多边形面积计算

二维多边形面积计算，代码参考如下：

public static float GetPolyArea2D(Vector2[] points)
{
	if (points == null) return 0;
	int  len = points.Length;
	if (len < 3) return 0;
	float areaTwice = 0;
	for (int i = 0; i < len; i++)
	{
		areaTwice = areaTwice + (points[i].x * points[(i + 1) % len].y - points[(i + 1) % len].x * points[i].y);
	}
	return Mathf.Abs(areaTwice * 0.5f);
}