一、实验目的和要求
熟悉光栅图形学中的相关直线段裁剪算法、理解Cohen-Sutherland裁剪算法。
二、实验内容
实现Cohen-Sutherland编码线段裁剪算法，能看到裁剪前后的屏幕显示效果。
（裁剪效果显示窗口，放大或最大化即可显示）
三、实验过程及代码
#include<GL/glut.h> #include<stdio.h> #include<math.h> class Point{public:float x, y;};bool hasLine = true, cut = false;float offset = 10;int winWidth = 400, winHeight = 300;Point clippingWindow[4], line[2], bound[4][2], inside[2];// 用于记录线段line位于裁剪窗口内的两个端点 // 初始化裁剪窗口的四个顶点坐标 void initClippingWindow(void){// 左上角 clippingWindow[0].x = -100;clippingWindow[0].y = 100;// 左下角 clippingWindow[1].x = -100;clippingWindow[1].y = -100;// 右下角 clippingWindow[2].x = 100;clippingWindow[2].y = -100;// 右上角 clippingWindow[3].x = 100;clippingWindow[3].y = 100;}// 绘制裁剪窗口 void drawClippingWindow(void){glColor3f(0, 0, 0);glBegin(GL_LINE_LOOP);for (int i = 0; i < 4; i++)glVertex2f(clippingWindow[i].x, clippingWindow[i].y);glEnd();}// 更新边界 void updateBound(void){// 由裁剪窗口的左上角顶点和左下角定点组成的线，即左边界 bound[0][0] = clippingWindow[0];bound[0][1] = clippingWindow[1];// 右边界 bound[1][0] = clippingWindow[2];bound[1][1] = clippingWindow[3];// 下边界 bound[2][0] = clippingWindow[1];bound[2][1] = clippingWindow[2];// 上边界 bound[3][0] = clippingWindow[0];bound[3][1] = clippingWindow[3];}// 初始化line void initLine(void){line[0].x = 0;line[0].y = 0;line[1].x = 0;line[1].y = 0;}// 用指定的颜色画线段 void drawLine(Point p1, Point p2, float red, float green, float blue){glLineWidth(5);glColor3f(red, green, blue);glBegin(GL_LINES);glVertex2f(p1.x, p1.y);glVertex2f(p2.x, p2.y);glEnd();}// 生成端点的区域码 int encode(Point point, Point clippingWindow[4]){int code = 0x0;if (point.x < clippingWindow[1].x)code = code | 0x1;if (point.x > clippingWindow[3].x)code = code | 0x2;if (point.y < clippingWindow[1].y)code = code | 0x4;if (point.y > clippingWindow[3].y)code = code | 0x8;return code;}// 求line1、line2的交点 Point getIntersection(Point line1[2], Point line2[2]){float dx1 = line1[1].x - line1[0].x, dy1 = line1[1].y - line1[0].y;float dx2 = line2[1].x - line2[0].x, dy2 = line2[1].y - line2[0].y;Point intersection;if (dx1 != 0 && dx2 != 0) // 如果两条直线都有斜率 {// 求直线的参数：y = ax + b float a1 = dy1 / dx1, b1 = line1[0].y - a1 * line1[0].x;float a2 = dy2 / dx2, b2 = line2[0].y - a2 * line2[0].x;intersection.x = (b2 - b1) / (a1 - a2);intersection.y = a1 * intersection.x + b1;}else if (dx1 == 0 && dx2 != 0) // 如果line1垂直于x轴 {float a2 = dy2 / dx2, b2 = line2[0].y - a2 * line2[0].x;intersection.x = line1[0].x;intersection.y = a2 * intersection.x + b2;}else if (dx1 != 0 && dx2 == 0) // 如果line2垂直于x轴 {float a1 = dy1 / dx1, b1 = line1[0].y - a1 * line1[0].x;intersection.x = line2[0].x;intersection.y = a1 * intersection.x + b1;}else // 如果两条直线都垂直于x轴，也就是平行，那么无交点。（NAN = not a number） {intersection.x = NAN;intersection.y = NAN;}return intersection;}// Cohen-Sutherland线段裁剪算法 void cohenSutherland(Point clippingWindow[4], Point line[2], int mode){int code0 = encode(line[0], clippingWindow);int code1 = encode(line[1], clippingWindow);if (code0 == 0 && code1 == 0) // 如果直线在裁剪窗口里面，则直接绘制直线 drawLine(line[0], line[1], 0, 1, 0);else{Point inside[2]; // 用于记录线段line位于裁剪窗口内的两个端点 inside[0] = line[0];inside[1] = line[1];// 4次循环分别代表4个边界处理：左、右、下、上 for (int i = 0; i < 4; i++){int temp = (int)pow(2, i);int current0 = (code0 & temp) >> i;int current1 = (code1 & temp) >> i;if (current0 == current1) // 两个端点都在边界的同一侧 {if (current0 == 1) // 两个端点都在边界的外侧 {if (mode == 0)drawLine(inside[0], inside[1], 1, 0, 0);return;}else // 两个端点都在边界的内侧 continue;}else // 两个端点在边界的两侧 {Point p = getIntersection(inside, bound[i]);if (p.x != NAN && p.y != NAN){if (current0 == 1) // 端点inside[0]在边界的外侧，则用交点p换掉端点inside[0] {if (mode == 0)drawLine(p, inside[0], 1, 0, 0);inside[0] = p;code0 = encode(inside[0], clippingWindow);}else // 端点inside[1]在边界的外侧，则用交点p换掉端点inside[1] {if (mode == 0)drawLine(p, inside[1], 1, 0, 0);inside[1] = p;code1 = encode(inside[1], clippingWindow);}}}}// 绘制裁剪窗口内的线段 drawLine(inside[0], inside[1], 0, 1, 0);}}// 绘制裁剪窗口和线段 void display1(void){glClear(GL_COLOR_BUFFER_BIT);drawClippingWindow();cohenSutherland(clippingWindow, line, 0);glFlush();}// 绘制裁剪窗口和线段 void display2(void){glClear(GL_COLOR_BUFFER_BIT);drawClippingWindow();cohenSutherland(clippingWindow, line, 1);glFlush();}// 平移裁剪窗口 void translateClippingWindow(float tx, float ty){for (int i = 0; i < 4; i++){clippingWindow[i].x += tx;clippingWindow[i].y += ty;}updateBound();}// 缩放裁剪窗口 void scaleClippingWindow(float sx, float sy){float centerX = (clippingWindow[1].x + clippingWindow[3].x) / 2;float centerY = (clippingWindow[1].y + clippingWindow[3].y) / 2;translateClippingWindow(-centerX, -centerY);for (int i = 0; i < 4; i++){clippingWindow[i].x *= sx;clippingWindow[i].y *= sy;}translateClippingWindow(centerX, centerY);updateBound();}void mouseEvent(int button, int state, int clickX, int clickY){float x = clickX - (winWidth / 2), y = (winHeight / 2) - clickY;if (button == GLUT_LEFT_BUTTON && state == GLUT_DOWN) // 左键画线 {if (hasLine){line[0].x = x;line[0].y = y;hasLine = false;cut = false;}else{line[1].x = x;line[1].y = y;hasLine = true;}}if (button == GLUT_RIGHT_BUTTON && state == GLUT_DOWN && hasLine) // 右键执行Cohen-Sutherland算法 {cut = true;glutPostRedisplay();}if (button == 3) // 向上滑动滚轮，放大裁剪窗口 {cut = false;scaleClippingWindow(1.1, 1.1);glutPostRedisplay();}if (button == 4) // 向下滑动滚轮，缩小裁剪窗口 {cut = false;scaleClippingWindow(0.9, 0.9);glutPostRedisplay();}}void passiveMotionEvent(int clickX, int clickY){if (!hasLine){line[1].x = clickX - (winWidth / 2);line[1].y = (winHeight / 2) - clickY;glutPostRedisplay();}}void keyEvent(unsigned char key, int clickX, int clickY){cut = false;float dx = 0, dy = 0;// w、a、s、d分别对应裁剪窗口的向上移动、向下移动、向左移动、向右移动操作 if (key == 'w' || key == 'W')dy = offset;if (key == 's' || key == 'S')dy = -offset;if (key == 'a' || key == 'A')dx = -offset;if (key == 'd' || key == 'D')dx = offset;// Enter键：将裁剪窗口恢复回初始状态 if (key == 13)initClippingWindow();translateClippingWindow(dx, dy);glutPostRedisplay();}void main(int argc, char** argv){glutInit(&argc, argv);//form1 glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB);glutInitWindowPosition(50, 100);glutInitWindowSize(winWidth, winHeight);glutCreateWindow("Cohen Sutherland - 控制窗口");glClearColor(1, 1, 1, 0);glMatrixMode(GL_PROJECTION);gluOrtho2D(-winWidth / 2, winWidth / 2, -winHeight / 2, winHeight / 2);initClippingWindow();updateBound();initLine();glutDisplayFunc(display1);glutMouseFunc(mouseEvent);glutKeyboardFunc(keyEvent);glutPassiveMotionFunc(passiveMotionEvent);//form2 glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB);glutInitWindowPosition(500, 100);glutInitWindowSize(winWidth, winHeight);glutCreateWindow("Cohen Sutherland - 裁剪效果显示窗口");glClearColor(1, 1, 1, 0);glMatrixMode(GL_PROJECTION);gluOrtho2D(-winWidth / 2, winWidth / 2, -winHeight / 2, winHeight / 2);initClippingWindow();updateBound();initLine();glutDisplayFunc(display2);glutMainLoop();}
四、实验运行结果截图
