frontface(3G) frontface(3G)
NAME
frontface - turns frontfacing polygon removal on and off
C SPECIFICATION
void frontface(b)
Boolean 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
frontface 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 frontface and backface are asserted simultaneously, no filled polygons
will be displayed.
SEE ALSO
backface, zbuffer
NOTE
On IRIS-4D G and B models frontface 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 frontface to do the opposite of what is intended.
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