[vtk-developers] vtkContourLineInterpolator bug

[Dean Inglis]

Hi all,

 

in working on the aforementioned shortest image path

classes, I've uncovered what I consider to be buggy

behaviour in vtkContourLineInterpolator and any of its

subclasses that do not re-implement the GetSpan method.

For example, if you lay out a terrain path using 

vtkPolygonalSurfaceContourLineInterpolator and then try

to pick and move the first node using vtkContourWidget,

you end up moving only the first node and the line segment

between it and the next interpolated point.  What I think should happen

is that then entire path up to the next node should be interpolated:

ie. reverse of if you move the next node to the first, the entire path

back to the first node is updated.  A fix is to 

use a modified version of the implementation used in 

vtkBezierContourLineInterpolator (edits are shown as comments):

 

//----------------------------------------------------------------------

void vtkBezierContourLineInterpolator::GetSpan( int nodeIndex,

                                          vtkIntArray *nodeIndices,

                                          vtkContourRepresentation *rep)

{

  int start = nodeIndex - 2;

 

// CHANGE TO 

// int start = nodeIndex - 1;

 

  int end   = nodeIndex - 1;

 

// CHANGE TO 

// int end   = nodeIndex;

 

 

  int index[2];

 

  // Clear the array

  nodeIndices->Reset();

  nodeIndices->Squeeze();

  nodeIndices->SetNumberOfComponents(2);

 

  for ( int i = 0; i < 4; i++ )

 

// CHANGE TO

//  for ( int i = 0; i < 3; i++ )

 

    {

    index[0] = start++;

    index[1] = end++;

 

    if ( rep->GetClosedLoop() )

      {

      if ( index[0] < 0 )

        {

        index[0] += rep->GetNumberOfNodes();

        }

      if ( index[1] < 0 )

        {

        index[1] += rep->GetNumberOfNodes();

        }

      if ( index[0] >= rep->GetNumberOfNodes() )

        {

        index[0] -= rep->GetNumberOfNodes();

        }

      if ( index[1] >= rep->GetNumberOfNodes() )

        {

        index[1] -= rep->GetNumberOfNodes();

        }

      }

 

    if ( index[0] >= 0 && index[0] < rep->GetNumberOfNodes() &&

         index[1] >= 0 && index[1] < rep->GetNumberOfNodes() )

      {

      nodeIndices->InsertNextTupleValue( index );

      }

    }

}

 

 

A bonus for shortest path interpolators that use graphs is that if an
intermediate 

node is moved, the paths to the preceding and following nodes are
re-calculated.

Currently, only the path back to the preceding node is re-calculated.

 

If there are no objections, I will commit this change to
vtkContourLineInterpolator.cxx

 

Dean

 

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