Pb with offscreen rendering

I use OpenGL on WinNT4 with a Matrox G400 Max
and I’ve got some rendering problems when
I try to perform an offscreen rendering
(thanks to PFD_DRAW_TO_BITMAP)
Some parts of polygons are not rendered,
leaving holes on the surface through which
other faces may be seen. I don’t think
whether hardware works at this stage.

These problems don’t occur when I render
directly the scene to the screen
(PFD_DRAW_TO_WINDOW) (accelerated),
or when I disable Z-buffer, sending polygons
offscreen in a suitable order.
Does anyone would have experienced similar
problems (Z-buffer software on WinNT
with offscreen rendering) ?
Thank you for any help you may give me.

[This message has been edited by PierreOlivier (edited 08-03-2000).]