[vtkusers] select cell whose edge goes through implicit function but no points

[David Gobbi]

On Sun, Feb 22, 2015 at 8:07 AM, redmod79 <michael.j.redmond at gmail.com>
wrote:

> I'm using vtkExtractGeometry to extract cells that lie within an implicit
> function.  I want to be able to select cells that not only have points
> within the implicit function, but also cells that do not have points within
> the implicit function, but have an edge that goes through it.  I'm
> currently
> not able to do this.  Any help would be greatly appreciated.  Thanks.
>

VTK only evaluates the function at the points, so other than subdividing the
edge to give the algorithm more points to check at, there isn't much to do.

What you really need is an algorithm that checks the function gradient at
each of the edge's endpoints to "guess" whether the endpoints bracket
a local minimum or maximum.  The algorithm would then have to find this
minimum (or maximum) to discover whether its sign is different from the
sign of the function evaluated at the endpoints.   VTK provides no such
algorithm, as far as I'm aware.

 - David
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