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       Perspective - visible portion of an interactor


       #include <InterViews/perspective.h>


       A perspective object describes which part of an interactor's total display area is visible
       in the interactor's canvas.  The main use of perspectives is for scrolling,  panning,  and
       zooming  interactors.  The member variables (x0, y0) define the origin of the interactor's
       display area; width and height define the total height of the area.  The member  variables
       (curx, cury) define the lower-left corner of the interactor's canvas relative to the total
       area; curwidth and curheight define how much of the total area is displayed in the canvas.

       The member variables sx, sy, lx, and  ly  specify  small  and  large  increments  in  each
       dimension.  These increments are used by a scroller to implement line and page scrolling.


              Define a new perspective with all member variables initialized to zero.

       void Attach(Interactor*)
              Add  to  the list of interactors that are interested in updates to the perspective.
              These interactors reflect the perspective visually; examples include scrollers  and

       void Detach(Interactor*)
              Remove  an interactor from the list of those interested in updates.  This operation
              should be called from the interactor's destructor.

       void Init(Coord x0, Coord y0, Coord width, Coord height)
              Initialize the perspective display area.

       void Update()
              Notify all the interactors interested in the perspective that it has changed.   The
              perspective's interactor should call Update after modifying its perspective.


       Adjuster(3I), Interactor(3I), Panner(3I), Scroller(3I)