Rotating an OpenGL primitive around its centre

> Object A -> trans 1, 0, 0 from origin
> Object B -> trans 0, 1, 0 from origin
> now you click Object A and rotate, it should rotate around its local center
> Object B should do the same rotation, but around A's centre and NOT around its own.


ok, here is the code:

trans_a -> trans 1, 0, 0
trans_b -> trans 0, 1, 0
rot     -> rotation calculated from your mouse-movement, here it is 10 degrees around axis z


    // render object A
    glPushMatrix();
    // first apply trans, then rot. makes object a rotating around its local center
    // apply trans_a
    glTranslatef( 1.0f, 0.0f, 0.0f );
    // apply rot
    glRotatef( 10.0f, 0.0f, 0.0f, 1.0f );
    render_object_a();
    glPopMatrix();

    // render object B
    glPushMatrix();
    // apply TRANS_A here makes object b rotate around object a rather then around itsself
    // or with other words, object a's center becomes vector {0,0,0}
    glTranslatef( 1.0f, 0.0f, 0.0f );
    // apply rot
    glRotatef( g_angle_z, 0.0f, 0.0f, 1.0f );
    // apply trans_b
    glTranslatef( 0.0f, 1.0f, 0.0f );
    // apply negated trans_a, *redo* displacement from first glTranslatef
    glTranslatef( -1.0f, 0.0f, 0.0f );
    render_object_b();
    glPopMatrix();
   
       
   
    so basically what happens is:
   
    (1) if you call it like this, the object rotates around its local center and stays at its position,
        as done for object a:
       
        glTranslatef
        glRotatef rot
        render_object
       
       
    (2) if you call it like this, the object rotates around worlds {0,0,0} it does not stay in its position
       
        glRotatef
        glTranslatef ..
        render_object
     

    now, you dont want to rotate object b around its local center neither around vector {0,0,0}. you
    want to rotate it around a given point, which is object a's center. in order to do so, we have
    to translate object b first to object a's position, then do all the rotations, and redo the displacement
    from first translation to put object b back to its position
       
        glTranslatef trans_a
        (2) glRotatef rot
        (2) glTranslatef trans_b
        glTranslatef -trans_a
        render_object_b

   i put the (2) above to make clear, we do the transformation explained in (2). we rotate object b around
   vector {0,0,0}. but the transformation before makes object a's center the new vector {0,0,0}
   
   
   
ike