[vtkusers] How to get the zoom ratio of vtkCamera?

[Shade]

Dear Everyone,

How can I get the volume rendering zoom ratio?
For example, a volume image is 256*256*256(data),
it can be rendered in the screen with 512*512*512(image),
so the zoom ratio should be 2,
but how can I get the zoom ratio or the screen size of the volume iamge?

Thanks.
Shade