im trying to set up lighting for my scene and apparently its not working…below is my calculate normal function to get normal for a square… is it correct?

```
void normal (point p1,point p2,point p3,point p4, point &normal)
{
point a, b,c;
// calculate the vectors A and B
// note that v[3] is defined with counterclockwise winding in mind
// a
a.x = p3.x - p2.x;
a.y = p3.y - p2.y;
a.z = p3.z - p2.z;
// b
b.x = p2.x - p1.x;
b.y = p2.y - p1.y;
b.z = p2.z - p1.z;
// c
c.x = p1.x - p4.x;
c.y = p1.y - p4.y;
c.z = p1.z - p4.z;
// calculate the cross product and place the resulting vector
// into the address specified by vertex_t *normal
normal.x = c.x*((a.y * b.z) - (a.z * b.y));
normal.y = c.y*((a.z * b.x) - (a.x * b.z));
normal.z = c.z*((a.x * b.y) - (a.y * b.x));
// normalize
float len = (float)(sqrt((double)((normal.x * normal.x) + (normal.y * normal.y) + (normal.z * normal.z))));
// avoid division by 0
if (len == 0.0f)
len = 1.0f;
// reduce to unit size
normal.x /= len;
normal.y /= len;
normal.z /= len;
}
```