backfa(3G)

NAME
     backfa - turns backfacing polygon removal on and off

FORTRAN 77 SPECIFICATION
     subroutine backfa(b)
     logical b

PARAMETERS
     b   expects either .TRUE. or .FALSE..

          .TRUE. suppresses the display of backfacing filled polygons.

          .FALSE. allows the display of backfacing filled polygons.

DESCRIPTION
     backfa 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 counter-
     clockwise 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
     zbuffe

NOTES
     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 backfa to do
     the opposite of what is intended.

     On IRIS-4D B and G models backfa 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.