NAME

backface − turns backfacing polygon removal on and off

SPECIFICATION

C
backface(b)
Boolean b;

FORTRAN
subroutine backfa(b)
logical b

Pascal
procedure backface(b: Boolean);

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 still 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 do the opposite of what was intended. 

SEE ALSO

zbuffer Programming Guide, Section 12.2, Backfacing Polygon Removal
 

Version 3.6  —  December 20, 1987