When clearing a room, the game has a chance of rewarding a pickup or trinket. This is based on many things, including your current luck and whether you have some particular items.

## Process[]

First, the game generates a random number using this formula:

*(RandomFloat * Luck * 0.1) + pickupPercent*

- RandomFloat and pickupPercent are random numbers between 0 and 1
- Luck is clamped to 0 if Isaac’s luck is less than 0 and 10 if Isaac’s luck is greater than 10
- This chance is modified by the following items/trinkets:
- Lucky Foot multiplies pickupPercent by 0.9, then adds 0.1 to it.
- Lucky Toe multiplies the final value by 0.9, then adds 0.1 to it. (But if Isaac also has Lucky Foot and your luck is greater than 0, the value is instead multiplied by 0.98, plus 0.02, with the Lucky Foot bonus still applying.)

That number is then used to determine the reward given after clearing the room, using the following index:

- Nothing (< 0.22, base 22% chance)
- A tarot card, pill, or trinket (0.22 - 0.3, base 8% chance)
- All three possibilities are equally likely to be chosen, giving them individual base 2.66% chances.

- A coin (0.3 - 0.45, base 15% chance)
- The range increases to 0.3 - 0.5 (base 20%) if Isaac has Rib of Greed

- A heart (0.45 - 0.6, base 15% chance)
- The range decreases to 0.5 - 0.6 (base 10%) if Isaac has Rib of Greed
- If Isaac has Daemon's Tail, the heart has an 80% chance of being replaced with a key.

- A key (0.6 - 0.8, base 20% chance)
- A bomb (0.8 - 0.95, base 15% chance)
- A chest (> 0.95, base 5% chance)
- If Isaac has positive luck, the chance for a chest increases and all other chances decrease.
- Lucky Foot/Toe decrease the chance of getting nothing, but do not themselves increase the chance for chests to spawn.

- If Isaac has positive luck, the chance for a chest increases and all other chances decrease.

The reward then has a chance to be replaced/modified by the following, in order:

- A lil' battery (base 5% chance, plus an additional 6.67% chance if Isaac has Watch Battery)
- A sack (2% chance, always in effect)

- A tarot card (10% chance if Isaac has Ace of Spades)
- A pill (10% chance if Isaac has Safety Cap)
- A bomb (10% chance if Isaac has Match Stick)
- A heart (10% chance if Isaac has Child's Heart or 2% if Isaac also has Daemon's Tail)
- A key (10% chance if Isaac has Rusted Key)
- If Isaac has multiple of these, the first one to activate is the one that replaces the room reward

- A trinket (2% chance if Isaac has Smelter)

- Guppy's Tail has a 33% chance to replace the reward with a chest (either gray or locked with equal chances) and a 22% chance to replace it with nothing.
- Contract From Below causes an extra copy of the reward to spawn, but has a 33% chance to replace it with nothing.
- Each extra contract Isaac has causes another copy of the reward to spawn and reduces the chance of the reward becoming nothing by 2/3.

- If playing on Hard Mode, heart rewards have a 66%/ 33% chance to be replaced with nothing.
- Broken Modem has a 25% chance to create an additional copy of the reward if it’s a key, coin, heart, bomb, or sack.

## Pseudo-code[]

The full logic is contained within the following pseudo-code:

local room = Game():GetLevel():GetCurrentRoom() local awardSeed = room.AwardSeed local player = Game():GetPlayer(0) local difficulty = Game().Difficulty local rng = RNG() rng:SetSeed(awardSeed, 35) --5, 9, 7 local pickupPercent = rng:RandomFloat() if (player:HasCollectible(COLLECTIBLE_LUCKYFOOT)) then pickupPercent = (pickupPercent * 0.9) + 0.1 end local luck = math.max(math.min(player.Luck, 10), 0) --Clamp to 0-10 --Max luck increases the pickupPercent range from 0-1 to 0-2 --That means the more luck you have, the more likely you are to get chests. pickupPercent = rng:RandomFloat() * luck * 0.1 + pickupPercent if (player:HasTrinket(TRINKET_LUCKY_TOE)) then if (player:HasCollectible(COLLECTIBLE_LUCKYFOOT) and luck > 0) then pickupPercent = (pickupPercent * 0.98) + 0.02 else pickupPercent = (pickupPercent * 0.9) + 0.1 end end local pickupAward = COLLECTIBLE_NULL local pickupCount = 1 if (pickupPercent > 0.22) then if (pickupPercent < 0.3) then if (rng:RandomInt(3) == 0) then pickupAward = PICKUP_TAROTCARD elseif (rng:RandomInt(2) == 0) then pickupAward = PICKUP_TRINKET else pickupAward = PICKUP_PILL end elseif (pickupPercent < 0.45) then pickupAward = PICKUP_COIN elseif (pickupPercent < 0.5 and player:HasTrinket(TRINKET_RIB_OF_GREED)) then pickupAward = PICKUP_COIN elseif (pickupPercent < 0.6 and (not player:HasTrinket(TRINKET_DAEMONS_TAIL) or rng:RandomInt(5) == 0)) then pickupAward = PICKUP_HEART elseif (pickupPercent < 0.8) then pickupAward = PICKUP_KEY elseif (pickupPercent < 0.95) then pickupAward = PICKUP_BOMB else pickupAward = PICKUP_CHEST end if (rng:RandomInt(20) == 0 or (rng:RandomInt(15) == 0 and player:HasTrinket(TRINKET_WATCH_BATTERY))) then pickupAward = PICKUP_LIL_BATTERY end if (rng:RandomInt(50) == 0) then pickupAward = PICKUP_GRAB_BAG end if (player:HasTrinket(TRINKET_ACE_SPADES) and rng:RandomInt(10) == 0) then pickupAward = PICKUP_TAROTCARD elseif (player:HasTrinket(TRINKET_SAFETY_CAP) and rng:RandomInt(10) == 0) then pickupAward = PICKUP_PILL elseif (player:HasTrinket(TRINKET_MATCH_STICK) and rng:RandomInt(10) == 0) then pickupAward = PICKUP_BOMB elseif (player:HasTrinket(TRINKET_CHILDS_HEART) and rng:RandomInt(10) == 0 and (not player:HasTrinket(TRINKET_DAEMONS_TAIL) or rng:RandomInt(5) == 0)) then pickupAward = PICKUP_HEART elseif (player:HasTrinket(TRINKET_RUSTED_KEY) and rng:RandomInt(10) == 0) then pickupAward = PICKUP_KEY end if (player:HasCollectible(COLLECTIBLE_SMELTER) and rng:RandomInt(50) == 0) then pickupAward = PICKUP_TRINKET end end if (player:HasCollectible(COLLECTIBLE_GUPPYS_TAIL)) then if (rng:RandomInt(3) != 0) then if (rng:RandomInt(3) == 0) then pickupAward = PICKUP_NULL end else if (rng:RandomInt(2) != 0) then pickupAward = PICKUP_LOCKEDCHEST else pickupAward = PICKUP_CHEST end end end if (player:HasCollectible(COLLECTIBLE_CONTRACT_FROM_BELOW) and pickupAward != PICKUP_TRINKET) then pickupCount = player:GetCollectibleNum(COLLECTIBLE_CONTRACT_FROM_BELOW) + 1 --The chance of getting nothing goes down with each contract exponentially local nothingChance = math.pow(0.666, pickupCount - 1) if (nothingChance * 0.5 > rng:NextFloat()) then pickupCount = 0 end end if (difficulty == 1 and pickupAward == PICKUP_HEART) then if rng:RandomInt(100) >= 35 then pickupAward = PICKUP_NULL end end if (player:HasCollectible(COLLECTIBLE_BROKEN_MODEM) and rng:RandomInt(4) == 0 and pickupCount >= 1 and (pickupAward == PICKUP_COIN or pickupAward == PICKUP_HEART or pickupAward == PICKUP_KEY or pickupAward == PICKUP_GRAB_BAG or pickupAward == PICKUP_BOMB) then pickupCount = pickupCount + 1 end if (pickupCount > 0 and pickupAward != PICKUP_NULL) then local subType = 0 for i=1, pickupCount do local ent = Game():Spawn(ENTITY_PICKUP, pickupAward, nearCenter, Vector(0, 0), 0, subtype, rng:Next()) subType = ent.SubType end end

## Notes[]

This pseudo-code is taken from Blade's GitHub Gist. (Blade is also known as blcd / Will.) He reverse engineered the game using a disassembler in order to create this pseudo-code.

The Binding of Isaac: Rebirth | |||||
---|---|---|---|---|---|

Achievements | Attributes | Bosses | Cards and Runes | Challenges | Chapters |

Characters | Co-op | Items | Item pools | Monsters | Objects |

Pickups | Pills | Rooms | Seeds | Transformations | Trinkets |