[vtkusers] Clipping geometry

[Will Schroeder]

Hi Brent-

At 12:38 PM 7/6/2000 -0500, Brent Burton wrote:

>Hello all,
>
>I have two PolyData sets, one contains line segments and the other has an
>isosurface.  Both sets are large, and to better manipulate the
>interesting quadrant of the volume I need to cull the rest.
>
>After trying several different clipping/extraction techniques, I got
>vtkClipPolyData working with a vtkImplicitBoolean function consisting of
>two vtkPlanes.  This clipper script is in TCL and works very well.
>
>So my question to the list:  Is the above clipper the most efficient
>way to trim geometric data?  Does anybody have a brief description
>of where the other filters (vtkExtractGeometry, vtkExtractVOI, etc)
>are more appropriate (and maybe more efficient) than vtkClipPolyData?

A couple of comments: the clipping based on implicit functions is limited in
accuracy because of the way the clip values are interpolated...so you will see
sloppy clips in some cases (e.g., big polygon, right angle plane combination).
On the other hand, the algorithm is robust. The Extract guys typically remove
cells, etc. on an atomic level, i.e., an entire cell is extracted, not just 
a piece.
This algorithm is also robust and probably faster. (Clipping is more accurate.)

There is no "exact" method to clip/cut in vtk (at least that I know of).

Will