\startMPpage

vardef MyShape(expr w, h) =
   save pth; path pth ;
   save pic; picture pic;
   pth = fullsquare xysized (w, h);
   pic := image (
       draw pth;
       % all kind of stuff here
   );
   setbounds pic to pth;
   pic
enddef ;

picture s ; s := MyShape(4cm, 2cm);

draw s;

drawdot center leftboundary s withpen pencircle scaled 2 withcolor green;
drawdot center rightboundary s withpen pencircle scaled 2 withcolor blue;
drawdot point .25 along topboundary s withpen pencircle scaled 2 withcolor yellow;
drawdot .25[llcorner s, lrcorner s] withpen pencircle scaled 2 withcolor magenta;
drawdot origin withpen pencircle scaled 2 withcolor red;

\stopMPpage