frontf(3G)

NAME
     frontf - turns frontfacing polygon removal on and off

FORTRAN 77 SPECIFICATION
     subroutine frontf(b)
     logical b

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

          .TRUE. suppresses the display of frontfacing filled polygons.

          .FALSE. allows the display of frontfacing filled polygons.

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

     A frontfacing polygon is defined as a polygon whose vertices are in
     counter-clockwise order in screen coordinates.  When frontfacing polygon
     removal is on, the system displays only polygons whose vertices are in
     clockwise order.  For complicated objects, this routine alone may not
     expose all hidden surfaces. To expose 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.)

     If frontf and backfa are asserted simultaneously, no filled polygons will
     be displayed.

SEE ALSO
     backfa, zbuffe

NOTE
     On IRIS-4D G and B models frontf 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.

     On all IRIS-4D models 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 frontf to do the opposite of what is intended.