[burchard: HPt3SubPt3()]

[munzner]

In the following function from
/u/gcg/ngrap/src/lib/geometry/hpoint3/hpoint3.c,
it seems like there is a both a sign error and a division by zero in  
the case that  p2->w == 0.  The division by zero will also bomb the  
case when  p1->w == p2->w  and both are zero (which should just  
result in vector subtraction).

void
HPt3SubPt3(register HPoint3 *p1, register HPoint3 *p2, register  
Point3 *v)
{
    if(p1->w == p2->w) {
	v->x = p1->x - p2->x; v->y = p1->y - p2->y; v->z = p1->z -  
p2->z;
    } else if(p1->w == 0) {
	*v = *(Point3 *)p1;
	return;
    } else if(p2->w == 0) {
	*v = *(Point3 *)p2;
    } else {
	float s = p2->w / p1->w;
	v->x = p1->x*s - p2->x; v->y = p1->y*s - p2->y; v->z =  
p1->z*s - p2->z;
    }
    if(p2->w != 1)
	v->x /= p2->w, v->y /= p2->w, v->z /= p2->w;
}