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这是一个关于多边形的裁剪算法，剪裁的是多边形，关于直线的剪裁，博主在https://blog.csdn.net/sinat_33916407/article/details/88918553 里面给出了梁友栋-Barskey裁剪算法的实现，在这里，博主给出的是多边形剪裁方法中的Sutherland-Hodgman方法，亲测可用，按下c键进行剪裁，按下r键会复原原图，按下x键会退出。
开发环境：win10 + opengl + visual studio2017
关于visual studio2017配置opengl是项目---->管理NuGet程序包--->输入nupengl，可以看见两个相应的包，下载即可。
附上代码：
| //本项目是计算机图形学中的Sutherland-Hodgman算法 //本算法的思想是从边框xl处开始，对所有线进行判断，更新新的顶点表，循环直到最后一个边界线判断结束，得到最后的顶点表
#include "pch.h" #include <iostream> #include<gl/glut.h> #include<vector> using namespace std;
//此处定义出了边框的边界 #define XL 100 #define XR 400 #define YL 100 #define YR 400
//定义点结构 struct Points { float x; float y; };
//定义边框 struct Border { float xl; float xr; float yl; float yr; };
//多边形的几个顶点 vector<Points> p = { {200,450},{450,300},{300,50},{50,200} }; Border rectangles = { XL,XR,YL,YR };
//画直线 void LineGL(Points &p1, Points &p2) { glBegin(GL_LINES); glColor3f(0.0f, 1.0f, 0.0f); glVertex2f(p1.x, p1.y); glColor3f(0.0f, 1.0f, 0.0f); glVertex2f(p2.x, p2.y); glEnd(); } //计算线与边界x的交点坐标 Points interactionx(float &borde,Points &x,Points &y) { float k, b; Points result; k = (y.y - x.y) / (y.x - x.x); b = (y.x*x.y - x.x*y.y) / (y.x - x.x); float yy = k * borde + b; result.x = borde; result.y = yy;
return result; } //计算线与边界y的交点坐标 Points interactiony(float &borde, Points &x, Points &y) { float k, b; Points result; k = (y.y - x.y) / (y.x - x.x); b = (y.x*x.y - x.x*y.y) / (y.x - x.x); float xx = (borde - b) / k; result.y = borde; result.x = xx;
return result; }
//核心算法 每条边按照一定的顺序进行相应的判断，然后更新这个新的含points的表，将最后的表再进行画出来 vector<Points> SutherlandHodgman(Border &border, vector<Points> &pp) {//输入边界和要剪切的图形 vector<Points> r1, r; //判断 从最左边的XL开始 for (int i = 0; i < pp.size(); i++) { //判断 由于一条线段是由两个点组成，所以相邻的两个点自发形成一条线段，最后的一个点与第一个点形成线段 if (i == pp.size() - 1) {//最后一个点与第一个点形成一条线段，单独先处理出来 int x1 = pp[i].x - border.xl; int x2 = pp[0].x - border.xl; if (x1 < 0 && x2 < 0) {
} else if (x1 < 0 && x2 >= 0) { Points poin; if (x2 == 0) { r1.push_back(pp[0]); } else { Points px = interactionx(border.xl, pp[i], pp[0]); r1.push_back(px); r1.push_back(pp[0]); } } else if (x1 >= 0 && x2 < 0) { if (x1 == 0) { r1.push_back(pp[i]); } else { Points px = interactionx(border.xl, pp[i], pp[0]); r1.push_back(px); } } else if (x1 >= 0 && x2 >= 0) { r1.push_back(pp[0]); } } else {//其余的线段正常处理 int x1 = pp[i].x - border.xl; int x2 = pp[i+1].x - border.xl; //处理四种情况 //第一种情况：两个点都在边界之外 if (x1 < 0 && x2 < 0) {
} //第二种情况：第一个点在边界外，第二个点在边界内或边界上 else if (x1 < 0 && x2 >= 0) { if (x2 == 0) { r1.push_back(pp[i + 1]); } else { Points px = interactionx(border.xl, pp[i], pp[i + 1]); r1.push_back(px); r1.push_back(pp[i + 1]); } } //第三种情况：第一个点在边界上或边界内，第二个点在边界外 else if (x1 >= 0 && x2 < 0) { if (x1 == 0) { r1.push_back(pp[i]); } else { Points px = interactionx(border.xl, pp[i], pp[i + 1]); r1.push_back(px); } } //第四种情况：两个点都在边界内或者边界上 else if(x1>=0 && x2>=0) { r1.push_back(pp[i + 1]); } } } r.assign(r1.begin(), r1.end()); r1.clear();
//最上边yr for (int i = 0; i < r.size(); i++) { if (i == r.size() - 1) { int x1 = border.yr - r[i].y; int x2 = border.yr - r[0].y; if (x1 < 0 && x2 < 0) {
} else if (x1 < 0 && x2 >= 0) { if (x2 == 0) { r1.push_back(r[0]); } else { Points px = interactiony(border.yr, r[i], r[0]); r1.push_back(px); r1.push_back(r[0]); } } else if (x1 >= 0 && x2 < 0) { if (x1 == 0) { r1.push_back(r[i]); } else { Points px = interactiony(border.yr, r[i], r[0]); r1.push_back(px); } } else if (x1 >= 0 && x2 >= 0) { r1.push_back(r[0]); } } else { int x1 = border.yr - r[i].y; int x2 = border.yr - r[i+1].y; if (x1 < 0 && x2 < 0) {
} else if (x1 < 0 && x2 >= 0) { Points poin; if (x2 == 0) { r1.push_back(r[i + 1]); } else { Points px = interactiony(border.yr, r[i], r[i + 1]); r1.push_back(px); r1.push_back(r[i + 1]); } } else if (x1 >= 0 && x2 < 0) { if (x1 == 0) { r1.push_back(r[i]); } else { Points px = interactiony(border.yr, r[i], r[i + 1]); r1.push_back(px); } } else if (x1 >= 0 && x2 >= 0) { r1.push_back(r[i + 1]); } } } r.clear(); r.assign(r1.begin(), r1.end()); r1.clear();
//最右边xr for (int i = 0; i < r.size(); i++) { if (i == r.size() - 1) { int x1 = border.xr - r[i].x; int x2 = border.xr - r[0].x; if (x1 < 0 && x2 < 0) {
} else if (x1 < 0 && x2 >= 0) { Points poin; if (x2 == 0) { r1.push_back(r[0]); } else { Points px = interactionx(border.xr, r[i], r[0]); r1.push_back(px); r1.push_back(r[0]); } } else if (x1 >= 0 && x2 < 0) { if (x1 == 0) { r1.push_back(r[i]); } else { Points px = interactionx(border.xr, r[i], r[0]); r1.push_back(px); } } else if (x1 >= 0 && x2 >= 0) { r1.push_back(r[0]); } } else { int x1 = border.xr - r[i].x; int x2 = border.xr - r[i + 1].x; if (x1 < 0 && x2 < 0) {
} else if (x1 < 0 && x2 >= 0) { if (x2 == 0) { r1.push_back(r[i + 1]); } else { Points px = interactionx(border.xr, r[i], r[i + 1]); r1.push_back(px); r1.push_back(r[i + 1]); } } else if (x1 >= 0 && x2 < 0) { if (x1 == 0) { r1.push_back(r[i]); } else { Points px = interactionx(border.xr, r[i], r[i + 1]); r1.push_back(px); } } else if (x1 >= 0 && x2 >= 0) { r1.push_back(r[i + 1]); } } } r.clear(); r.assign(r1.begin(), r1.end()); r1.clear();
//最下边yl for (int i = 0; i < r.size(); i++) { if (i == r.size() - 1) { int x1 = r[i].y - border.yl; int x2 = r[0].y - border.yl; if (x1 < 0 && x2 < 0) {
} else if (x1 < 0 && x2 >= 0) { Points poin; if (x2 == 0) { r1.push_back(r[0]); } else { Points px = interactiony(border.yl, r[i], r[0]); r1.push_back(px); r1.push_back(r[0]); } } else if (x1 >= 0 && x2 < 0) { if (x1 == 0) { r1.push_back(r[i]); } else { Points px = interactiony(border.yl, r[i], r[0]); r1.push_back(px); } } else if (x1 >= 0 && x2 >= 0) { r1.push_back(r[0]); } } else { int x1 = r[i].y - border.yl; int x2 = r[i+1].y - border.yl; if (x1 < 0 && x2 < 0) {
} else if (x1 < 0 && x2 >= 0) { Points poin; if (x2 == 0) { r1.push_back(r[i + 1]); } else { Points px = interactiony(border.yl, r[i], r[i + 1]); r1.push_back(px); r1.push_back(r[i + 1]); } } else if (x1 >= 0 && x2 < 0) { if (x1 == 0) { r1.push_back(r[i]); } else { Points px = interactiony(border.yl, r[i], r[i + 1]); r1.push_back(px); } } else if (x1 >= 0 && x2 >= 0) { r1.push_back(r[i + 1]); } } } r.clear(); r.assign(r1.begin(), r1.end()); return r; }
//初始化整个界面 void Init() { glClearColor(0.0, 0.0, 0.0, 0.0); glShadeModel(GL_FLAT); //按下c键会进行剪裁，按下r键会复原，按下x键会退出窗口 printf("Press key 'c' to Clip!\nPress key 'r' to Recover\nPress key 'x' to Exit\n"); }
//窗口大小的改变函数 void Reshape(int w, int h) { glViewport(0, 0, (GLsizei)w, (GLsizei)h); glMatrixMode(GL_PROJECTION); glLoadIdentity(); gluOrtho2D(0.0, (GLdouble)w, 0.0, (GLdouble)h); }
//未剪裁前的显示 void display() { glClear(GL_COLOR_BUFFER_BIT); glColor3f(1.0f, 0.0f, 0.0f); glPointSize(2); //画矩形 使用的是红色 glBegin(GL_LINE_LOOP); glVertex2f(XL, YL); glVertex2f(XR, YL); glVertex2f(XR, YR); glVertex2f(XL, YR); glEnd();
//画出最先开始的直线，没有经过任何修改 for (int i = 0; i < p.size(); i++) { if (i == p.size() - 1) { LineGL(p[p.size() - 1], p[0]); } else { LineGL(p[i], p[i + 1]); } } glFlush(); }
//键盘监听事件 void keyboard(unsigned char key, int x, int y) { switch (key) { case 'c': //按下c键以后，会进行裁剪工作 p = SutherlandHodgman(rectangles, p);//调用函数得到最新的点 glutPostRedisplay();//当前窗口重新绘制 break; case 'r': //按下r键以后，原来的直线恢复，出现原本的线 p = { {200,450},{450,300},{300,50},{50,200} }; Init(); glutPostRedisplay(); break; case 'x'://退出 exit(0); break; default: break; } }
int main(int argc, char* argv[]) { glutInit(&argc, argv);//从main函数传递参数 //设置显示模式，单缓冲，RGB glutInitDisplayMode(GLUT_RGB | GLUT_SINGLE); glutInitWindowPosition(400, 200);//窗口位置 glutInitWindowSize(500, 500);//窗口大小 glutCreateWindow("Sutherland_Hodgman");//窗口标题
Init();//初始化
glutDisplayFunc(&display);//渲染函数 glutReshapeFunc(Reshape); //窗口函数发生改变时调用函数 glutKeyboardFunc(&keyboard);//调用键盘监听事件 glutMainLoop();//循环消息机制
return 0; }
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关于算法的原理，一部分在博主给出的代码可看懂，另一部分给出一个博主当时看的博客https://blog.csdn.net/damotiansheng/article/details/43274183 其余的也可以自己寻找。如果有什么问题可以给博主留言，博主会尽快回复！
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