backface(3G) backface(3G)
NAME
backface - turns backfacing polygon removal on and off
SPECIFICATION
C backface(b)
Boolean b;
FORTRAN subroutine backfa(b)
logical b
DESCRIPTION
backface initiates or terminates backfacing filled polygon
removal. A backfacing polygon is a polygon whose vertices
appear in clockwise order in screen coordinates. When
backfacing polygon removal is on, the system displays only
polygons whose vertices appear in counterclockwise order.
The backface utility improves the performance of programs
that represent solid objects as collections of polygons.
Backfacing polygon removal does not always remove all hidden
surfaces because some frontfacing polygons can also be
obscured. When a polygon shrinks to the point where its
vertices are coincident, its orientation is indeterminate
and it is displayed. Backface removal is useful for simple
convex objects. For more general images, you can achieve
hidden surface removal using another technique, perhaps in
conjunction with backface removal.
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 to the opposite of what was intended.
SEE ALSO
zbuffer
Programming Guide, Section 12.2, Backfacing Polygon Removal
Page 1 (printed 8/20/87)