImageStencil? If vtkPolyDataToImageStencil raterizes the
> extrusion along X Axis, I do not get a volume?
>
> Your help is greatly appreciated!
>
> Thanks,
>
> Frank
>
> -----Original Message-----
> From: David Gobbi [mailto:dgobbi at atamai.com]
> Sent: Tuesday, May 22, 2007 5:09 PM
> To: gao at mdimaging.com
> Cc: Vtkusers at Vtk.Org
> Subject: Re: [vtkusers] Possible Bug in vtkPolyDataToImageStencil
>
> Hi Frank,
>
> Sure thing.   Let me know how it works for you.
>
>  - David
>
>
>
> Frank Gao wrote:
>
>> Hi David,
>>
>> I am using vtkPolyDataToImageStencil, and saw an earlier email thread as
>>
> below.
>
>> Most of the time I am getting good results but have same issue as Jesus.
>>
> Would it be possible for you to send me a copy of another
> vtkPolyDataToImageStencil class you mentioned in your post?
>
>> Thanks in advance,
>>
>> Frank Gao
>>
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