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：
代码语言：javascript复制#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 8
struct Rectangle
{
float xmin, xmax, ymin, ymax;
};
Rectangle 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, Rectangle rect)
{
int code = 0x00;
if (y < rect.ymin)
code = code | 4;//即为1000
if (y > rect.ymax)
code = code | 8;//即为0100
if (x > rect.xmax)
code = code | 2;
if (x < rect.xmin)
code = code | 1;
return code;
}
bool cohensutherlandlineclip(Rectangle 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;
else
codeout = 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：
代码语言：javascript复制y0 = height - y;请解释这行代码的含义和作用。
