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       al_get_backbuffer - Allegro 5 API


              #include <allegro5/allegro.h>

              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
       tranformation  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.