public Vector3f calculateNormal(Vector3f normal) {
        if (normal == null) {
            normal = new Vector3f();
        }

        Vector3f v1 = c.subtract(b);
        Vector3f v2 = a.subtract(b);
        normal.set(v1.crossLocal(v2).normalizeLocal());

        return normal;
    }
Returns a normal vector, calculated by the formula
<pre>
     (C - B) x (B - A)
N = -------------------
    |(C - B) x (B - A)|
</pre>

@param normal storage for the normal, or null for a new Vector3f
@return the normal direction (either {@code normal} or a new Vector3f)
jme3-core/src/main/java/com/jme3/math/Rectangle.java