[vtkusers] picking objects from a subset of a grid
Stephen Langer
stephen.langer at nist.gov
Thu Jan 19 13:57:17 EST 2012
Hi David --
Following your suggestion I've been looking at using selections and vtkExtractSelectedFrustum, but I'm getting more confused.
Is the selection list that's set by SetSelectionList used for input or output? The examples that I've seen (e.g., http://vtk.org/gitweb?p=VTK.git;a=blob;f=Graphics/Testing/Cxx/TestExtraction.cxx) fill the list before calling SetSelectionList. Is that just simulating a selection so that the extraction process can be demonstrated? Does the selection list contain the selected items, or does it contain the set of items from which the selection will be made? Do I need to call SetSelectionList at all, since I want vtk to make the selection?
I still don't understand how to select edges and vertices. The extracted grid contains the tetrahedra from the source grid that intersect the frustum, and its Points array contains only the points from those tets, but those points are not in the frustum.
Here's what I've been trying:
vtkSmartPointer<vtkSelectionNode> selectionNode =
vtkSmartPointer<vtkSelectionNode>::New();
selectionNode->Initialize();
selectionNode->SetFieldType(vtkSelectionNode::POINT);
selectionNode->SetContentType(vtkSelectionNode::INDICES);
// I don't understand what the next two lines do. I don't see any change in
// behavior if I remove them.
vtkSmartPointer<vtkIdTypeArray> ids = vtkSmartPointer<vtkIdTypeArray>::New();
selectionNode->SetSelectionList(ids);
vtkSmartPointer<vtkSelection> selection =
vtkSmartPointer<vtkSelection>::New();
selection->AddNode(selectionNode);
// Using vtkAreaPicker here seems like it must be inefficient, but it's easier
// than doing the geometry myself, for now.
vtkSmartPointer<vtkAreaPicker> picker = vtkSmartPointer<vtkAreaPicker>::New();
picker->AreaPick(x, y, x+1, y+1, renderer); // x and y are given screen coordinates
vtkSmartPointer<vtkPlanes> planes = picker->GetFrustum();
vtkSmartPointer<vtkExtractSelectedFrustum> extractor =
vtkSmartPointer<vtkExtractSelectedFrustum>::New();
extractor->SetFrustum(planes);
extractor->PreserveTopologyOff();
vtkSmartPointer<vtkDataSet> dataset = layer->get_pickable_dataset("elements");
extractor->SetInputConnection(0, dataset->GetProducerPort());
extractor->SetInputConnection(1, selection->GetProducerPort());
extractor->Update();
vtkSmartPointer<vtkUnstructuredGrid> selected =
vtkSmartPointer<vtkUnstructuredGrid>::New();
selected->ShallowCopy(extractor->GetOutput());
At this point the "selected" grid contains tets that intersect the frustum of the screen pixel x,y, even though the selection field type was POINT. What am I missing?
I also don't see how this helps to find the point or edge that was clicked on, if the user is trying to select a vertex or edge of the grid.
Thanks again,
Steve
On Jan 16, 2012, at 11:52 AM, David E DeMarle wrote:
> I suggest using vtkExtractSelectedFrustum instead of picking. See
> http://www.vtk.org/Wiki/VTK/Examples/Cxx/GeometricObjects/Frustum
> and
> http://vtk.org/gitweb?p=VTK.git;a=blob;f=Rendering/Testing/Cxx/TestAreaSelections.cxx
> for examples
>
> Picking is, with the exception of picking multiple actors, more useful for identifying the one thing underneath one pixel.
>
> Selection is more useful for picking multiple things (both under multiple pixels and deeper underneath them).
>
> Note you can select points directly and then turn on a flag to include the cells that contain those points. Also note that the filter behind FrustumSelection is defined in world space, not screen space and works on cells or points directly, so there shouldn't be any need to make cells to represent edges or vertices. Lastly note that you can manipulate the frustum however you like to be more selective about what is chosen.
>
> David E DeMarle
> Kitware, Inc.
> R&D Engineer
> 21 Corporate Drive
> Clifton Park, NY 12065-8662
> Phone: 518-881-4909
>
>
> On Fri, Jan 13, 2012 at 4:40 PM, Stephen Langer <stephen.langer at nist.gov> wrote:
> Hi --
>
> My program displays a vtkUnstructuredGrid, along with other objects.
> I'd like users to be able to pick nodes, edges, faces, and elements of
> the grid with mouse clicks. The interior of the grid needs to be
> visible, and sometimes only a subset of the grid is displayed (ie,
> only elements meeting some user-specified criterion). Currently I've
> more or less figured out how to click on and select elements and
> nodes, but I'm having some difficulties and would greatly appreciate
> some advice.
>
> If the vtkUnstructuredGrid contains only 3D (eg, tetrahedral) cells,
> then the internal edges aren't drawn in a wire frame representation,
> so to display the grid I'm using the vtkExtractEdges filter. If I
> only display the edges, then vtkCellPicker won't pick anything, so I
> add a second actor that displays the same grid using an almost
> completely transparent surface representation. Then the picker can
> work on the second actor when choosing elements. In both cases cells
> in the grid are created only for the subset that's being displayed.
> This works, but it seems inefficient. Is there a better way?
>
> When picking nodes I'd like to use a vtkPointPicker because then I
> don't have to explicitly create vertex cells in the grid. However,
> then the picker can pick points that aren't in the displayed subset of
> the grid. How can I restrict the picker so that it only finds points
> that are in the displayed tets? Do I have to make a new vtkPoints
> list? I'd rather not do that for memory reasons. (I can't use a
> vtkCellPicker to pick a tet and then select the closest node of that
> tet, because I might be trying to pick a node that's visible through
> some number of intervening tets.)
>
> Is it possible to get the vtkPointPicker to return multiple points for
> one vtkActor? If it returned all points within a tolerance of the
> ray, then I could choose the closest one that's in the displayed
> subset. vtkPicker.GetPickedPositions returns a set of positions, but
> it seems to include just one per vtkActor, and all of my points are in
> the same vtkActor. Is it necessary to create a separate vtkActor for
> each point?
>
> I tried creating a separate vtkUnstructuredGrid containing only vertex
> cells from the elements in the subset, but vtkCellPicker doesn't seem
> to work on vertices. Is that correct? Is there a way of picking
> cells from a grid that contains only vertices?
>
> In case it matters, I'm not using a vtkRenderWindowInteractor. I'm
> using vtk inside a gtk program. The vtk render window is redirected
> to a gtk drawing area, and the mouse click coordinates are returned
> from there.
>
> Thanks for any advice. I hope my questions aren't too elementary.
> I've looked for answers in the kitware books and online without much
> success.
>
> -- Steve
>
> --
> -- EMail: stephen.langer at nist.gov Phone: (301) 975-5423 --
> -- http://math.nist.gov/~SLanger/ Fax: (301) 975-3553 --
> -- Mail: NIST; 100 Bureau Drive -- Stop 8910; Gaithersburg, Md 20899-8910 --
>
> -- "I don't think this will work. That's why it's science." --
> -- Naomi Langer (age 6), 17 Feb 2003 --
>
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--
-- EMail: stephen.langer at nist.gov Phone: (301) 975-5423 --
-- http://math.nist.gov/~SLanger/ Fax: (301) 975-3553 --
-- Mail: NIST; 100 Bureau Drive -- Stop 8910; Gaithersburg, Md 20899-8910 --
-- "I don't think this will work. That's why it's science." --
-- Naomi Langer (age 6), 17 Feb 2003 --
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