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       screen  -  Global  pointer  to  the screen hardware video memory. Allegro game programming


       #include <allegro.h>

       extern BITMAP *screen;


       Global  pointer  to  a  bitmap,  sized  VIRTUAL_W  x  VIRTUAL_H.  This   is   created   by
       set_gfx_mode(),  and represents the hardware video memory. Only a part of this bitmap will
       actually be visible, sized SCREEN_W x SCREEN_H.  Normally this is the top left  corner  of
       the  larger  virtual  screen,  so  you  can ignore the extra invisible virtual size of the
       bitmap if you aren't interested in hardware  scrolling  or  page  flipping.  To  move  the
       visible  window  to other parts of the screen bitmap, call scroll_screen().  Initially the
       clipping rectangle will be limited to the physical screen size, so if  you  want  to  draw
       onto  a  larger  virtual  screen space outside this rectangle, you will need to adjust the

       For example, to draw a pixel onto the screen you would write:

          putpixel(screen, x, y, color);

       Or to implement a double-buffered system:

          /* Make a bitmap in RAM. */
          BITMAP *bmp = create_bitmap(320, 200);
          /* Clean the memory bitmap. */
          /* Draw onto the memory bitmap. */
          putpixel(bmp, x, y, color);
          /* Copy it to the screen. */
          blit(bmp, screen, 0, 0, 0, 0, 320, 200);

       Warning: be very careful when using this pointer at the same time as any  bitmaps  created
       by  the  create_video_bitmap()  function  (see  the  description of this function for more
       detailed information). And never try to destroy it with destroy_bitmap().


       set_gfx_mode(3alleg4),      is_screen_bitmap(3alleg4),       create_video_bitmap(3alleg4),