如何在opengl中修复墙上的时钟？

Question

我有一个时钟和一个立方体（作为一个房间）。我想把时钟放在其中一面墙上。但是当我旋转或移动相机时，屏幕上的时钟位置是固定的。我怎样才能在其中一面墙上修理时钟？

 void room(){
    if(flag){
        glRotatef(rt,0.0f,1.0f,0.0f);
    }
    glEnable(GL_NORMALIZE);
    glPushMatrix();
    glBegin(GL_QUADS);

    glColor3f(1.0, 1.0 ,1.0);  //roof(white)
    glNormal3f(0,-1,0);
    glVertex3f( 3.0, 3.0,-3.0); 
    glVertex3f(-3.0, 3.0,-3.0); 
    glVertex3f(-3.0, 3.0, 3.0); 
    glVertex3f( 3.0, 3.0, 3.0);

    glColor3f(1.0, 1.0 ,0.0);  // floor(yellow)
    glNormal3f(0,1,0);
    glVertex3f( 3.0,-3.0, 3.0); 
    glVertex3f(-3.0,-3.0, 3.0); 
    glVertex3f(-3.0,-3.0,-3.0); 
    glVertex3f( 3.0,-3.0,-3.0); 

    glColor3f(1.0, 0.0 ,0.0); // front (red)
    glNormal3f(0,0,-1);
    glVertex3f( 3.0, 3.0, 3.0);  
    glVertex3f(-3.0, 3.0, 3.0);  
    glVertex3f(-3.0,-3.0, 3.0);  
    glVertex3f( 3.0,-3.0, 3.0);  

    glColor3f( 0.0, 1.0,1.0);   // behind (cyan)
    glNormal3f(0,0,1);
    glVertex3f( 3.0,-3.0,-3.0); 
    glVertex3f(-3.0,-3.0,-3.0); 
    glVertex3f(-3.0, 3.0,-3.0);  
    glVertex3f( 3.0, 3.0,-3.0); 

    glColor3f(0.0 ,0.0,1.0);  //left 
    glNormal3f(1,0,0);
    glVertex3f(-3.0, 3.0, 3.0); 
    glVertex3f(-3.0, 3.0,-3.0); 
    glVertex3f(-3.0,-3.0,-3.0); 
    glVertex3f(-3.0,-3.0, 3.0); 

    glColor3f( 0.2, 0.4, 0.0); // right 
    glNormal3f(-1,0,0);
    glVertex3f( 3.0, 3.0,-3.0);  
    glVertex3f( 3.0, 3.0, 3.0);  
    glVertex3f( 3.0,-3.0, 3.0); 
    glVertex3f( 3.0,-3.0,-3.0);

    glEnd();
    glPopMatrix();
}
    void drawClock() {
    char buff[100];
    GLfloat x1=0.0;
    GLfloat y1=2.5;
    GLfloat z1= -2.9;
    GLfloat radius = 1.5;   
    int angle;

    glLoadIdentity();

    // Draw circle
    glPushMatrix();
    glBegin(GL_TRIANGLE_FAN);
    glColor3f(0.5,1.0,1.0);   
    glVertex3f(x1, y1, z1);
    for(angle=0; angle <=360; angle +=1)  
        glVertex3f(x1 + cos(angle * PI/180.0f)* radius, y1 + sin(angle * PI/180.0f) * radius, z1);
    glEnd();
    glPopMatrix();

    // draw ticks
    glPushMatrix();
    glColor3f(0.2,0.2,0.2);
    glLineWidth(2);   
    for(angle=0; angle <= 360; angle +=30)
    {
        glBegin(GL_LINES);
        //printf("Angle: %d\n", angle);
        if(angle == 0.0 || angle==90.0 || angle == 180.0 || angle == 270.0 || angle == 360.0 )
        {
            glColor3f(1.0,0.2,0.2);   
            glVertex3f(x1 + cos(angle * PI/180.0f)* (radius+0.2), y1 + sin(angle * PI/180.0f) * (radius+0.2), z1);
            glVertex3f(x1 + cos(angle * PI/180.0f)* radius, y1 + sin(angle * PI/180.0f) * radius, z1);
        }
        else
        {
            glColor3f(0.2,0.2,0.2);
            glVertex3f(x1 + cos(angle * PI/180.0f)* (radius+0.05), y1 + sin(angle * PI/180.0f) * (radius+0.05), z1);
            glVertex3f(x1 + cos(angle * PI/180.0f)* radius, y1 + sin(angle * PI/180.0f) * radius, z1);
        }
        glEnd();
    }
    glPopMatrix();

    // Draw seconds line
    glColor3f(1.0,1.0,1.0);
    glLineWidth(2);
    glPushMatrix();
    glTranslatef(0.0,1.7,0.0);
    glRotatef(-secAngle, 0.0, 0.0, 1.0);  
    glBegin(GL_LINES);
    glVertex3f(0.0, 0.0 , -2);
    glVertex3f(0.0, 1.0 , -2);
    glEnd();
    glPopMatrix();

    // Draw minutes line
    glColor3f(1.0,0.0,0.0);
    glLineWidth(2);
    glPushMatrix();
    glTranslatef(0.0,1.7,0.0);
    glRotatef(-minAngle, 0.0, 0.0, 1.0);
    glBegin(GL_LINES);
    glVertex3f(0.0, 0.0, -2);
    glVertex3f(0.0, 0.8, -2);
    glEnd();
    glPopMatrix();

    // Draw hour line
    glColor3f(0.0,0.0,0.0);
    glLineWidth(2);
    glPushMatrix();
    glTranslatef(0.0,1.7,0.0);
    glRotatef(-hourAngle, 0.0, 0.0, 1.0);
    glBegin(GL_LINES);
    glVertex3f(0.0, 0.0, -2.0);
    glVertex3f(0.0, 0.6, -2.0);
    glEnd();

    // Draw central point
    glColor3f(0.0,0.0,0.0);
    glPointSize(10.0);
    glPushMatrix();
    glTranslatef(0.0,1.7,0.0);
    glBegin(GL_POINTS);
    glVertex3f(0.0, 0.0, 2.0);
    glEnd();
    glPopMatrix();
    glFlush();
}  

我运行项目时如下：

然后旋转相机：

我这两张照片的时钟位置是固定的 。我怎样才能把它固定在墙上？