polygon_offset
Allows displacement of polygon fragments, relative to the maximum absolute slope in z (window coords.):
Allows lines (or points) and polygons in the same plane to be rendered without interaction.
Active only in polygon fill mode.
void
glPolygonOffsetEXT
(GLfloat factor, GLfloat bias);
Applications:
Rendering hidden-line images
Rendering solids with highlighted edges
Applying `decals' to surfaces