frontf(3G)frontf(3G)NAMEfrontf - 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.
DESCRIPTIONfrontf 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.
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