code:

void gldisplay2(struct poly * p)
{
   struct poly *q;
   int i,j;
   
   glEnable(GL_STENCIL_TEST);
   glStencilFunc(GL_ALWAYS, 0x1, 0x1);
   glStencilOp(GL_INVERT, GL_INVERT, GL_INVERT);
   glClear(GL_STENCIL_BUFFER_BIT);
   glClearStencil(0x0);
   glColorMask(GL_FALSE, GL_FALSE, GL_FALSE, GL_FALSE);
   struct vector ori;
   struct vector pt1;
   struct vector pt2;
   q = p->f[0].p;
   ori.x = q->origin.x + q->f[0].t * q->d.x;
   ori.y = q->origin.y + q->f[0].t * q->d.y;
   ori.z = q->origin.z + q->f[0].t * q->d.z;
   // draw the triangles using GL_INVERT to get a stencil
   // of the face
   for(i = 1; i < p->curnum; i++) {
	q = p->f[i].p;
        pt1.x = q->origin.x + q->f[0].t * q->d.x;
        pt1.y = q->origin.y + q->f[0].t * q->d.y;
        pt1.z = q->origin.z + q->f[0].t * q->d.z;
	pt2.x = q->origin.x + q->f[1].t * q->d.x;
        pt2.y = q->origin.y + q->f[1].t * q->d.y;
        pt2.z = q->origin.z + q->f[1].t * q->d.z;
	if(!(PTEQU(ori, pt1)) && !(PTEQU(ori, pt2))) {
		//glNormal3f(p->n.x, p->n.y, p->n.z);
		glBegin(GL_POLYGON);
		glVertex3f(ori.x, ori.y, ori.z);
		glVertex3f(pt1.x, pt1.y, pt1.z);
		glVertex3f(pt2.x, pt2.y, pt2.z);
		glEnd();
	}
   }
   // redraw the triangles via color buffer
   glColorMask(GL_TRUE, GL_TRUE, GL_TRUE, GL_TRUE);
   glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
   glStencilFunc(GL_EQUAL, 0x1, 0x1);
   glStencilOp(GL_KEEP, GL_KEEP, GL_KEEP);
   for(i = 1; i < p->curnum; i++) {
	q = p->f[i].p;
        pt1.x = q->origin.x + q->f[0].t * q->d.x;
        pt1.y = q->origin.y + q->f[0].t * q->d.y;
        pt1.z = q->origin.z + q->f[0].t * q->d.z;
	pt2.x = q->origin.x + q->f[1].t * q->d.x;
        pt2.y = q->origin.y + q->f[1].t * q->d.y;
        pt2.z = q->origin.z + q->f[1].t * q->d.z;
	if(!(PTEQU(ori, pt1)) && !(PTEQU(ori, pt2))) {
		//glNormal3f(p->n.x, p->n.y, p->n.z);
		glBegin(GL_POLYGON);
		glVertex3f(ori.x, ori.y, ori.z);
		glVertex3f(pt1.x, pt1.y, pt1.z);
		glVertex3f(pt2.x, pt2.y, pt2.z);
		glEnd();
	}
   }
   glDisable(GL_STENCIL_TEST);
}