/// <summary>
/// 多角形の面積を取得する
/// </summary>
public float GetArea()
{
float crossSum = 0f; //外積の合計
int size = vertices.Count;
//頂点を巡回
for (int i = 0; i < size; ++i)
{
Vector2 v1 = vertices[i];
Vector2 v2 = vertices[(i + 1) % size];
//外積を計算
float cross = GeomUtil.Cross(v1, v2);
//外積を加算
crossSum += cross;
}
return(Mathf.Abs(crossSum / 2f));
}