al_get_backbuffer - Allegro 5 API
Return a special bitmap representing the back-buffer of the display.
ALLEGRO_BITMAP *al_get_backbuffer(ALLEGRO_DISPLAY *display)
Care should be taken when using the backbuffer bitmap (and its
sub-bitmaps) as the source bitmap (e.g as the bitmap argument to
al_draw_bitmap(3)). Only untransformed operations are hardware accelerated.
These consist of al_draw_bitmap(3) and al_draw_bitmap_region(3) when the
current transformation is the identity. If the transformation is not the
identity, or some other drawing operation is used, the call will be routed
through the memory bitmap routines, which are slow. If you need those
operations to be accelerated, then first copy a region of the backbuffer
into a temporary bitmap (via the al_draw_bitmap(3) and
al_draw_bitmap_region(3)), and then use that temporary bitmap as the source