how can glFlush() affect rendering correctness?

Question

or, more correctly, what is the fundamental bug with my classic untrendy non-shader-VBO-stuff?

    cdef struct xyz:
        float x, y, z

    cdef inline void _normal(xyz b,xyz a):
        glNormal3f(a.x-b.x,a.y-b.y,a.z-b.z)

    cdef inline void _draw_quad(xyz a,xyz b,xyz c,xyz d):
        glVertex3f(a.x,a.y,a.z)
        glVertex3f(b.x,b.y,b.z)
        glVertex3f(c.x,c.y,c.z)
        glVertex3f(d.x,d.y,d.z)

    cdef void _draw_grid(xyz a,xyz b,xyz c,xyz d):
        glBegin(GL_LINE_LOOP)
        _draw_quad(a,b,c,d)
        glEnd()


    .... # main loop goes through my data array issuing the appropriate functions

    while self._buf.remaining() > 0:
        op = self._buf.read_char()
        if op == _COLOR:
            col = self._buf.read_rgb()
            #print col
            glColor3f(col.r,col.g,col.b)
        elif op in (_BOX,_GRID):                    
            tl,tr,br,bl,trb,brb,tlb,blb = self._buf.read_xyz(),self._buf.read_xyz(),\
                self._buf.read_xyz(),self._buf.read_xyz(),\
                self._buf.read_xyz(),self._buf.read_xyz(),\
                self._buf.read_xyz(),self._buf.read_xyz()
            if op == _BOX:
                #print "box",col
                glBegin(GL_QUADS)
                func = _draw_quad
            else:
                #print "grid",col
                func = _draw_grid
            _normal(tlb,tl)
            func(tl,tr,br,bl)
            _normal(tl,tr)
            func(tr,trb,brb,br)
            _normal(tr,tl)
            func(tl,tlb,blb,bl)
            _normal(tr,tl)
            func(tl,tlb,trb,tr)
            _normal(tl,tr)
            func(bl,blb,brb,br)
            _normal(tl,tlb)
            func(tlb,trb,brb,blb)
            if op == _BOX:
                glEnd()
            #glFlush()
        else:
            raise Exception("corrupt serialisation; got %x"%op)

if flush after each cube or wireframe, I get this CORRECT rendering:

if I omit the flush - and I obviously don't want to be flushing, even if I am not treading the most optimal opengl path - then I get this INCORRECT rendering, and this is the bug I don't understand:

For the curious, here is how glutSolidCube and wires do it: http://www.google.com/codesearch/p?hl=en#xbii4fg5bFw/trunk/FDS/trunk/SMV_5/source/glut-3.7.6/glut_shapes.c&q=glutSolidCube%20lang:c&sa=N&cd=4&ct=rc

Answer 1

I can't say precisely why not calling glFlush() causes undesired results, but you may want to take a look at a question about the difference between glFlush ad glFinish I asked some time ago. It may have some useful info.

As for solutions, perhaps try putting a glFlush() after you're done rendering the entire scene, as opposed to each cube.