[vtkusers] CellLocator, Octrees and Segfaults...

[Philipp Kemmeter]

Ahaa !

That's helpful! Cool, I'll try it. Thanks a lot :)

Phil

2006/10/25, Obada Mahdi <omahdi at gmx.de>:
> Hi Radu!
>
> On 10/24/06, Radu Bogdan Rusu <rusu at cs.tum.edu> wrote:
> > Yeah, what I don't understand is why:
> >
> > this->MarkParents((void*)VTK_CELL_INSIDE,i,j,k,ndivs,this->Level);
> >
> > ...in: void vtkCellLocator::BuildLocator()
> > ...where: #define VTK_CELL_INSIDE 1
> > ...because: void vtkCellLocator::MarkParents(void* a, int i, int j, int k, int ndivs, int
> > level)
> > ...contains: this->Tree[parentIdx] = (vtkIdList *)a;
>
> I am not sure that I understand the question :-)  Here is what I think
> is going on, please excuse the traffic if I am just repeating what you
> already know:
>
> When the locator is built, cells are distributed over 8^l leaf nodes
> of an octree, where l is the height of the tree (which is computed
> automatically, based on the number of cells, unless given explicitly).
>  These leaf nodes contain either NULL if they are empty, or hold a
> pointer to an instance of vtkIdList otherwise.  These are the only
> nodes that can carry cell IDs.
>
> In addition, inner nodes of the octree are marked as empty (NULL,
> value from initialization) or non-empty (VTK_CELL_INSIDE), depending
> on whether at least one child (leaf) node contains cells or not.  This
> is done by MarkParents(), by tagging every node on the path towards
> the root node.  This is required for e.g. speeding up intersection
> tests (eliminating parts of the tree by top-down testing intersection
> with bounding boxes).
>
> The octree maintained by vtkCellLocator is quite simple and has a very
> specific structure--maybe that is why there is no more elaborate
> interface for navigating through the tree, but frankly, I do not
> understand that interface, too.  It is also not picked up by
> vtkOBBTree, which directly inherits from vtkCellLocator and does not
> provide methods for accessing tree nodes at all (as far as I can see).
>  Maybe the best thing is to follow the suggestion from the
> documentation of vtkCellLocator, which states that it is meant to be
> subclassed.
>
> > Is there any other way of actually "iterating" through the octants, then ?
>
> If you want to exploit the structure of the flattened octree
> representation to iterate just over leaf nodes, you can use something
> like:
>
> // fetch total number of nodes (inner+leaf)
> int numNodes = locator->GetNumberOfBuckets();
> // compute 8^(tree height), which is the number of leaf nodes
> int numLeafNodes = ( 2 << (3 * locator->GetLevel()) );
> for ( int i = numNodes - numLeafNodes; i < numNodes; i++ ) {
>   idList = locator->GetCells(i);
>   if ( idList != NULL ) ...
> }
>
>
> HTH,
>
> Obada
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