glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
glTranslatef(0, 0, -cam_dist);
glRotatef(cam_phi, 1, 0, 0);
glRotatef(cam_theta, 0, 1, 0);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
glTranslatef(0, 0, -cam_dist);
glRotatef(cam_phi, 1, 0, 0);
glRotatef(cam_theta, 0, 1, 0);