如何让我的物体围绕着自己的中心点旋转在OpenGL中？

Question

我现在有一个机器人， 机器人的中心点是(0.5f,0.0f,-1.0f) 我可以围绕着中心点旋转， 但是当我移动机器人到新的位置后再旋转的话就不会围绕着中心点转动了。

void draw(){
    glPushMatrix();
    //update the central point of the quad
    newX = 0.5 + xr; 
    newZ = -1.0 + zr;

    glTranslatef(newX, 0.0, newZ);
    glRotatef(robotAngle, 0.0, 1.0, 0.0);
    glTranslatef(-newX, 0.0, -newZ);
    drawQuad();
    glPopMatrix();
}

void drawQuad(){
    glPushMatrix();
    // Front
    glBegin(GL_QUADS);
    glColor3f(1.0f, 0.0f, 0.0f);    
    glVertex3f(0.0f + xr, 0.0f, 0.0f + zr);
    glVertex3f(1.0f + xr, 0.0f, 0.0f + zr);
    glVertex3f(1.0f + xr, 1.0f, 0.0f + zr);
    glVertex3f(0.0f + xr, 1.0f, 0.0f + zr);
    glEnd();
    // Back
    glBegin(GL_QUADS);
    glColor3f(1.0f, 0.0f, 0.0f);     
    glVertex3f(0.0f + xr, 0.0f, -2.0f + zr);
    glVertex3f(1.0f + xr, 0.0f, -2.0f + zr);
    glVertex3f(1.0f + xr, 1.0f, -2.0f + zr);
    glVertex3f(0.0f + xr, 1.0f, -2.0f + zr);
    glEnd();

    glColor3f(1, 0, 1);
    glBegin(GL_QUAD_STRIP);
    glVertex3f(0.0f + xr, 0.0f, 0.0f + zr);
    glVertex3f(0.0f + xr, 0.0f, -2.0f + zr);

    glVertex3f(1.0f + xr, 0.0f, 0.0f + zr);
    glVertex3f(1.0f + xr, 0.0f, -2.0f + zr);

    glVertex3f(1.0f + xr, 1.0f, 0.0f + zr);
    glVertex3f(1.0f + xr, 1.0f, -2.0f + zr);

    glVertex3f(0.0f + xr, 1.0f, 0.0f + zr);
    glVertex3f(0.0f + xr, 1.0f, -2.0f + zr);
    glEnd();
}

void specialkey(unsigned char key, int x, int y)
{
    switch (key) {
    case 's':
        robotAngle += 1.0;
        glutPostRedisplay();
        break;
    case 'S':
        robotAngle -= 1.0;
        glutPostRedisplay();
        break;
        }
}

void directionkey(int key, int x, int y)
{
    switch (key) {
    case GLUT_KEY_LEFT:
        xr -= 0.05;
        glutPostRedisplay();
        break;
    case GLUT_KEY_RIGHT:
        xr += 0.05;
        glutPostRedisplay();
        break;
   }
}

应该如何解决这个问题 让我可以随时旋转我的机器人围绕着他的中心点？