[vtkusers] compute polygon centroids and normals

[David.Pont at ForestResearch.co.nz]

Matt wrote:

   I am computing the centroid of each polygon in a
   vtkpolydata (which are triangles) as well as the
   normal of each triangle. I have done this by
   extracting the verticies of each triangle and
   computing the centroid by taking the average of the 3
   verticies. I am fitting a plane to the verticies to
   get the normal of each triangle. I feel like I am
   reinventing the wheel - I have looked but I haven't
   found a vtk function that does this. Any suggestions?

Take a look at vtkCellCenters
http://public.kitware.com/VTK/doc/release/4.0/html/classvtkCellCenters.html

For normals check vtkPolyDataNormals as already suggested. If you really
need to calculate normals on a cell by cell basis you can create an
instance of vtkPolygon, then use the ComputeNormal method, passing it the
points from your cell with something like: cell->GetPoints().


   Also- how can I order verticies in my polydata such
   that they are always referenced clockwise or
   counterclockwise for each triangle? I am thinking this
   may help with providing consistently oriented normals
   (ie: outward or inward). Any suggestions would be
   greatly appreciated!

Try ConsistencyOn with vtkPolyDataNormals which should "... reorder
polygons to insure consistent orientation ..."

regards, Dave Pont