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<H2><A NAME="SECTION04053000000000000000">
Viewport</A>
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Class <B>viewport</B> performs three-dimensional viewport clipping in
the normalized device coordinates, and maps the result into the device
dependent coordinates.
The viewport is the term representing the visible rectangular area
on a screen.
The physical size (dots in x and y) of a viewport should be given with 
<B>:init</B> message as the <EM>:width</EM> and <EM>:height<