定义一个简单形体（OpenGL提供的基础形体，如立方体、球体等），并生成正交投影及透视投影视图

以下是使用OpenGL生成一个立方体并生成正交投影和透视投影视图的示例代码：

#include <GL/glut.h>

void display()
{
    // 清除屏幕
    glClear(GL_COLOR_BUFFER_BIT);

    // 设置正交投影视图
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    glOrtho(-1, 1, -1, 1, -1, 1);

    // 绘制立方体
    glBegin(GL_QUADS);
        // 上面
        glColor3f(1, 0, 0);
        glVertex3f(-0.5f, 0.5f, -0.5f);
        glVertex3f(-0.5f, 0.5f, 0.5f);
        glVertex3f(0.5f, 0.5f, 0.5f);
        glVertex3f(0.5f, 0.5f, -0.5f);

        // 下面
        glColor3f(0, 1, 0);
        glVertex3f(-0.5f, -0.5f, -0.5f);
        glVertex3f(0.5f, -0.5f, -0.5f);
        glVertex3f(0.5f, -0.5f, 0.5f);
        glVertex3f(-0.5f, -0.5f, 0.5f);

        // 前面
        glColor3f(0, 0, 1);
        glVertex3f(-0.5f, -0.5f, 0.5f);
        glVertex3f(0.5f, -0.5f, 0.5f);
        glVertex3f(0.5f, 0.5f, 0.5f);
        glVertex3f(-0.5f, 0.5f, 0.5f);

        // 后面
        glColor3f(1, 1, 0);
        glVertex3f(-0.5f, -0.5f, -0.5f);
        glVertex3f(-0.5f, 0.5f, -0.5f);
        glVertex3f(0.5f, 0.5f, -0.5f);
        glVertex3f(0.5f, -0.5f, -0.5f);

        // 左面
        glColor3f(0, 1, 1);
        glVertex3f(-0.5f, -0.5f, -0.5f);
        glVertex3f(-0.5f, -0.5f, 0.5f);
        glVertex3f(-0.5f, 0.5f, 0.5f);
        glVertex3f(-0.5f, 0.5f, -0.5f);

        // 右面
        glColor3f(1, 0, 1);
        glVertex3f(0.5f, -0.5f, -0.5f);
        glVertex3f(0.5f, 0.5f, -0.5f);
        glVertex3f(0.5f, 0.5f, 0.5f);
        glVertex3f(0.5f, -0.5f, 0.5f);
    glEnd();

    // 设置透视投影视图
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    gluPerspective(60.0f, 1.0f, 1.0f, 100.0f);
    glMatrixMode(GL_MODELVIEW);
    glLoadIdentity();
    gluLookAt(0.0f, 0.0f, 5.0f, 0.0f, 0.0f, 0.0f, 0.0f, 1.0f, 0.0f);

    // 绘制立方体
    glBegin(GL_QUADS);
        // 上面
        glColor3f(1, 0, 0);
        glVertex3f(-0.5f, 0.5f, -0.5f);
        glVertex3f(-0.5f, 0.5f, 0.5f);
        glVertex3f(0.5f, 0.5f, 0.5f);
        glVertex3f(0.5f, 0.5f, -0.5f);

        // 下面
        glColor3f(0, 1, 0);
        glVertex3f(-0.5f, -0.5f, -0.5f);
        glVertex3f(0.5f, -0.5f, -0.5f);
        glVertex3f(0.5f, -0.5f, 0.5f);
        glVertex3f(-0.5f, -0.5f, 0.5f);

        // 前面
        glColor3f(0, 0, 1);
        glVertex3f(-0.5f, -0.5f, 0.5f);
        glVertex3f(0.5f, -0.5f, 0.5f);
        glVertex3f(0.5f, 0.5f, 0.5f);
        glVertex3f(-0.5f, 0.5f, 0.5f);

        // 后面
        glColor3f(1, 1, 0);
        glVertex3f(-0.5f, -0.5f, -0.5f);
        glVertex3f(-0.5f, 0.5f, -0.5f);
        glVertex3f(0.5f, 0.5f, -0.5f);
        glVertex3f(0.5f, -0.5f, -0.5f);

        // 左面
        glColor3f(0, 1, 1);
        glVertex3f(-0.5f, -0.5f, -0.5f);
        glVertex3f(-0.5f, -0.5f, 0.5f);
        glVertex3f(-0.5f, 0.5f, 0.5f);
        glVertex3f(-0.5f, 0.5f, -0.5f);

        // 右面
        glColor3f(1, 0, 1);
        glVertex3f(0.5f, -0.5f, -0.5f);
        glVertex3f(0.5f, 0.5f, -0.5f);
        glVertex3f(0.5f, 0.5f, 0.5f);
        glVertex3f(0.5f, -0.5f, 0.5f);
    glEnd();

    // 刷新屏幕
    glutSwapBuffers();
}

int main(int argc, char** argv)
{
    // 初始化GLUT
    glutInit(&argc, argv);
    glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGBA);
    glutInitWindowSize(800, 800);
    glutCreateWindow("OpenGL");

    // 设置显示回调函数
    glutDisplayFunc(display);

    // 进入主循环
    glutMainLoop();

    return 0;
}

在该示例中，我们首先使用正交投影视图绘制了一个立方体，然后使用透视投影视图再次绘制了相同的立方体。在透视投影视图中，我们使用了`gluPerspective()`函数来设置透视投影矩阵，并使用`gluLookAt()`函数设置相机位置和方向。