backfa(3G)backfa(3G)NAMEbackfa - 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.
DESCRIPTIONbackfa 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.
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