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A_BubbleSpawn is an action used by the Air Bubble Patch that, if underwater, spawns a randomly sized bubble at the actor's location. Normally, the actor randomly chooses between spawning an Object of type MT_SMALLBUBBLE, an Object of type MT_MEDIUMBUBBLE, or spawning nothing at all otherwise – however, if the action is called within the first 8 tics of every third second of time passed in the level, an Object of type MT_EXTRALARGEBUBBLE (the large, breathable bubble) is spawned instead. Note that A_BubbleSpawn should not be called more frequently than every 8 tics, otherwise multiple MT_EXTRALARGEBUBBLE Objects can potentially be spawned within only a few tics of each other (given the 8 tic window they can spawn in every third second).

If the actor is not underwater when the action is called, the actor is given the MF2_DONTDRAW flag, becoming invisible, and will not spawn any bubbles; if the actor is underwater again the next time the action is called, the MF2_DONTDRAW flag is removed and the actor will continue spawning bubbles again. To save memory, bubbles are also not spawned if there is no player nearby: Var1 determines the distance in fracunits to search for players around the actor; if the actor cannot find any players within this distance, no bubbles will be spawned. This behavior is disabled if the actor has MF_AMBUSH in its flags.

Bubble spawn chances

The exact chances of each type of bubble (or nothing) spawning are given below. Note that the result of a single P_RandomByte call (a number betwen 0 and 255 inclusive) is used by A_BubbleSpawn to determine what is spawned, except in the case of MT_EXTRALARGEBUBBLE.

Object type Sprite P_RandomByte results Chance of occurring
129 to 255 127/256, or ~49.6%
97 to 127 31/256, or ~12.1%
Nothing (None) 0 to 96, 128 98/256, or ~38.3%
N/A 100% if action called when leveltime % (3*TICRATE) < 8
0% otherwise

  Actions – Spawners [view]