anges
      made to the layout often requires a manual refresh.
      However, all commands that change any aspect of the 3D
      display will also cause a refresh.
   </BLOCKQUOTE>

<H3>Implementation Notes:</H3>
   <BLOCKQUOTE>
      <B>refresh</B> is implemented as a built-in <B>wind3d</B>
	window command in <B>magic</B>.  It can only be invoked from
	a window created with the <B>specialopen wind3d</B> command,
	or the <B>render</B> procedure.  Note that the <B>wind3d</B>
	window is only available when 