[vtk-developers] 2 ways of representing lines

[Dean Inglis]

Hi all,

I have a workaround for a long-standing pb with representing straight lines
in VTK.  I recently added cross-hairs to vtkImagePlaneWidget wherein the
pipeline for the cross-hairs is basically:

<to store it>
  vtkPoints          *CursorPoints;
  vtkPolyData        *CursorPolyData;
  vtkPolyDataMapper  *CursorMapper;
  vtkActor           *CursorActor;

<and to generate it>
  // Construct initial points
  this->CursorPoints->SetNumberOfPoints(4);

  this->CursorPolyData->SetPoints(this->CursorPoints);

  this->CursorMapper->SetInput(this->CursorPolyData);
  this->CursorMapper->SetResolveCoincidentTopologyToPolygonOffset();

  this->CursorActor->SetMapper(this->CursorMapper);
  this->CursorActor->PickableOff();

  vtkCellArray *cells = vtkCellArray::New();
  cells->Allocate(cells->EstimateSize(2,2));
  vtkIdType pts[2];
  pts[0] = 0; pts[1] = 1;       // horizontal segment
  cells->InsertNextCell(2,pts);
  pts[0] = 2; pts[1] = 3;       // vertical segment
  cells->InsertNextCell(2,pts);

  this->CursorPolyData->SetLines(cells);
  this->CursorPolyData->Modified();
  cells->Delete();

<snip>

Well, if one zooms in on a vtkImagePlaneWidget, the image data can still be
queried (i.e. the picking is functional) but the cross-hairs disappear.  Try
/Examples/GUI/Tcl/ImagePlaneWidget.tcl without the line:
<snip>
[ren1 GetCullers] RemoveAllItems
<snip>

Hmmmmm.  I am considering changing the representation of the cross-hairs to
avoid this messing about with the cullers:
<new representation>
  vtkLineSource      *CursorLine1;
  vtkLineSource      *CursorLine2;
  vtkAppendPolyData  *CursorPolyData;
  vtkPolyDataMapper  *CursorMapper;
  vtkActor           *CursorActor;
<snip>

I've tested and it works.  Any thoughts before I make this change?

Dean

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Dean Inglis, Ph.D.
Clinical Research Unit
25 Charlton Avenue East, Unit 610
Hamilton, ONT, L8N 1Y2
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~