Item Pool

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Item Pool
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An item pool is the set of items that can be generated under a given circumstance. There are many item pools that exist within The Binding of Isaac: Rebirth.

Item Weight

  • Each item that is in an item pool has a weight associated with it. The weight represents how likely it is for the game to spawn the item. Nearly every item in the game has a weight of 1.
    • For example, there are 339 items in the Treasure Room pool. Each has a weight of 1, which means that there is a 1 in 339 (0.29%) chance to get any particular item.
  • Only a few items in the game do not have a weight of 1:

Weight Decrementation

  • When you see an item spawn, the weight of that item is automatically decremented inside of the respective item pool. However, the amount of the decrementation varies depending on the specific item pool.
  • If an item happens to be in more than one item pool, the weight decrementation will also apply to other pools.
  • Touching an item will remove it from all item pools.

The following item pools are fully decremented on sight:

The following item pools are decremented by 50% (0.5) on sight:

The following item pools are decremented by 40% (0.4) on sight:

The following item pools are decremented by 10% (0.1) on sight:

Examples

  • All items in the Treasure Room pool are completely decremented on sight. Thus, seeing a 15►Notched Axe Notched Axe in the Basement 1 Treasure Room will make it impossible to see another Notched Axe in a future Treasure Room. (In this case, players do not have to touch the Notched Axe in order to prevent the game from ever spawning another one.)
  • All items in the shop pool are decremented by 0.5 on sight. Thus, if you use a Restock Machine to reroll a 15►Treasure Map Treasure Map, you have a chance for another Treasure Map to appear. However, after the Treasure Map has already appeared twice on a run, it is completely decremented from the shop pool and it will never appear again.
  • If the player sees 15►Telepathy for Dummies Telepathy for Dummies in the Basement 1 Treasure Room, it is completely removed from all other pools for the rest of the run even without touching it. Seeing it in a Library and not touching it, however, allows it still to be encountered as a Treasure item, with slightly lowered weight.

Unused Item Pools

In the itempools.xml file, we can see that there are some unused item pools:

Set Drops

  • Some specific collectible items are hard-coded to have a chance to drop from certain special events. These are referred to as "set drops".
  • Set drops are not affected by item pools. For example, if you touch 15►The Virus The Virus in the Treasure Room on Basement 1, and then you fight Lust in the Mini-Boss Room of Basement 2, Lust will still have the normal chance to drop 15►The Virus The Virus. If it does, you can take it and have two Viruses.
  • Similarly, just seeing set drops does not decrement them from other pools. For example, if you fight Lust in the Mini-Boss Room of Basement 1, it drops 15►The Virus The Virus and the item remains uncollected, it will remain in the Treasure Room item pool and it can still be found in a Treasure Room later on in the run. Touching the drop will, as expected, remove it from all pools.

The set drops are as follows:

Note that Blood Donation Machines will always spawn a 15►Blood Bag Blood Bag instead of an 15►IV Bag IV Bag if you have already touched an IV Bag during the run.

Pseudo-Set Drops

  • Destroying a pot, mushroom, or skull has a 1% chance to spawn a specific collectible; see below.
  • Like set drops, these items are hard coded and exist outside of the item pools.
  • However, unlike set drops, these collectibles will not spawn if they have already been fully decremented out of their respective pools.
    • For example, if a player sees 15►A Quarter A Quarter in the Basement 1 Treasure Room and does not touch it, then they will not be able to get A Quarter from an urn for the remainder of the run.
  • Furthermore, if a psuedo-set drop is successfully spawned from a urn/mushroom/skull, it will be also be decremented from the respective pool.
    • For example, if a player gets A Quarter from destroying an urn, then they will not be able to get A Quarter from another urn for the remainder of the run.

The pseudo-set drops as as follows:

Notes

  • Item pools can be exhausted by having the game create every item in that specific pool. Items do not need to be picked up for this to happen, the game simply has to generate them (rerolling the item with 15►The D6 The D6 for example).
    • When an item pool is exhausted, the game will generate items from the Treasure Room pool instead.
    • If the Treasure Room pool is exhausted, the only item that can be generated is 15►Breakfast Breakfast.


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