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Creating sub-bitmap of locked bitmap
Felix-The-Ghost
Member #9,729
April 2008
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I'm looking to combine two for loops in my code, it would mean creating several sub-bitmaps of a bitmap that is currently locked to read pixels from. I couldn't tell from the manual if this is error prone/dangerous in any way. Is it safe?

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Edgar Reynaldo
Member #8,592
May 2007
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It looks safe enough to create a sub bitmap on a locked parent from the function definition. All al_create_sub_bitmap does is allocate a new ALLEGRO_BITMAP and populate it with default values.

bitmap.c#SelectExpand
431/* Function: al_create_sub_bitmap 432 */ 433ALLEGRO_BITMAP *al_create_sub_bitmap(ALLEGRO_BITMAP *parent, 434 int x, int y, int w, int h) 435{ 436 ALLEGRO_BITMAP *bitmap; 437 438 if (parent->parent) { 439 x += parent->xofs; 440 y += parent->yofs; 441 parent = parent->parent; 442 } 443 444 bitmap = al_calloc(1, sizeof *bitmap); 445 bitmap->vt = parent->vt; 446 447 /* Sub-bitmap inherits these from the parent. 448 * Leave these unchanged so they can be detected if improperly accessed 449 * directly. */ 450 bitmap->_format = 0; 451 bitmap->_flags = 0; 452 bitmap->_display = (ALLEGRO_DISPLAY*)0x1; 453 454 bitmap->w = w; 455 bitmap->h = h; 456 bitmap->locked = false; 457 bitmap->cl = bitmap->ct = 0; 458 bitmap->cr_excl = w; 459 bitmap->cb_excl = h; 460 al_identity_transform(&bitmap->transform); 461 al_identity_transform(&bitmap->inverse_transform); 462 bitmap->inverse_transform_dirty = false; 463 al_identity_transform(&bitmap->proj_transform); 464 al_orthographic_transform(&bitmap->proj_transform, 0, 0, -1.0, w, h, 1.0); 465 bitmap->shader = NULL; 466 bitmap->parent = parent; 467 bitmap->xofs = x; 468 bitmap->yofs = y; 469 bitmap->memory = NULL; 470 471 bitmap->dtor_item = _al_register_destructor(_al_dtor_list, "sub_bitmap", bitmap, 472 (void (*)(void *))al_destroy_bitmap); 473 474 return bitmap; 475}

What do you need to do with the sub-bitmaps while the parent is locked? Just create them? That should be safe.

Felix-The-Ghost
Member #9,729
April 2008
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Edgar, my hero!

Yes I am just creating them, then after all are created I unlock the parent. Nice.

==========================
<--- The ghost with the most!
---------------------------
[Website] [Youtube]

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