实验3 直线裁剪算法

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实验3 直线裁剪算法

1．实验目的：

• 理解直线裁剪的基本原理；
• 验证直线的编码裁剪算法，参考网络资料实现梁友栋-Barsky裁剪算法；
• 了解与掌握OpenGL鼠标操作。

2．实验内容：

本次实验主要结合鼠标画线程序来验证编码裁剪算法和实现梁友栋-Barsky裁剪算法，具体步骤如下：

（1） 阅读学习所给的编码裁剪示范代码，了解程序使用方法，并结合三种不同类型直线对其进行裁剪测试，将测试结果存为图1-3，保存至word实验文档中（30分钟）；

（2） 为示范代码增加梁友栋-Barsky裁剪算法，并通过键盘按键“L”来控制，即按键盘“L”键时，用梁友栋-Barsky算法裁剪程序窗口内的直线。同样测试三种不同类型的直线，将测试结果存为图4-6，，保存至word实验文档中（60分钟）；

（3） 整理图1-6，并增加程序代码合并到一个word文档，将其命名为“序号-姓名-Prj2.doc”，电子版提交至雨课堂，A4打印稿下一次课前或实验课前提交。

3．实验原理：

示范代码中编码裁剪算法原理参见教材裁剪内容，有关鼠标操作知识请参考OpenGL编程 基础篇（四）与鼠标的交互。

4．实验代码：

#include <GL/glut.h>
#include <stdio.h>
#include <stdlib.h>#define LEFT_EDGE   1
#define RIGHT_EDGE  2
#define BOTTOM_EDGE 4
#define TOP_EDGE    8struct MyRect
{float xmin, xmax, ymin, ymax;
};MyRect  rect;
int x0, y0, x1, y1;
bool bDrawLine = true;
int width = 640, height = 480;void LineGL(int x0, int  y0, int x1, int y1)
{glBegin(GL_LINES);glColor3f(1.0f, 0.0f, 0.0f);   glVertex2f(x0, y0);glColor3f(1.0f, 0.0f, 0.0f);   glVertex2f(x1, y1);glEnd();
}//求p点的编码
int CompCode(int x, int y, MyRect rect)
{int code = 0x00;if (y < rect.ymin)code = code | 4;//即为1000if (y > rect.ymax)code = code | 8;//即为0100if (x > rect.xmax)code = code | 2;if (x < rect.xmin)code = code | 1;return code;
}bool cohensutherlandlineclip(MyRect  rect, int &x0, int & y0, int &x1, int &y1)
{bool accept, done;float x, y;accept = false;done = false;int code0, code1, codeout;code0 = CompCode(x0, y0, rect);code1 = CompCode(x1, y1, rect);do {if (!(code0 | code1)){accept = true;done = true;}else if (code0 & code1)done = true;else{if (code0 != 0)codeout = code0;elsecodeout = code1;if (codeout&LEFT_EDGE) {y = y0 + (y1 - y0)*(rect.xmin - x0) / (x1 - x0);x = (float)rect.xmin;}else if (codeout&RIGHT_EDGE) {y = y0 + (y1 - y0)*(rect.xmax - x0) / (x1 - x0);x = (float)rect.xmax;}else if (codeout&BOTTOM_EDGE) {x = x0 + (x1 - x0)*(rect.ymin - y0) / (y1 - y0);y = (float)rect.ymin;}else if (codeout&TOP_EDGE) {x = x0 + (x1 - x0)*(rect.ymax - y0) / (y1 - y0);y = (float)rect.ymax;}if (codeout == code0){x0 = x; y0 = y;code0 = CompCode(x0, y0, rect);}else{x1 = x; y1 = y;code1 = CompCode(x1, y1, rect);}}} while (!done);return accept;
}void myDisplay()
{glClear(GL_COLOR_BUFFER_BIT);glColor3f(1.0f, 1.0f, 0.0f);glRectf(rect.xmin, rect.ymin, rect.xmax, rect.ymax);if (bDrawLine)LineGL(x0, y0, x1, y1);glFlush();
}void Init()
{glClearColor(0.0, 0.0, 0.0, 0.0);glShadeModel(GL_FLAT);rect.xmin = 100; //窗口的大小rect.xmax = 300;rect.ymin = 100;rect.ymax = 300;x0 = 300, y0 = 50, x1 = 0, y1 = 450;//裁剪前的直线端点printf("Press key 'c' to Clip!\n");printf("Please Click left button of mouse to input the points of Line!!\n");
}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);
}void keyboard(unsigned char key, int x, int y)
{switch (key){case 'c':bDrawLine = cohensutherlandlineclip(rect, x0, y0, x1, y1);glutPostRedisplay();//重画break;case 'x':exit(0);break;default:break;}
}void mouse(int button, int state, int x, int y)
{switch (button){case GLUT_LEFT_BUTTON:if (state == GLUT_DOWN){if (bDrawLine){x0 = x;y0 = height - y;bDrawLine = false;glutPostRedisplay();//}else{x1 = x;y1 = height - y;bDrawLine = true;glutPostRedisplay();//}}break;default:break;}
}int main(int argc, char *argv[])
{glutInit(&argc, argv);glutInitDisplayMode(GLUT_RGB | GLUT_SINGLE);glutInitWindowPosition(100, 100);glutInitWindowSize(width, height);glutCreateWindow("Hello World!");Init();glutDisplayFunc(myDisplay);glutReshapeFunc(Reshape);glutKeyboardFunc(keyboard);glutMouseFunc(mouse);glutMainLoop();return 0;
}

5．实验思考

实验示范代码的第162行：

y0 = height - y;

请解释这行代码的含义和作用。