backface(3G)							  backface(3G)

NAME    [Toc]    [Back]

     backface -	turns backfacing polygon removal on and	off

C SPECIFICATION    [Toc]    [Back]

     void backface(b)
     Boolean b;

PARAMETERS    [Toc]    [Back]

     b	 expects either	TRUE or	FALSE.

	 TRUE suppresses the display of	backfacing filled polygons.

	 FALSE allows the display of backfacing	filled polygons.

DESCRIPTION    [Toc]    [Back]

     backface allows or	suppresses the display of backfacing filled polygons.
     If	your programs represent	solid objects as collections of	polygons, you
     can use this routine to remove hidden surfaces.  This routine works best
     for simple	convex objects that do not obscure other objects.

     A backfacing polygon is defined as	a polygon whose	vertices are in
     clockwise order in	screen coordinates.  When backfacing polygon removal
     is	on, the	system displays	only polygons whose vertices are in counterclockwise
 order.  For complicated objects,	this routine alone may not
     remove all	hidden surfaces. To remove hidden surfaces for more
     complicated objects or groups of objects, your routine needs to check the
     relative distances	of the object from the viewer (z values).  (See
     ``Hidden Surface Removal''	in the Graphics	Library	Programming Guide.)

SEE ALSO    [Toc]    [Back]

     zbuffer

NOTES    [Toc]    [Back]

     Matrices that negate coordinates, such as scale(-1.0, 1.0,	1.0), reverse
     the directional order of a	polygon's points and can cause backface	to do
     the opposite of what is intended.

     On	IRIS-4D	B and G	models backface	does not work well when	a polygon
     shrinks to	the point where	its vertices are coincident.  Under these
     conditions, the routine cannot determine the orientation of the polygon
     and so displays the polygon by default.


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