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acquire_bitmap(3)               Allegro manual               acquire_bitmap(3)

NAME
       acquire_bitmap - Locks the bitmap before drawing onto it. Allegro game
       programming library.

SYNOPSIS
       #include <allegro.h>

       void acquire_bitmap(BITMAP *bmp);

DESCRIPTION
       Acquires the specified video bitmap prior to drawing onto it. You never
       need to call the function explicitly as it is low level, and will only
       give you a speed up if you know what you are doing. Using it wrongly
       may cause slowdown, or even lock up your program.

       Note: You do never need to use acquire_bitmap on a memory bitmap, i.e.
       a normal bitmap created with create_bitmap. It will simply do nothing
       in that case.

       It still can be useful, because e.g. under the current DirectDraw
       driver of Allegro, most drawing functions need to lock a video bitmap
       before drawing to it. But doing this is very slow, so you will get much
       better performance if you acquire the screen just once at the start of
       your main redraw function, then call multiple drawing operations which
       need the bitmap locked, and only release it when done.

       Multiple acquire calls may be nested, but you must make sure to match
       up the acquire_bitmap and release_bitmap calls. Be warned that DirectX
       and X11 programs activate a mutex lock whenever a surface is locked,
       which prevents them from getting any input messages, so you must be
       sure to release all your bitmaps before using any timer, keyboard, or
       other non-graphics routines!

       Note that if you are using hardware accelerated VRAM->VRAM functions,
       you should not call acquire_bitmap(). Such functions need an unlocked
       target bitmap under DirectX, so there is now just the opposite case
       from before - if the bitmap is already locked with acquire_bitmap, the
       drawing operation has to unlock it.

       Note: For backwards compatibility, the unlocking behavior of such
       functions is permanent. That is, if you call acquire_bitmap first, then
       call e.g. an accelerated blit, the DirectX bitmap will be unlocked
       internally (it won't affect the nesting counter of acquire/release
       calls).

       There is no clear cross-platform way in this Allegro version to know
       which drawing operations need a locked/unlocked state. For example a
       normal rectfill most probably is accelerated under DirectX, and
       therefore needs the screen unlocked, but an XOR rectfill, or one with
       blending activated, most probably is not, and therefore locks the
       screen. And while the DirectX driver will do automatic unlocking, there
       is no such thing under X11, where the function is used to synchronize
       X11 calls from different threads. Your best bet is to never use
       acquire_bitmap - changes are you are doing something in the wrong way
       if you think you need it.

       Warning: This function can be very dangerous to use, since the whole
       program may get locked while the bitmap is locked. So the lock should
       only be held for a short time, and you should not call anything but
       drawing operations onto the locked video bitmap while a lock is in
       place. Especially don't call things like show_mouse (or scare_mouse
       which calls that) or readkey, since it will most likely deadlock your
       entire program.

SEE ALSO
       release_bitmap(3), acquire_screen(3), release_screen(3), ex3buf(3),
       exaccel(3), expat(3), exquat(3), exscroll(3), exswitch(3), exupdate(3)

Allegro                          version 4.4.2               acquire_bitmap(3)