al_lock_bitmap_region - Allegro 5 API

       Like  al_lock_bitmap(3alleg5),  but  only  locks a specific area of the bitmap.  If the bitmap is a video
       bitmap, only that area of the texture will be updated when it is unlocked.  Locking only the  region  you
       indend to modify will be faster than locking the whole bitmap.

              Note:  Using  the  ALLEGRO_LOCK_WRITEONLY  with  a  blocked  pixel  format (i.e. formats for which
              al_get_pixel_block_width(3alleg5) or al_get_pixel_block_height(3alleg5) do not return 1)  requires
              you  to have the region be aligned to the block width for optimal performance.  If it is not, then
              the function will have to lock the region with the ALLEGRO_LOCK_READWRITE instead in order to  pad
              this region with valid data.

       al_lock_bitmap_region - Allegro 5 API

ALLEGRO_LOCKED_REGION(3alleg5), ALLEGRO_PIXEL_FORMAT(3alleg5), al_unlock_bitmap(3alleg5)

Allegro reference manual                                                          al_lock_bitmap_region(3alleg5)

              #include <allegro5/allegro.h>

              ALLEGRO_LOCKED_REGION *al_lock_bitmap_region(ALLEGRO_BITMAP *bitmap,
                 int x, int y, int width, int height, int format, int flags)