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Binding of Isaac Rebirth Mechanics Guide
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mechanics.md

Binding of Isaac Mechanics Guide

Stats

Health

Each character starts with the following health

  • Isaac: 3
  • Maggy: 4
  • Cain: 2
  • Judas: 1
  • ???: 3
  • Eve: 2
  • Sam: 1
  • Az: 0
  • Laz: 3
  • Eden: ?
  • Lost: 0

+1 HP Up

  • Lunch
  • Dinner
  • Dessert
  • Breakfast
  • Rotten Meat
  • Magic Mushroom
  • Odd Mushroom (Large)
  • The Halo
  • <3
  • Stem Cells
  • Holy Grail
  • MEAT!
  • SMB Super Fan!
  • Stigmata
  • Blood Bag

+2 HP Up

  • Raw Liver
  • Bucket of Lard

+3 HP Up

  • The Body

Movement Speed

  • Base speed: 1
  • Modifiers (Max 2.0 Speed)
    • Speed items here

Shot Speed

  • Base: 1
  • Modifiers:
    • Speed Ball: +0.2
    • Cat-o-nine-tails: +0.23
    • Stem Cells: +0.16
    • Tooth Picks: +0.16
    • Magic 8: +0.16
    • Sacred Heart: -0.25
    • Samson: +0.31

Tears

  • Tears Modifiers

    • Number One: 1.5
    • Wire Coat: 0.7
    • Pact: 0.7
    • Sad Onion: 0.7
    • Odd Mushroom (Small): 1.7
    • Squeezy: 0.4
    • Wiggle: 0.4
    • Sacred Heart: -0.4
    • Halo: 0.2
    • Small Rock: 0.2
    • SMB Super Fan!: 0.2
    • Samson: -0.25

  • Calculation:

    • delay = 16 - 6 * sqrt(modifiers * 1.3 + 1) If your modifier is 0 > x > -0.77
    • delay = 16 - 6 * sqrt(modifiers * 1.3 + 1) - modifiers * 6 If your modifier is < -0.77
    • delay = 16 - modifiers * 6

  • Tear delay rounds in your favour down to next whole number

    • e.g If you have a +0.7 modifier from a Sad Onion, your delay would be reduced to 7.7, which is rounded down to 7.

  • Post-calculation Multipliers

    • Chocolate Milk: delay = delay * 0.8
    • Inner Eye, Ipecac, Polyphemus, or Mutant Spider: delay = delay * 2.1 + 3
    • Cancer: delay = delay - 2

Damage

Damage Sources

  • Devil Card or Book of Belial: 2
  • Blood of the Martyr: 1
  • Small Rock: 1
  • Money = Power: coins * 0.04
  • Whore of Babylon:
  • Steven: 1
  • Pentagram: 1
  • Growth Hormones: 1
  • The Mark: 1
  • The Pact: 0.5
  • Max's Head: 0.5
  • Jesus Juice: 0.5
  • Magic Mushroom: 0.3
  • The Halo: 0.3
  • Odd Mushroom (large): 0.3
  • Stigmata: 0.3
  • MEAT!: 0.3
  • SMB Super Fan!: 0.3
  • Chemical Peel: 2.0 (Left eye tears only)
  • The Nail (Used): 0.7
  • Bloody Lust:
  • Razor Blade:

Main Calculations

  • Sum all the above effects
  • damage = 3.5 * sqrt(1 + damage * 1.2)
  • damage = character_mod * damage
    • Isaac: 1.00
    • Maggy: 1.00
    • Cain: 1.20
    • Judas: 1.35
    • ???: 1.05
    • Eve: 0.75
    • Sam: 1.00
    • Az:
    • Laz:
    • Eden:
    • Lost:
  • Damage Multipliers
    • Magic Mushroom: damage = damage * 1.5
    • Book of Belial AND Blood of the Martyr: damage = damage * 1.5
    • Polyphemus: damage = 2 * (damage + 4)
    • Sacred Heart: damage = damage * 2.3 + 1
    • Tough love: damage = damage * 3.2
    • Odd Mushroom (Thin): damage = damage * 0.9 - 0.4
    • Technology 2: damage = damage * 0.65
  • More damage applications
    • Curved Horn: damage = damage + 2
    • Lump of Coal:

Non-basic damage

  • Lasers
    • Brimstone *
    • Technology *
    • Technology 2 *
  • Mom's Knife *

Luck

  • Luck starts at 0

Item Mechanics

Cancelled Effects

  • Over-riding items go here...

Health Restoration

+0.5 hearts

  • Bucket of Lard

+1 heart

  • Lunch
  • Dinner
  • Dessert
  • Breakfast
  • Rotten Meat
  • MEAT!
  • Stigmata
  • SMB Super Fan!
  • The Halo
  • Super Bandage
  • Fate (if you complete an eternal heart)

+1.5 hearts

  • Stem cells

+2 hearts

  • PHD

+5 hearts

  • Blood bag

Full restoration

  • Sacred Heart
  • Magic Mushroom
  • <3
  • Raw Liver

Spirit Hearts

From item pickups

  • Super Bandage: 2
  • Sacred Heart: 1
  • Rosary: 3
  • The Mark: 1
  • The Pact: 2

Eternal Hearts

  • Fate: 1/2 heart

Gives Stuff

  • Bombs
    • Pyro: 99
    • Remote Detonator: 5 (first time only)
    • Bobs Curse: 5
    • Bobby - Bomb: 5
    • Mr. Mega: 5
    • Boom!: 10
  • Keys
    • Skeleton Key: 99
  • Coins
    • A dollar!: 99
    • A Quarter: 25

Familiars

  • Tear shooting:
    • Brother bobby
    • Sister Maggy
    • The Robo-Baby
    • Little Gish
    • Little Steve
    • Demon Baby
    • Ghost Baby
    • Harlequin Baby
    • Rainbow Baby
    • Abel
    • Isaac's Head
    • ???'s Soul
  • Other combat
    • Cube of Meat
    • Meatboy
    • Full Meatboy
    • Bandage Girl
    • Full Bandage Girl
    • Guardian Angel
    • The Peeper
    • Guppy's Hairball
    • Sacrificial Dagger
    • Dead Bird
    • Daddy Longlegs
    • The Pinking Shears
    • Halo of Flies
    • Distant Admiration
    • Forever Alone
    • Lil Chubby
  • Non-combat
    • Bum Friend
    • Dark Bum
    • Dead Cat
    • 1up!
    • Sack of Pennies
    • Little Chad
    • The Relic
    • Bomb Bag
    • Holy Water

Pills

  • PHD automatically identifies all pills.
  • If you find a negative pill and then acquire PHD, it will become its positive counter-part (Full Health being Bad Trip's).
  • All above Up/Down pills have an equal chance of spawning the Up or Down version
  • Pills effects
    • Tears Up: +0.35
    • Tears Down: -0.28
    • Range Up: +2.5
    • Range Down: -2.0
    • Speed Up: +0.15
    • Speed Down: -0.12
    • Luck Up: +1.0
    • Luck Down: -1.0

Cards

  • Card effects
    • Strength: Magic Mushroom
    • The Devil: Book of Belial effect

Trinkets

Trinket Pool

  • Counterfeit Penny is not in the pool, but has a 1/x chance to spawn from golden poop
  • Trinket pool behaviour goes here...

Fish Head

Every time you get hit, you spawn one blue fly

Pinky Eye

  • Chance to make a poison tear (Is this affected by luck?)

Push Pin

  • Chance to make a spectral tear (1/10 chance?)

Liberty Cap

1/4 chance to give one of the following effects: * Compass * Mini Mush * Odd Mushroom (thin) * Odd Mushroom (thick)

Umbilical Cord

If you take damage that leaves you on 1/2 heart left, you gain Little Steve for the remainder of the room.

Pickup-boosting trinkets

  • x/x chance to give you an item at the end of a room
  • x/x chance to give you an item when opening a chest
  • Types
    • Child's Heart: Heart
    • Rusted Key: Key
    • Match Stick: Bomb
    • Safety Cap: Pill
    • Ace of Spades: Card

Curved Horn

+2 damage (Post formula, flat damage)

Goat Hoof

+0.15 speed

Mom's Pearl

1/10 additional chance for any given heart drop to become a spirit heart

Cancer

Fire delay decreased by 2

Red Patch

x/x chance to get +1.8 damage for the remainder of the room

Lucky Toe

+1 luck stat

Cursed Skull

Returns you to the previous room when you are hit with 1/2 heart remaining

Isaac's Fork

x/x chance to gain a half heart after clearing a room

A Missing Page

1/20 chance to deal 40 damage to all enemies in the room when taking damage

Pennies

  • 1/2 chance to gain one of the following pickups when picking up any coin
    • Bloody Penny: Heart
    • Burnt Penny: Bomb
    • Flat Penny: Key
    • Counterfeit Penny: +1 coin (automatic)
  • These items do not have a chance to be upgraded (see Pickups section)

The Tick

  • When an enemy spawns, if it has over 60 hp, 15% of that is removed immediately
  • When you enter a boss room and ?firsttime is not set, you recover 1 heart

Isaac's Head

Creates a familiar that fires projectiles (See Familiars section)

Maggy's Faith

Grants 1/2 eternal heart at the beginning of every level

Judas's Tongue

Items that cost 2 hearts in a deal with the Devil will be reduced to 1 heart instead

???'s Soul

Creates a familiar that moves around the room

Pickups

Hearts

  • Chance for a normal heart to become a spirit heart when dropped
    • Base 1/10 chance
    • Additional 1/16 chance if you are Eve
    • Additional 1/10 chance if you have Mom's Pearl
    • Additional 1/2 chance if you have Mitre
  • Chance for a normal heart to become an eternal heart when dropped: 1/50
  • If you are in the super secret room that has an eternal heart in it, all hearts that spawn in that room will be eternal, i.e., through slot machines placed with Wheel of Fortune card.
  • 1/2 chance to set col = 2 (full heart vs half?)

Keys

  • 1/20 chance for golden key

Bombs

  • 1/7 chance to set col = 2 (1+1 free?)
  • 1/10 chance to set col = 3 (troll bomb?)
  • 1/50 chance to set col = 5 (homing troll bomb?)
  • Damage
    • Base: 10
    • Mr. Mega: +50

Coins

  • 1/20 chance to be a nickel
  • 1/100 chance to be a dime

Items

How random items are selected

  • Item selection + item pool info here

Incidental item pool removal

  • If this still happens with some items, info here

Item Pools

  • Item pools go here

Specific Items

Dead Sea Scrolls

  • On use, uses one of the following items:
    • The Book of Belial
    • The Necronomicon
    • Mr. Boom!
    • Tammys Head
    • Moms Bra
    • kamikaze!
    • Moms Pad
    • Bobs Rotten Head
    • Teleport!
    • Yum Heart
    • Lemon Mishap
    • Shoop da Whoop!
    • Book of shadows
    • Wiggle Worm
    • Anarchist Cookbook
    • The Hourglass
    • My little Unicorn
    • Book of Revelations
    • the Nail
    • We need to go deeper!
    • Deck of Cards
    • Monstros Tooth
    • The Gamekid
    • The Book of Sin
    • Moms Bottle of Pills
    • The Pinking Shears

The Bible

  • Using The Bible on the Chapter 9 boss (Satan or Isaac) will deal 1000 damage to the player.

Dead Cat

  • Special dead cat info here

Rosary

  • Adds Bibles to shop and devil room (more info here)

9 Volt

  • Automatically gives you 1 bar of charge on your spacebar item when you use it

Guppy's Hairball

  • Each floor reset size
  • More hairball mechanics here

Monster Maneul

  • Gives one of the following for the room (will not pick one that you already have unless it's Cube of Meat or Ball of Bandages):
    • Brother Bobby
    • Cube of Meat
    • Ball of Bandages
    • Halo of Flies
    • Distant Admiration
    • Sister Maggy
    • Lil Chubby
    • Robo-Baby
    • Little Gish
    • Little Steve
    • Guardian Angel
    • Demon Baby
    • Forever alone

3 Dollar Bill

  • One of the following random effects each room:
    • The Sad Onion
    • The Inner Eye
    • Spoon Bender
    • My Reflection
    • Number One
    • Lucky Foot
    • Magneto
    • Mom's Contact
    • Ouija Board
    • Speed Ball
    • Tough Love

Slots

Skull Game

Slot Machine

Fortune Machine

Blood Donation Machine

  • 1/x chance to break
  • Otherwise you get 1-3 coins (+1 coin if you have PHD)
  • When it breaks, you normally get the Blood Bag, but there is a 1/2 chance to get the IV Bag if you do not have it already.

Beggars?

Rooms and Floors

Spawning

  • Floor spawning mechanics and individual room spawning mechanics (i.e. Library, Arcade)

Clearing a Room

  • End of room drops + trinkets that augment it

Super Secret Rooms

Types

Devil Rooms

  • Spawning mechanics

Angel Rooms

  • Spawning mechanics

Deals with the Devil

Mini-boss Rooms

Boss Rooms

  • Boss selection

Curses

  • Curse selection & how it changes with advancing in the game

Objects

Red Chest

Brown Chests, Gold Chests, and Tinted Rocks (?)

Tinted Rocks

Poop

  • Info about golden/rainbow poop spawning goes here

Misc

Challenge modes

  1. Pitch Black
    • No special starting items
  • End stage: Floor 6 (Mom's Foot / Depths 2)
    • No shops
    • 100% Curse of Darkness
  1. High Brow
  • Starting Items: Butt Bombs, Number One, E. Coli, FLUSH!
  • End stage: Floor 6 (Mom's Foot / Depths 2)
    • No item rooms
    • Start with Petrified Poop trinket.
  1. Head Trauma
  • Starting Items: Tiny Planet, Soy Milk, Iron Bar, Small Rock
    • End stage: Floor 6 (Mom's Foot / Depths 2)
    • No item rooms
  1. Darkness Falls
  • Starting Items: Razor Blade, Dark Matter, Pentagram, Sacrificial Dagger
    • End stage: Floor 9 (Isaac / Cathedral)
    • No item rooms
    • Start as Eve
    • Required achievements: Eve character unlocked, It Lives unlocked (10 Mom's Heart kills)
  1. The Tank
  • Starting Items: Thunder Thighs, Bucket of Lard, Infamy
    • End stage: Floor 6 (Mom's Foot / Depths 2)
    • No item rooms
    • Start as Maggy
    • Required achievements: Maggy character unlocked
  1. Solar System
  • Starting Items: Distant Admiration, Forever Alone, Halo of Flies, Transcedence
    • End stage: Floor 8 (Mom's Heart / Womb 2)
    • No item rooms
    • Required achievements: Transcendence item unlocked (Beat Mom's Heart 3 times)
    • Blindfold item (Isaac can't shoot)
  1. Suicide King
  • Starting Items: IPECAC, My Reflection, Mr. Mega
    • End stage: Floor 9 (Isaac / Cathedral)
    • No item rooms
    • Start as Lazarus
    • Required Achievements: It Lives unlocked (10 Mom's Heart kills), Lazarus character unlocked.
  1. Cat Got Your Tongue
  • Starting Items: Guppy's Tail,
  1. Demo Man
  1. Cursed!
  1. Glass Cannon
  1. When Life Gives You Lemons
  1. BEANS!
  1. Its In The Cards
  1. Slow Roll
  1. Computer Savy
  1. WAKA WAKA
  1. The Host
  1. The Family Man
  1. The Purist

Everything is Terrible

  • Increased chance to get a curse
  • Less heart drops from bosses
  • Other effects go here

Other notes

Credits

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