[vtkusers] How to draw the path of the movement? Please help!!

[german]

Dear all VTK user:
I want to update the position of the object by the encoder signal and show the path of the object move!  I try to renWin->EraseOff but it doesn't work!  But I find that function was work in ..\cowwalk.tcl.  I try to use the user interface to change the cow position, and the image of the cows will keep on the screen.  Why...?
My code was something like that:--------

for ( i = 1; i < 2000; i++)
 { 
 ren1->AddActor(joinActor);
 ren1->AddActor(cutterActor);
 ren1->SetBackground(0.1,0.2,0.4);
 renWin->SetSize(500,500);
 renWin->Render();

 //print out the input
 cout << "please enter x, y, z" <<"\n";
 cin >>x>>y>>z;
 a=x+a;
 b=y+b;
 c=z+c;

 cutterActor->SetPosition(a, b ,c);
 ren1->AddActor(cutterActor);
 renWin->Render();
 renWin->Render();
 renWin->EraseOff;

 
 }
-------------------------
the for loop just simulate the encoder signal input!

Please help!  
Thanks for any advise!

Best regards,
German Leung 
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