OpenGL 坐标点大小，视点问题（无法显示三维图形）

Question

本人大三初学Opengl，尝试着写了一下显示三维空间里的三个多边形的代码：

#include
void init()
{glClearColor(1.0,1.0,1.0,0.0);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
gluLookAt(2.0f,0.0f,4.0f,2.0f,0.0f,0.0f,2.0f,4.0f,0.0f);
}
void CGTest()
{
glClear(GL_COLOR_BUFFER_BIT);
glColor3f(0.0f,0.0f,0.0f);

glBegin(GL_QUADS);
glVertex3f(1.5f,0.0f,2.5f);
glVertex3f(2.5f,0.0f,2.5f);
glVertex3f(2.5f,0.5f,2.5f);
glVertex3f(1.5f,0.5f,2.5f);
glEnd();

glBegin(GL_QUADS);
glVertex3f(1.5f,0.5f,2.0f);
glVertex3f(2.5f,0.5f,2.0f);
glVertex3f(2.5f,0.5f,2.5f);
glVertex3f(1.5f,0.5f,2.5f);
glEnd();

glBegin(GL_QUADS);
glVertex3f(2.5f,0.5f,2.5f);
glVertex3f(2.5f,0.5f,2.0f);
glVertex3f(2.5f,0.0f,2.0f);
glVertex3f(2.5f,0.0f,2.5f);
glEnd();

glFlush();
}
void main(int argc,char** argv)
{glutInit(&argc,argv);
glutInitDisplayMode(GLUT_SINGLE|GLUT_RGB);
glutInitWindowPosition(50,100);
glutInitWindowSize(400,400);
glutCreateWindow("An Example OpenGL Program");
init();
glutDisplayFunc(CGTest);
glutMainLoop();
}

但是生成后没有显示任何黑色的多边形，是跟坐标点的位置有关么？还是说LookAt函数的参数不对呢？