acquire_bitmap(3) Allegro manual acquire_bitmap(3)NAMEacquire_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 func‐
tions 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 there‐
fore needs the screen unlocked, but an XOR rectfill, or one with blend‐
ing 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 ALSOrelease_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)
