Allegro 5 api
#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)
Like al_lock_bitmap(3alleg5), but only locks a specific area of the bitmap. If the bitmap is a display 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.
ALLEGRO_LOCKED_REGION(3alleg5), ALLEGRO_PIXEL_FORMAT(3alleg5), al_unlock_bitmap(3alleg5)