[vtkusers] Isosurfaces disappears from renderers

[Tatti Emanuele]

Hi all,
i use latest vtk from cvs, qt 4.1.1 an vtkqt from matthias koenig; when 
i try to render the output of a vtkcontourfilter, it works without 
problems, but after few resizes and/or refreshes, the isosurface 
disappear from the renderer. There is no error on output and there is 
enough free memory; i tried to debug the mapper, the actor, the renderer 
and the renderwindow, but no errors.

Here is part of the code...

    filter = vtkContourFilter::New();
    filter->SetInput(input);
    filter->SetNumberOfContours(1);
    filter->SetValue(0,value);
    filter->SetArrayComponent(comp-1);
    filter->Update();
    vtkActor* surfActor = vtkActor::New();
    vtkPolyDataMapper *surfMapper = vtkPolyDataMapper::New();
    surfMapper->SetInput(filter->GetOutput());
    surfMapper->ScalarVisibilityOff();
    surfActor->SetMapper(surfMapper);
    renderer->AddActor(a);
    renderer->Render();
    renWin->update();



Ps. i tried only on ati hardware
how can i investigate the problem?