#include <bits/stdc++.h> using namespace std; const double PI = acos(-1); const double eps = 1e-9; struct Point { double x, y; Point(double x = 0, double y = 0): x(x), y(y) {} }; typedef Point Vector; double dcmp(double x) { if (fabs(x) < eps) return 0; return x < 0 ? -1 : 1; } Vector operator + (const Point& A, const Point& B) { return Vector(A.x + B.x, A.y + B.y); } Vector operator - (const Point& A, const Point& B) { return Vector(A.x - B.x, A.y - B.y); } Vector operator * (const Point& A, double a) { return Vector(A.x * a, A.y * a); } Vector operator / (const Point& A, double a) { return Vector(A.x / a, A.y / a); } double Cross(const Vector& A, const Vector& B) { return A.x * B.y - A.y * B.x; } double Dot(const Vector& A, const Vector& B) { return A.x * B.x + A.y * B.y; } double Length(const Vector& A) { return sqrt(Dot(A, A)); } bool operator < (const Point& A, const Point& B) { return A.x < B.x || (A.x == B.x && A.y < B.y); } bool operator == (const Point& A, const Point& B) { return A.x == B.x && A.y == B.y; } Point GetLineIntersection(Point P, Point v, Point Q, Point w) { Point u = P - Q; double t = Cross(w, u) / Cross(v, w); return P + v * t; } bool SegmentProperIntersection(const Point& a1, const Point& a2, const Point& b1, const Point& b2) { double c1 = Cross(a2 - a1, b1 - a1); double c2 = Cross(a2 - a1, b2 - a1); double c3 = Cross(b2 - b1, a1 - b1); double c4 = Cross(b2 - b1, a2 - b1); return dcmp(c1) * dcmp(c2) < 0 && dcmp(c3) * dcmp(c4) < 0; } bool OnSegment(const Point& p, const Point& a1, const Point& a2) { return dcmp(Cross(a1 - p, a2 - p)) == 0 && dcmp(Dot(a1 - p, a2 - p)) < 0; } const int N = 10005; const int M = 205; Point P[M], res[N], temp[M]; double ang[M], s; int n, cnt; void add(Point a, Point b) { temp[cnt] = a; ang[cnt] = atan2(temp[cnt].y - b.y, temp[cnt].x - b.x) - s; while (dcmp(ang[cnt]) <= 0) ang[cnt] += 2 * PI; cnt++; } int main() { while (scanf("%d", &n) == 1) { int minid = 0; for (int i = 0; i < n; i++) { scanf("%lf%lf", &P[i].x, &P[i].y); if (P[i] < P[minid]) minid = i; } res[0] = P[minid]; int num = 1; s = -PI; while (1) { cnt = 0; for (int i = 0; i < n; i++) { if (res[num - 1] == P[i]) { add(P[(i + 1) % n], res[num - 1]); add(P[(i + n - 1) % n], res[num - 1]); break; } } for (int i = 0; i < n; i++) { if (OnSegment(res[num - 1], P[i], P[(i + 1) % n])) { add(P[(i + 1) % n], res[num - 1]); add(P[i], res[num - 1]); } } int id = 0; for (int i = 0; i < cnt; i++) if (ang[i] < ang[id]) id = i; double minlen = 1e9; Point RP = temp[id], its; for (int i = 0; i < n; i++) { if (SegmentProperIntersection(temp[id], res[num - 1], P[i], P[(i + 1) % n])) { its = GetLineIntersection(temp[id], temp[id] - res[num - 1], P[i], P[i] - P[(i + 1) % n]); if (Length(its - res[num - 1]) < minlen) { minlen = Length(its - res[num - 1]); RP = its; } } } res[num] = RP; s = atan2(res[num - 1].y - res[num].y, res[num - 1].x - res[num].x); num++; if (res[num - 1] == res[0]) break; } printf("%d\n", num - 1); for (int i = 0; i < num - 1; i++) printf("%.4lf %.4lf\n", res[i].x, res[i].y); } return 0; }
转载来自:http://blog.csdn.net/hyczms/article/details/47155427
先选一个起点,即最左下角的点,再选第一条出边,即角度最小点边,开始走,找到所有与该边相交的点但不和该边起点一样交点,找到最近的一个,如果有多个,找到出去方向拐向最右的一个,不断地找,直到回到最初起点为止。。