[vtkusers] how to display image without zooming

[diaoxianfen]

Hello,every one
I tried to display 3D image data slice by slice along z axial.The program is as follows.

 metaImageReader->SetFileName("E:/TempImage/0.1resolution/ConThreshSegDdicomSe3Edge.mhd");//e:/tempimage/sampledcochleaConThreshSeg.mhd
  metaImageReader->Update();
  int xyzdim[3];
  metaImageReader->GetOutput()->GetDimensions(xyzdim);
  int xd=xyzdim[0]-1;
  int yd=xyzdim[1]-1;
  int zd=xyzdim[2]-1;
imageActor->SetInput(metaImageReader->GetOutput());
  imageActor->VisibilityOn();
  imageActor->SetDisplayExtent(0,xd,0,yd,120,120);  
  imageActor->InterpolateOff();
  Renderer->AddActor(imageActor);
  Renderer->SetBackground(0.0,0.1,0.2);
  Interactor->SetRenderWindow(RenderWindow);

I really get what I want.But the image is zoomed.I want to remove the zooming function.But I don't know how to do.That is,how many pixel in the image slice,there are the same number pixel point on the screen.
Moreover,I want to display the image from the (0,0)point on the screen.How can I get that?
Any information will be appreciated.
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