判断点在线的左边还是右边还是在线上
这个算法遇到了，虽然有点思路，但是之前没写过
我考虑应该是一条直线把平面划分成两块，将询问点的坐标带入直线，大于0和小于0则说明询问点在直线的两侧
然而后来我还是选择了直接百度。。
以下内容来自https://www.cnblogs.com/carekee/articles/2299546.html,复制到这里备忘，这位兄总结的非常精炼
判断点在线的左边还是右边在构建三角网时是非常重要的
以及两点p1(x1,y1),p2(x2,y2),判断点p(x,y)在线的左边还是右边。
1.
bool LeftOfLine(const ZCoord2D& p, const ZCoord2& p1, const ZCoord2D& p2)
{
double tmpx = (p1.x – p2.x) / (p1.y – p2.y) * (p.y – p2.y) + p2.x;
if (tmpx > p.x)//当tmpx>p.x的时候，说明点在线的左边，小于在右边，等于则在线上。
return true;
return false;
}
2.
另外一种方法：
Tmp = (y1 – y2) * x + (x2 – x1) * y + x1 * y2 – x2 * y1
Tmp > 0 在左侧
Tmp = 0 在线上
Tmp < 0 在右侧
3.
设线段端点为从 A(x1, y1)到 B(x2, y2), 线外一点 P(x0，y0)，
判断该点位于有向线 A→B 的那一侧。
a = ( x2-x1, y2-y1)
b = (x0-x1, y0-y1)
a x b = | a | | b | sinφ (φ为两向量的夹角)
| a | | b | ≠ 0 时， a x b 决定点 P的位置
所以 a x b 的 z 方向大小决定 P位置
(x2-x1)(y0-y1) – (y2-y1)(x0-x1) > 0 左侧
(x2-x1)(y0-y1) – (y2-y1)(x0-x1) < 0 右侧
(x2-x1)(y0-y1) – (y2-y1)(x0-x1) = 0 线段上
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