al_get_backbuffer - Allegro 5 API
ALLEGRO_BITMAP *al_get_backbuffer(ALLEGRO_DISPLAY *display)
Return a special bitmap representing the back-buffer of the 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(3alleg5)). Only untransformed operations are hardware
accelerated. These consist of al_draw_bitmap(3alleg5) and
al_draw_bitmap_region(3alleg5) 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(3alleg5) and al_draw_bitmap_region(3alleg5)), and then use
that temporary bitmap as the source bitmap.