C# 判断点是否在多边形内

/// 

    /// 判断点是否在多边形内.
    /// ----------原理----------
    /// 注意到如果从P作水平向左的射线的话，如果P在多边形内部，那么这条射线与多边形的交点必为奇数，
    /// 如果P在多边形外部，则交点个数必为偶数(0也在内)。
    /// 
    public bool IsInPolygon(Vector3 checkPoint, List<Vector2> polygonPoints)
    {
        bool inside = false;
        
        int pointCount = polygonPoints.Count;
        Vector2 p1, p2;
        for (int i = 0, j = pointCount - 1; i < pointCount; j = i, i++)
        {
            p1 = polygonPoints[i];
            p2 = polygonPoints[j];
            if (checkPoint.y < p2.y)
            {
                if (p1.y <= checkPoint.y)
                {
                    if ((checkPoint.y - p1.y) * (p2.x - p1.x) > (checkPoint.x - p1.x) * (p2.y - p1.y))
                    {
                        inside = !inside;
                    }
                }
            }
            else if (checkPoint.y < p1.y)
            {
                if ((checkPoint.y - p1.y) * (p2.x - p1.x) < (checkPoint.x - p1.x) * (p2.y - p1.y))
                {
                    inside = !inside;
                }
            }
        }

        return inside;
    }

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