[Paraview] Re: Stream tracing

[Stefan Murphy]

To be honest, I am not much of a programmer, and I would rather not
get into the guts of ParaView. Also, it would be preferable for me and
my group to use a stable release version of ParaView.

Is there anything I can do with scattered points? Since this is how my
data is stored in my program, it would be very desirable for me.




On Feb 18, 2008 12:16 PM, John Biddiscombe <biddisco at cscs.ch> wrote:
>
>  Further to what ken wrote....
>
>  "Many VTK algorithms, including stream tracing, require your mesh to be
> conforming. "
>
>  in fact the stream tracer, uses FindCell to locate the integration position
> - this can take a cell as start point and uses the neighbours to locate the
> next cell as the point moves ....which is (I think) why it fails on your
> data.
>
>  you can rewrite the core of the stream tracer to use a locator instead of
> straight findcell to make it work on your data. (Assuming this is preferable
> to re-dividing your mesh to get a conforming one!) - It might only be
> vtkInterpolatedVelocityField that needs work, rather than the tracer
> itself.....Berk will know for sure...
>
>  JB
>
>
>  Stefan,
>
> The idea of a hexahedron having more than 6 neighbors is not allowed. The
> mesh example you gave is one that is not "conforming" (or not consistent
> depending on your nomenclature). Many VTK algorithms, including stream
> tracing, require your mesh to be conforming. There are multiple problems
> that occur with non-conforming meshes. As Berk mentioned, the mesh tends not
> to be "water-tight." That is, it is not reasonable to represent 3 collinear
> points exactly in finite precision numbers. The edge represented by 2 nodes
> and the edges with 3 nodes usually do not match up perfectly, leading to
> "cracks" in the mesh.
>
> The second large problem is that connectivity is ill-defined. In your
> example, is the large cell really a neighbor with the two smaller cells?
> They do not actually share faces. The large cell has a face connecting the
> top and bottom nodes. The other two cells each have a face with the middle
> node. What if you moved that middle node a little to the right? You would no
> longer consider these neighbors, right? That would be a geometric change,
> not a topological change. The connectivity should be based entirely on the
> topology. Trying to introduce geometric position into the connectivity
> computations opens a Pandora's box of problems.
>
> In short, VTK does not, and cannot, consider these mismatched faces
> neighbors. In the case of the stream tracer, if the stream reaches one of
> these interfaces, it will assume that the stream left the mesh and terminate
> that stream.
>
> -Ken
>
>
>
>  -----Original Message-----
> From: paraview-bounces+kmorel=sandia.gov at paraview.org [mailto:paraview-
> bounces+kmorel=sandia.gov at paraview.org] On Behalf Of Stefan Murphy
> Sent: Sunday, February 17, 2008 10:13 AM
> To: paraview at paraview.org
> Subject: [Paraview] Re: Stream tracing
>
> I had a previous post looking for help with a stream tracing problem.
> I sent some data to Berk and he said that my problem lies with my
> grid. He said: "I am seeing problems with the way you wrote this mesh.
> It looks like you are creating duplicate points or something like
> that. The mesh is not "water-tight". When you load it in paraview,
> change the representation to wireframe. This is supposed to show only
> the external surfaces as wireframe. Instead, it shows a lot of
> internal boundaries."
>
> I ditched my VTK output routine and adopted some functions that have
> been successfully used to create OpenDX files. With this routine I
> shouldn't have any issues with duplicate points, etc. With the new
> routine I am having the same problem with my mesh. I am wondering if
> my problem could have anything to do with the following:
>
> My grid is cartesian and unstructured (vtk cell type 11). There are 6
> sides per cell, but it is possible to have more than 6 neighbouring
> cells. I attached a picture to show what I mean. I would think this is
> a common thing with unstructured cartesian grids, but I am starting to
> scrape the bottom of the barrel here and I am wondering if anyone
> thinks this could be a problem.
>
> Has anyone been successful in implementing my type of grid in ParaView
> before?
>
>
> Stefan
>
>  _______________________________________________
> ParaView mailing list
> ParaView at paraview.org
> http://www.paraview.org/mailman/listinfo/paraview
>
>
>
>  --
> John Biddiscombe, email:biddisco @ cscs.ch
> http://www.cscs.ch/about/BJohn.php
> CSCS, Swiss National Supercomputing Centre | Tel: +41 (91) 610.82.07
> Via Cantonale, 6928 Manno, Switzerland | Fax: +41 (91) 610.82.82
>