g coordinates
and the projection, (3) a viewport for clipping in NDC and
the transformation from NDC to physical screen coordinates,
and (4) a viewsurface which performs drawing functions on
a physical display device.
A <B>viewer</B> object holds a viewing, a viewport and a viewsurface object,
and controls successive coordinates transformation. 
Functions <B>draw</B> and <B>hid</B> described in section <A HREF="#Drawings">17.5</A>
use the instances of viewer.

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