al_lock_bitmap_region_blocked - Allegro 5 API

       Like   al_lock_bitmap_blocked(3alleg5),  but  allows  locking  a  sub-region,  for  performance.   Unlike
       al_lock_bitmap_region(3alleg5) the region specified in terms of blocks and not pixels.

       al_lock_bitmap_region_blocked - Allegro 5 API

al_lock_bitmap_region(3alleg5), al_lock_bitmap_blocked(3alleg5)

Allegro reference manual                                                  al_lock_bitmap_region_blocked(3alleg5)

       5.1.9

              #include <allegro5/allegro.h>

              ALLEGRO_LOCKED_REGION *al_lock_bitmap_region_blocked(ALLEGRO_BITMAP *bitmap,
                 int x_block, int y_block, int width_block, int height_block, int flags)