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Level generation is the process the game uses to create each floor. Each floor has its own requirements of size, special rooms, and dead ends that must be met.

Process[ | ]

First, the game determines how many rooms will generate on the floor. Later floors have more rooms, and Curse of the LabyrinthCurse of the Labyrinth makes the floor much larger.

  • If the floor is not XL or (in Afterbirth † and Repentance)The VoidThe Void:
    • NumberOfRooms = 3.33 × FloorDepth + 5-6 (maximum of 20)
  • If the floor is an XL floor:
    • NumberOfRooms = 1.8 × (3.33 × FloorDepth + 5-6) (maximum of 45)
  • If the floor is The Void:
    • NumberOfRooms = 50-59
  • 2-3 more rooms are added if playing on Hard Mode
  • 4 more rooms are added if the floor has Curse of the Lost.

Then the minimum number of dead ends the floor must generate is set.

  • All floors have at least 5 dead ends:
    • 1 dead end is added to all floors except the first floor.
    • 1 dead end is added to XL floors.
    • 2 dead ends are added to The Void.
    • (in Repentance) 1 dead end is added if Isaac has Collectible Voodoo Head iconVoodoo Head.

Next, special rooms are generated in dead ends.

Special rooms generate in the following order:

  • Boss RoomBoss Room (guaranteed).
    • If it is an XL floor, two boss rooms will be generated back to back.
    • (in Afterbirth † and Repentance) Multiple Boss Rooms are generated in The VoidThe Void.
  • Super Secret RoomSuper Secret Room (guaranteed).
    • (in Repentance) Two Super Secret Rooms will be generated if Isaac has Collectible Luna iconLuna.
  • ShopShop (guaranteed for the first 3 Chapters).
  • Treasure RoomTreasure Room (guaranteed for the first 3 Chapters).
  • (in Repentance) PlanetariumPlanetarium (base 1% chance to be generated in Chapter 1 through Chapter 3, but will not be generated in later chapters; skipping Treasure Rooms, and obtaining one of a number of items and trinkets will modify this chance).
  • Dice RoomDice Room or Sacrifice RoomSacrifice Room (1/7 (~14%) base chance, plus a second 1/4 chance for a total of 5/14 (~36%) if Isaac’s total health is equal to or greater than his total Heart Containers).
    • If a Sacrifice Room is successfully generated, it has a 1/50 base chance to become a Dice Room instead, plus a second 1/5 chance for a total of 27/125 (~22%) if Isaac has 2 or more Keys.
  • LibraryLibrary (1/20 chance).
    • The code specifically checks for a flag that is supposed to increase Library generation rate if a book was touched, but the flag is never set.
  • Curse RoomCurse Room (1/2 chance, plus a second 1/4 chance for a total of 5/8 (~63%) if Isaac has entered a Devil RoomDevil Room this run).
    • (in Repentance) An additional Curse Room is generated on each floor if Isaac has Collectible Voodoo Head iconVoodoo Head; an extra dead end is also generated to accommodate it.
  • Mini-Boss RoomMini-Boss Room (1/4 chance, plus a second 1/3 chance for a total of 1/2 (50%) if it is the first floor).
  • Challenge RoomChallenge Room (guaranteed if Isaac’s total health is equal to or greater than his total heart containers; will not be generated otherwise).
    • Will not be generated on the first floor, and has a 50% chance on the second floor.
    • Will always be a Boss Challenge RoomBoss Challenge Room on the second floor of a chapter, and a normal Challenge Room otherwise.
  • VaultVault or ArcadeArcade (guaranteed if Isaac has 5 or more Coins and it’s the second floor of a Chapter; will not be generated otherwise).
    • If an Arcade is successfully generated, it has a 1/10 chance to be a Vault instead, plus a second 1/3 chance for a total of 2/5 (40%) if Isaac has 2 or more Keys.
    • (in Repentance) If Isaac has 2 or more Keys but less than 5 Coins, a Vault can still be generated. If Isaac has 5 or more Coins but less than 2 Keys, and if the 1/10 chance to become a Vault passes, then neither an Arcade or a Vault will be generated.
  • Clean BedroomClean Bedroom or Dirty BedroomDirty Bedroom (1/50 chance, plus a second 1/5 chance for a total of 27/125 (~22%) if Isaac has 1/2 or fewer Red HeartRed Hearts and no Soul HeartSoul Hearts, or 1 or fewer Soul Hearts and no Red/(in Afterbirth † and Repentance) Bone HeartBone Hearts).
    • Equal chance to be a Clean or Dirty Bedroom.
    • Will only be generated in the first 3 Chapters.
  • Secret RoomSecret Room (guaranteed).
    • Each location candidate is given a starting weight of 10-14, reduced by 3 if it has only 2 neighboring rooms, and reduced by 6 if it has only 1 neighboring room. The highest weight candidate is picked.
      • In practice, this means a Secret Room will be extremely likely to have at least 2 rooms adjacent to it, unless it's the very first floor, where occasionally one will be placed at a dead end for lack of anywhere else to put it.
    • Will not be placed next to a Boss Room, Super Secret Room, another Secret Room, or the starting room if it's The Chest or Dark Room.
    • (in Afterbirth † and Repentance) An additional Secret Room will be generated if Isaac has Fragmented CardFragmented Card.
    • (in Repentance) An additional Secret Room will be generated if Isaac has Collectible Luna iconLuna, three if he also has Fragmented Card.
  • (in Repentance) Ultra Secret RoomUltra Secret Room (guaranteed).
  • (in Afterbirth † and Repentance) A Grave Room is placed if the floor is Dark Room.
  • Normal Room layouts are chosen.
    • Each room has a set difficulty number, which is chosen based on the current floor of the chapter and if it's Hard Mode or not.
      • Normal Mode floor 1: 1-5
      • Normal Mode floor 2: 5-10
      • Hard Mode floor 1: (except in Repentance) 5 only / (in Repentance) 1-10 (1 and 10 are less common)
      • Hard Mode floor 2: (in Rebirth and Afterbirth) 10 / (in Afterbirth †) 10-15 / (in Repentance) 5-15 (5 is less common)
      • XL Floors count as (except in Repentance) floor 1 / (in Repentance) both floors
      • Chapters 5 and 6 count as (except in Repentance) floor 1 / (in Repentance) floor 2
      • The Void on Normal Mode: 5-10 ((in Repentance) and 20)
      • The Void on Hard Mode: 5-15 / (in Repentance) 5-20

Pseudo-Code[ | ]

Some parts of the level generation are based on the following pseudo-code:

--Pseudo lua code for getting the number of rooms for a floor.

NumberOfRooms = Min(20, Rand(0, 1) + 5 + Floor(StageId * 10 / 3))
if CurseOfTheLabyrinth then
 NumberOfRooms = Min(45, Floor(NumberOfRooms * 1.8))
elseif CurseOfTheLost then
 NumberOfRooms += 4
end
if StageId == 12 then --The Void
 NumberOfRooms = 50 + (Rand() % 10)
end
if IsHardDifficulty then
 NumberOfRooms += 2 + Rand(0, 1)
end

--Pseudo lua code for getting the number of deadends a floor must have.

MinDeadEnds = 5
if StageId ~= 1 then
 MinDeadEnds += 1
end
if CurseOfTheLabyrinth then
 MinDeadEnds += 1
end
if StageId == 12 then
	MinDeadEnds += 2
end


--Deadends are sorted by distance from the starting room descending.
--e.g. Boss room is placed in the farthest deadend, super secret is placed in the next farthest deadend

--DequeueDeadend also resizes the deadend. This is important to note because that means most deadends get changed to a 1x1 by the end of this.

PlaceRoom(ROOM_BOSS, DequeueDeadend())
--Super Secret
PlaceRoom(ROOM_SUPERSECRET, DequeueDeadend())
--Shop
if StageId < 7 or (StageId < 9 and HasTrinket(SilverDollar)) and VictoryLap < 3 then
	PlaceRoom(ROOM_SHOP, DequeueDeadend())
end
--Treasure
if StageId < 7 or (StageId < 9 and HasTrinket(BloodyCrown)) then
	PlaceRoom(ROOM_TREASURE, DequeueDeadend())
end
--Dice and Sacrifice
if StageId < 12 then
	local deadend = DequeueDeadend() --resizes the room :(
	if deadend ~= nil then
		local roomType = nil
		if Rng() % 50 == 0 or (Rng() % 5 == 0 and GetNumKeys() > 1) then 
			roomType = ROOM_DICE
		else
			roomType = ROOM_SACRIFICE
		end
		if Rng() % 7 == 0 or (Rng() & 3 == 0 and GetHearts() + GetSoulHearts() >= GetMaxHearts()) then
			PlaceRoom(roomType)
		else
			QueueDeadend(deadend)
		end
	end
end
--Library
local deadend = DequeueDeadend()
if deadend ~= nil then
	if Rng() % 20 == 0 or (Rng() & 3 == 0 and GetStateFlag(STATE_BOOK_PICKED_UP)) then
		PlaceRoom(ROOM_LIBRARY, deadend)
	else
		QueueDeadend(deadend)
	end
end
--Curse
local deadend = DequeueDeadend()
if deadend ~= nil then
	if Rng() & 1 == 0 or (Rng() & 3 == 0 and GetStateFlag(STATE_DEVILROOM_VISITED)) then
		PlaceRoom(ROOM_CURSE, deadend)
	else
		QueueDeadend(deadend)
	end
end
--MiniBoss
local boss = GetMiniBossRoom() --mini boss is picked based on floor, STATE_ULTRAPRIDE_SPAWNED and rng
local deadend = DequeueDeadend()
if deadend ~= nil then
	if Rng() & 3 == 0 or (Rng() & 2 == 0 and StageId == 1) then
		PlaceRoom(boss, deadend)
	else
		QueueDeadend(deadend)
	end
end
--Challenge
local deadend = DequeueDeadend()
if deadend ~= nil then
	if (Rng() & 1 == 0 or StageId > 2) and GetHearts() + GetSoulHearts() >= GetMaxHearts() and StageId > 1 then
		PlaceRoom(boss, deadend)
	else
		QueueDeadend(deadend)
	end
end
--Chest and Arcade
local deadend = DequeueDeadend()
if deadend ~= nil then
	local roomType = nil
	if Rng() % 10 == 0 or (Rng() % 3 == 0 and GetNumKeys() > 1) then 
		roomType = ROOM_CHEST
	else
		roomType = ROOM_ARCADE
	end
	if GetNumCoins() >= 5 and (StageId == 2 or StageId == 4 or StageId == 6 or StageId == 8) then
		PlaceRoom(roomType)
	else
		QueueDeadend(deadend)
	end
end
--Bedroom
if StageId < 7 then
	local deadend = DequeueDeadend()
	if deadend ~= nil then
		local roomType = nil
		if Rng() & 1 == 0 then
			roomType = ROOM_ISAACS
		else
			roomType = ROOM_BARREN
		end
		
		local maxHearts = nil
		if GetPlayerType() == PLAYER_THELOST or GetPlayerType() == PLAYER_XXX or GetPlayerType() == PLAYER_THESOUL then
			maxHearts = GetMaxHearts()
		else
			maxHearts = GetMaxHearts() + GetBoneHearts() * 2
		end
		
		local second = false
		if (GetHearts() < 2 and GetSoulHearts() <= 0) or (maxHearts <= 0 and GetSoulHearts() <= 2) then
			second = true
		end

		if Rng() % 50 == 0 or (Rng() % 5 == 0 and second) then
			PlaceRoom(roomType)
		else
			QueueDeadend(deadend)
		end
	end
end
--Secret
TryPlacingSecret()
if HasTrinket(TRINKET_FRAGMENTED_CARD) or HasItem(ITEM_LUNA) then
	TryPlacingSecret()
end
-- Grave Room
if StageId == 11 and StageType == 0 then --Dark Room
	PlaceGrave(DequeueDeadend())
end

Notes[ | ]

  • The algorithm is from Blade's GitHub Gist, also known as blcd / Will. He reverse engineered the game using a disassembler in order to create the pseudo-code.
  • The second chances for certain types of rooms are separate, independent rolls: the probabilities are not added together directly.


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