[vtkusers] How to get the framebuffer

[ling Hai]

Hi,
I am new to vtk and I am trying to get the front framebuffer from the
renderwindow like this:
int * dim = renderWindow->GetSize();
unsigned char* pixels =
renderWindow->GetPixelData(0,0,dim[0]-1,dim[1]-1,true);

but in pixels there is only a null value. I am using this on a 3d render if
that makes a difference the above segment of code called after rendering
the volume. What am I missing? Any help is appreciated thanks.
Linghai
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