Cohen-Sutherland线段裁剪算法

通过一个矩形的裁剪区域将整个屏幕分成9个部分，并为每一个部分赋予相应的区域码，然后根据端点的位置确定这个端点的区域码。

先判断能否完全接受或者完全排除一条线段，若以上2个判断无法直接得出，则逐步裁剪，选取一个位于裁剪区外的端点，把端点的区域码和裁剪边界的区域码进行逻辑与运算，若结果为真，则端点在该裁剪边界外部，这时将端点移向线段和该边界的交点处，如此循环，直到裁剪结束。

代码基本和书上一样，加了点自己的注释

 

 

 

#include <Windows.h> #include <gl/glut.h> ////////////////////////////////////////////////////////////////////////// //区域码 const GLint leftBitCode=0x1; const GLint rightBitCode=0x2; const GLint buttonBitCode=0x4; const GLint topBitCode=0x8; GLint winWidth=640,winHeight=480; class screenPT { public: GLfloat x,y; }; inline GLint inside(GLint code){return GLint(!code);} //判断点是否在裁剪区内 inline GLint reject(GLint code1,GLint code2){return GLint(code1&code2);} //判断能否完全排除一条线段 inline GLint accept(GLint code1,GLint code2){return GLint(!(code1 | code2));} //判断能否完全接受一条线段 inline void swapPT(screenPT& a,screenPT& b){screenPT t=a;a=b;b=t;} //交换两个点 inline void swapCode(GLubyte& a,GLubyte& b){GLubyte t=a;a=b;b=t;} //交换两个区域码 //确定一个点所在位置的区域码 GLubyte encode(const screenPT& p,const screenPT& winMin,const screenPT& winMax) { GLubyte code=0x00; if(p.x<winMin.x) code |= leftBitCode; if(p.x>winMax.x) code |= rightBitCode; if(p.y<winMin.y) code |= buttonBitCode; if(p.y>winMax.y) code |= topBitCode; return code; } //在屏幕上画一条未裁剪的线，由裁剪函数调用 void drawOneLine(const screenPT& a,const screenPT& b) { glBegin(GL_LINES); glVertex2f(a.x,a.y); glVertex2f(b.x,b.y); glEnd(); } //裁剪函数 void lineClip(screenPT winMin,screenPT winMax,screenPT lineBegin,screenPT lineEnd) { GLubyte code1,code2; //保存两个端点的区域码 GLboolean done=false,plotLine=false; //判断裁剪是否结束和是否要绘制直线 GLfloat k; //斜率 while(!done) { code1 = encode(lineBegin,winMin,winMax); code2 = encode(lineEnd,winMin,winMax); if(accept(code1,code2)) //当前直线能完全绘制 { done=true; plotLine=true; } else { if(reject(code1,code2)) //当前直线能完全排除 done = true; else { if(inside(code1)) //若lineBegin端点在裁剪区内则交换两个端点使它在裁剪区外 { swapPT(lineBegin,lineEnd); swapCode(code1,code2); } //计算斜率 if(lineBegin.x != lineEnd.x) k = (lineEnd.y-lineBegin.y)/(lineEnd.x-lineBegin.x); //开始裁剪,以下与运算若结果为真， //则lineBegin在边界外，此时将lineBegin移向直线与该边界的交点 if(code1 & leftBitCode) { lineBegin.y += (winMin.x-lineBegin.x)*k; lineBegin.x = winMin.x; } else if(code1 & rightBitCode) { lineBegin.y += (winMax.x-lineBegin.x)*k; lineBegin.x = winMax.x; } else if(code1 & buttonBitCode) { if(lineBegin.x != lineEnd.x) lineBegin.x += (winMin.y-lineBegin.y)/k; lineBegin.y = winMin.y; } else if(code1 & topBitCode) { if(lineBegin.x != lineEnd.x) lineBegin.x += (winMax.y-lineBegin.y)/k; lineBegin.y = winMax.y; } } } } if(plotLine) drawOneLine(lineBegin,lineEnd); //绘制裁剪好的直线 } ////////////////////////////////////////////////////////////////////////// void rect(screenPT winMin,screenPT winMax) { glBegin(GL_LINE_LOOP); glVertex2f(winMin.x,winMin.y); glVertex2f(winMax.x,winMin.y); glVertex2f(winMax.x,winMax.y); glVertex2f(winMin.x,winMax.y); glEnd(); } void init() { glViewport(0,0,winWidth,winHeight); glClearColor(1.0,1.0,1.0,0.0); glMatrixMode(GL_PROJECTION); glLoadIdentity(); gluOrtho2D(0,winWidth,0,winHeight); glMatrixMode(GL_MODELVIEW); } void display() { screenPT winMin,winMax,lineBegin,lineEnd; winMin.x=100.0; winMin.y=50.0; winMax.x=400.0; winMax.y=300.0; lineBegin.x=0.0; lineBegin.y=0.0; lineEnd.x=winWidth; lineEnd.y=winHeight; glClear(GL_COLOR_BUFFER_BIT); glColor3f(0.0,0.0,0.0); rect(winMin,winMax); //为裁剪区域绘制一个边框 lineClip(winMin,winMax,lineBegin,lineEnd); lineBegin.y=240.0; lineEnd.y=240.0; lineClip(winMin,winMax,lineBegin,lineEnd); lineBegin.x=320.0; lineBegin.y=0.0; lineEnd.x=320.0; lineEnd.y=winHeight; lineClip(winMin,winMax,lineBegin,lineEnd); glFlush(); } int main(int argc,char** argv) { glutInit(&argc,argv); glutInitWindowPosition(100,100); glutInitWindowSize(winWidth,winHeight); glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB); glutCreateWindow("my app"); init(); glutDisplayFunc(display); glutMainLoop(); return 0; }

 

 

参考《计算机图形学》  赫恩