[vtkusers] X values of GetPositionCoordinate() & GetEventPosition() differ by 1 pixel

[John Platt]

Hi Users,

 

I have a 2-D actor rendering polydata representing the border of a
rectangle as 1 polyline. The points for the polydata are created using
the position of the 2-D actor in viewport coordinates as follows

 

   int *pos1, *pos2;

   pos1 = this->BorderActor->GetPositionCoordinate()->

              GetComputedViewportValue(this->CurrentRenderer);

   pos2 = this->BorderActor->GetPosition2Coordinate()->

              GetComputedViewportValue(this->CurrentRenderer);

 

   vtkPoints* pts = this->BorderPolyData->GetPoints();

   int width  = pos2[0] - pos1[0];

   int height = pos2[1] - pos1[1];

   pts->SetPoint( 0,     0,      0, 0 );

   pts->SetPoint( 1, width,      0, 0 );

   pts->SetPoint( 2, width, height, 0 );

   pts->SetPoint( 3,     0, height, 0 );

 

When trying to select the left hand edge of the rectangle (0,3), the
display value returned from the interactor using GetEventPosition()[0]
is always 1 pixel less than the actors GetPositionCoordinate()->
GetComputedViewportValue(this->CurrentRenderer)[0].

 

Also, when the border is drawn using the default actor 2-D position (
(0,0) & (0.5,0.5) ), the left hand edge is not visible.

 

There are no problems with any of the other 3 edges - picking or
displaying.

 

Can anybody explain this behaviour.

 

Many thanks.

 

John.

 

 

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