<p align="center"> <img src="docs/media/banner.png" alt="The Binding of Isaac — Native Recomp" width="100%"> </p> <p align="center"> <img alt="Windows" src="https://img.shields.io/badge/Windows-64--bit-8c1610?style=for-the-badge"> <img alt="Linux" src="https://img.shields.io/badge/Linux-64--bit-8c1610?style=for-the-badge"> <img alt="Android" src="https://img.shields.io/badge/Android-7.0+-8c1610?style=for-the-badge"> <img alt="Nintendo Switch homebrew" src="https://img.shields.io/badge/Switch-homebrew-8c1610?style=for-the-badge"> <img alt="60 fps" src="https://img.shields.io/badge/60_fps-1080p-28180f?style=for-the-badge"> <img alt="Requires the original game" src="https://img.shields.io/badge/requires-the_Steam_game-e8dcc8?style=for-the-badge&labelColor=28180f"> </p> <p align="center"> <b>The Binding of Isaac (Flash), rebuilt as a native program.</b><br> No Flash Player, no emulator: the same game you know, at 60 fps and in high resolution,<br> on PC, Android and Nintendo Switch. </p>

[!IMPORTANT] This recomp was made by whylurking with the help of Artificial Intelligence. It is a free, non-profit fan project with no affiliation to Edmund McMillen, Florian Himsl, Nicalis, Valve or Nintendo.

The game is not included. To play, you need your own original copy from Steam: The Binding of Isaac and, for the expansion, Wrath of the Lamb.

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😢 What is this?

The Binding of Isaac came out in 2011 as a Flash game. Flash is gone, and the Steam version still relies on an old player that runs at 30 fps, in a small window, with the performance limits of its time.

This project translates all of the game's original code into C++ and runs it on a modern, native engine written from scratch. Graphics, sounds and music are read from your own Steam Isaac.exe at startup, so you need the original game. What changes is how it runs, and where:

	Original Flash	Recomp
Frame rate	30 fps	60 fps (or whatever your display does: 120, 144…)
Resolution	800×600, upscaled	Native (1080p, 1440p, 4K…)
Game logic	30 Hz	Exactly 30 Hz: same physics and patterns, checked frame by frame against the original
Performance	Can stutter with many enemies	Smooth, with drawing spread across CPU cores
Controllers	Needed JoyToKey	Supported out of the box, with analog movement
Versions	One per install	Original, Wrath of the Lamb and Eternal Edition in one program
Platforms	Windows / Mac	Windows, Linux, Android and Nintendo Switch (homebrew)

📥 Downloads

Everything is on the Releases page. Pick the package for your device:

Platform	File	Guide
Windows (64-bit)	IsaacRecomp_Win64.zip	Installation (PC)
Linux (64-bit)	IsaacRecomp_Linux64.tar.gz	Installation (PC)
Android (phone or tablet)	IsaacRecomp.apk	Android
Nintendo Switch	IsaacRecomp_Switch.zip	Nintendo Switch

On every platform you also need the game's Isaac.exe from your Steam copy (see Get the original game).

🕹️ Comparisons

30 fps → 60 fps

The logic still runs at 30 ticks per second, just like the original, so the game feels exactly the same. Every in-between frame is drawn by interpolating between the last two ticks: movement, animations, fades and room transitions all play at your display's refresh rate.

<p align="center"> <img src="docs/media/compare_60fps_slowmo.gif" alt="30 fps vs 60 fps (slow motion)" width="100%"><br> _{Shown in slow motion (×0.5) so the difference is visible on any screen. <a href="https://github.com/user-attachments/assets/cd45b188-6449-4ee5-8da7-b818399a5756">Real-time 60 fps video</a>.} </p>

Real high resolution

Vector art is rasterized at your screen's actual resolution, and the layers the game used to paint at 640×480 (floor, blood, shadows, minimap) are generated at 1440×1080.

<p align="center"> <img src="docs/media/sharpness.png" alt="Original window upscaled vs native 1080p" width="100%"> </p>

All three Steam versions

If you have more than one version of the game, the program recognizes each one by its content and lets you pick at startup. Each version keeps its own save file.

<p align="center"> <img src="docs/media/versions.png" alt="Original, Wrath of the Lamb and Eternal Edition" width="100%"> </p> <p align="center"> <img src="docs/media/selector.png" alt="Version selection screen" width="60%"> </p>

✨ What's new compared to the original

• 60 fps or more, with the game logic untouched at 30 Hz.
• Native 1080p / 4K, with no thin white seams between colored areas (shapes are rasterized the way Flash does it).
• No pop-in: graphics are prepared in the background while the opening cutscene plays.
• Smooth room transitions, with no sudden jumps after a hiccup.
• Controller support without external tools. Every menu works with the controller alone: the selected option is highlighted in pink, A/✕ presses it and B/◯ goes back.
• Control rebinding inside the game (Options → Controls → Rebind controls) for keyboard, controller and touch.
• Analog movement: with a stick, Isaac's speed follows how far you push it, like in Rebirth (it can be turned off).
• Touch controls on Android and on Switch in handheld mode: floating joysticks, tappable HUD and a pause button.
• Low input latency: input keeps being read while the program waits for the display.
• Cleaned-up options menu: options that depended on Flash and no longer do anything are gone.
• Fullscreen or windowed (F11 or Alt+Enter), fitted to your desktop's usable area.
• Built-in performance overlay (F3).
• Original, Wrath of the Lamb and Eternal Edition in a single program.
• Save converter to bring your progress from the Flash version (and back).
• Mods, turned on and off with a mod manager: StatOn shows your stats and the devil / angel room chances on screen as in Repentance, GenesisConsole is a Rebirth-style debug console, WideSaac fills wide screens (with Hyper Zoom, the room fills the screen, and Minimap+), PillLog shows the effect of the pills you already know, Itemnomicon tells you what an item, trinket or card does before you pick it up, BaseTime adds speedrun timers, The Gone is a new character in the spirit of The Lost, and RainbowVision recolours the game for colour-blind players.

📦 Installation (PC)

1. Get the original game

Buy The Binding of Isaac on Steam (and Wrath of the Lamb if you want the expansion). Install it and open its folder: in Steam, right-click the game → Manage → Browse local files.

There are two executables in there:

File	What it is
Isaac.exe	The game itself: it contains all of its graphics, sounds and code. This is the one you need.
Binding_of_Isaac.exe	Just the Steam launcher. Not useful here.

2. Download the recomp

Go to Releases and grab the package for your system:

• Windows (64-bit): IsaacRecomp_Win64.zip
• Linux (64-bit): IsaacRecomp_Linux64.tar.gz

3. Put them together and play

1. Extract the recomp anywhere you like.
2. Copy Isaac.exe from Steam next to IsaacRecomp.exe (on Linux, next to isaac).
3. Run IsaacRecomp.exe (on Linux: ./isaac).

[!TIP] Several versions at once? On Steam they are all called Isaac.exe. Copy them with different names, for example Isaac-Original.exe, Isaac-WotL.exe and Isaac-Eternal.exe. The name doesn't matter: each version is recognized by its content, and the program asks which one to open.

Requirements: 64-bit Windows or Linux, and a graphics card that supports OpenGL 3.3.

💾 Bringing your Flash saves over

Every package includes tools/save_converter.html: open it in any web browser (it works offline, nothing is uploaded). It turns the Flash game's so.sol into the IsaacRecomp save file and back, for the original game, Wrath of the Lamb and Eternal Edition. Stats, items, secrets, bosses and unlock order carry over. Close the game and back up your save before replacing it. The converter was originally made by g333nza.

🧩 Mods

Mods are small optional changes, kept as .mod files in the mods folder next to the program (Android: IsaacRecomp/mods/; Switch: sd:/switch/isaac/mods/; the program puts the mods that come with it there the first time it sees them, and one you delete stays deleted). They are all off until you turn them on.

Which mods are on, and their options, are in one text file: mods/mods.txt (the game writes it the first time it starts). Change it with any text editor - one [id] section per mod, on = 1 or on = 0, and its options, each explained on the line above - or open tools/mod_manager.html in any web browser (offline, nothing to install, no folder to choose): flip the switches, change the options, save mods.txt and put it in the mods folder in place of the old one (open your current one there first to start from it). Mods are read when the game starts, and log.txt lists which ones are on.

Mod	What it does
StatOn (staton)	Your stats on the left of the screen during a run, each with its symbol, as in Repentance: speed, tears, damage, range, shot speed and luck (luck only in Wrath of the Lamb and Eternal Edition), and the chances that this floor's boss room opens the devil room and the angel room, with the game's own formula. When one changes, the difference shows next to it for a moment, green up and red down. Options: show_changes (0 hides the differences), show_deals (0 hides the devil / angel chances), opacity (10-100), size (1-5, 3 normal), margin (1-5: distance from the left edge). It only reads the game: runs play exactly the same with it on.
GenesisConsole (genesis)	A console to type commands during a run, like Rebirth's: give items, spawn enemies, bosses, pickups, trinkets and cards, change your stats, hearts and pickups, god mode, open every door, teleport to any room or floor. Open it with the key above Tab, the controller's Back / Minus button, or a three-finger tap; the game pauses while it's open. Everything it does is explained in tools/genesis_wiki.html. While it's on, nothing is saved (no achievements, unlocks, statistics or collection; the save itself isn't touched). Its unlock command unlocks everything for that session only (not saved).
WideSaac (widesaac)	The whole screen on wide screens (16:9, 16:10, 21:9...) and any other shape: black bars by default, or with fill = 1 the game's own background fills the sides and the menus fill the whole screen (more of their own sheet, nothing stretched); screen-wide shading (pause, fades) covers the whole screen too. Hyper Zoom (hyper_zoom, a tick box in the mod manager): while you play, the room itself fills the screen; the HUD band and the map go, and your hearts (top right), active item with its charges, coins / bombs / keys and pill / card are drawn on the room's right side. It stays like that the whole run, paused and on the death screen too; a boss's "VS" intro, the cutscenes between floors and the endings are shown in the room's space with black sides, so the zoom never changes. With Hyper Zoom, M (L3 on a controller, a four-finger tap) shows or hides Minimap+: the floor's map, larger and see-through, in the middle of the room (minimap = 0: no map at all). It only changes what is drawn.
PillLog (pilllog)	Pills you already took this run are known by sight: their effect is written over every pill of that colour in the room or for sale (green good, red bad), the pickup banner says the effect instead of "Pills here!", and the pause screen lists the six pill colours of the run with what each one does ("???" until taken). The names are the game's own, as its HUD shows them. Options: labels, banner, pause_list (1/0).
Itemnomicon (itemnomicon)	Walk up to an item, a trinket or a card you can pick up or buy, and a card over it tells you what it is: its name (in the colour of its quality, as Rebirth rates the same items: grey 0, white 1, blue 2, violet 3, gold 4), passive or active item and its quality, the stats it changes (green up, red down) and what it does. The stat changes are the game's own, measured in each version (for Isaac with nothing else). Options: range (0.5-4 tiles, 1 by default), opacity (30-100).
BaseTime (basetime)	Speedrun timers, left of the map (with Hyper Zoom, at the room's bottom left): the game time (mm:ss, while you can move: it stops while paused, in cutscenes and while loading) and below it the total time (mm:ss:mmm, everything since the run started). The main menu shows, on a note of its own, your best game time - the fastest run to your furthest ending (Mom, then Mom's Heart, then Isaac or Satan, then The Chest) - and below it your last run's total time, with "win" or "died".
The Gone (thegone)	A new character in the spirit of Rebirth's The Lost, one more place on the character select screen, with his own card like the others' (name, stats, item): a pale Isaac (3.5 damage) who starts with Spirit of the Night (in the original game, which doesn't have it, Transcendence and the Ouija Board) and has no hearts at all: no hearts, heart containers or extra lives, and the first hit kills him. In a devil room he can take one deal for free; then the other deals disappear. He has to be unlocked first (three floors in a row without a hit or a heart), and he unlocks four new things of his own: the card Negative Isaac (Mom's Heart: a 15-second shield), Nothing (Mom's Heart with no item but his own: some enemies just aren't there), The Void Shard (Isaac: enemy shots may go through you) and Black Mirror (Satan: the room in negative, the enemies fight each other), each with its own "You unlocked" card. Kept in saves/thegone_<game>.txt.
RainbowVision (rainbowvision)	Colours for colour-blind players: the whole picture is recoloured (daltonization) so colours you can't tell apart look different. type: deuteranopia (green-blind, the most common), protanopia (red-blind) or tritanopia (blue-blind); strength (0-100). Only the colours on screen change.

A .mod file is plain text: id, name, version, author, description and the mod's own options as key = value lines. The code behind each mod is built into the program (rt/mods.*, rt/mod_staton.cpp, rt/mod_genesis.cpp, rt/mod_widesaac.cpp, rt/mod_pilllog.cpp, rt/mod_itemnomicon.cpp, rt/mod_basetime.cpp, rt/mod_thegone.cpp), so for now new mods come with new versions of IsaacRecomp.

🎮 PC controls

The original controls, plus controller support. Controller buttons adapt to whether you are playing, paused or in a menu:

Action	Keyboard	Controller (in game)	Controller (menus / pause)
Move	W A S D	Left stick (analog) / D-pad	Choose: left stick / D-pad
Shoot	Arrow keys	Right stick / A B X Y	—
Bomb	E / Shift	LB / left trigger	—
Active item	Space	RB / right trigger	—
Card / pill	Q	Right stick click	—
Pause	P / Esc	Start / Back / left stick click	Start / Back (resume)
Confirm / back	Mouse	—	A / B
Fullscreen	F11 / Alt+Enter	—	—
Screenshot	F12	—	—
Performance overlay	F3	—	—

In menus, the selected option is highlighted in pink; the D-pad or left stick chooses another, the controller presses it with A (✕ on PlayStation controllers) and goes back with B (◯), which also closes the pause screen.

Rebinding: Options → Controls → Rebind controls (from the main menu or the pause screen) changes the controls of the device you are playing with. Pick an action and press the new key or button; Reset to defaults restores the original layout. With a controller you can also choose which stick moves.

📱 Android

The same native recomp, packaged as an APK. It runs the game on the 960×720 stage used on Switch, rendered with OpenGL ES 3.0 and scaled to fill the screen.

You need:

• A 64-bit ARM phone or tablet (practically every device since 2017) with Android 7.0 or newer and OpenGL ES 3.0.
• Your Isaac.exe from your Steam copy of the game (see step 1 of the PC installation).

Installation:

1. Download IsaacRecomp.apk from Releases and install it. Android will ask you to allow "install unknown apps" for your browser or file manager.
2. Create a folder called IsaacRecomp at the root of the internal storage (next to Download, DCIM…) and copy Isaac.exe into it. Several versions work as on PC (different names, a menu to choose).
3. Open Isaac Recomp. The first time, it asks for All files access (Android 11 and newer) or for access to your files: the game, isaac.ini, the saves and log.txt all live in that folder.

Touch controls:

Action	Touch
Move	Finger on the left half: a joystick appears where it lands (analog)
Shoot	Finger on the right half: a second joystick
Bomb	Tap the bomb counter
Active item	Tap the "SPACE" box
Card / pill	Tap it (top right, when you have one)
Pause	The button next to -LIFE-, or the back button / gesture
Menus	Tap the options; on screens without options (cutscenes, title cards) a tap continues

The joysticks are semi-transparent, stay where your finger first landed and disappear when you lift it. Which side moves, the joystick size and its type (fixed base, or a base that follows the finger) can be changed in Options → Controls → Rebind controls. Bluetooth and USB controllers work exactly as on PC, and can be mixed with touch at any time.

Performance: the program renders at 720 lines and, if the device can't keep 60 fps during play, steps down to 600, 540 or 480 by itself (remembered in isaac.ini). It runs at the screen's refresh rate (90 or 120 Hz screens included), with the logic and rendering pinned to the fastest CPU cores, and audio mixed at the device's own rate through Android's low-latency audio path. To choose the resolution yourself, set render_height in isaac.ini.

[!NOTE] While the game is still loading its graphics (the first seconds, during the opening cutscene) touches are ignored; touching shows the progress. Saves are written when the app goes to the background, and the run is paused automatically.

🎮 Nintendo Switch (homebrew)

The same native recomp, built as a homebrew application (.nro, version 1.5.0). It contains no code or assets from Nintendo or from the game: you provide the game's Isaac.exe yourself, exactly as on PC.

You need:

• A Switch that can run homebrew, with the Homebrew Menu.
• Your Isaac.exe from your Steam copy of the game (see step 1 of the PC installation).

Installation:

1. Download IsaacRecomp_Switch.zip from Releases and copy its switch folder to the root of your SD card. You should end up with sd:/switch/isaac/isaac.nro.
2. Copy Isaac.exe into that same folder: sd:/switch/isaac/Isaac.exe. For several versions, use different names (for example Isaac-Original.exe, Isaac-WotL.exe, Isaac-Eternal.exe).
3. Open the Homebrew Menu in application mode: hold R while launching any installed game. Then pick The Binding of Isaac.

[!WARNING] Don't launch it from the Album (applet mode). That mode only gives homebrew about 400 MB of memory, which isn't enough. Application mode has more than 3 GB.

Performance: 720p in handheld mode and 1080p when docked, at 60 fps with interpolation. The game logic runs on its own CPU core, and new graphics are prepared in the background on another core.

Switch controls:

Action	Button
Move	Left stick (analog) / D-pad
Shoot	Right stick, or A / B / X / Y (→ ↓ ↑ ←)
Bomb	L
Active item	ZL
Card / pill	R
Restart the current run	Hold ZR for 5 seconds
Pause	+ or −
Menus	Stick / D-pad choose (in red), A to accept, B to go back
Performance overlay	L3 + R3

Buttons can be changed in Options → Controls → Rebind controls.

Touch screen (handheld mode): the same touch controls as on Android: joysticks on the left and right halves of the screen, taps on the bomb counter, active item and card/pill, a pause button next to -LIFE-, and taps in menus. The pause button is only shown in handheld mode.

⚙️ Configuration

isaac.ini is created on first launch next to the program (Android: in the IsaacRecomp folder; Switch: sd:/switch/isaac/isaac.ini).

Option	Default	What it does
swf	(empty)	Path to Isaac.exe, if it isn't next to the program
game	(empty)	Last chosen version: og, wotl or eternal
fullscreen	1	Borderless fullscreen (PC)
vsync	1	Vertical sync
smooth	1	Interpolation to the display refresh rate (0 = original 30 fps)
filters	1	Blur effects in cutscenes
volume	100	Master volume (0-100)
cache_mb	depends	Video memory for already rasterized graphics
low_latency	1	Keep reading input while waiting for the display (less input lag)
interp_lead	0	Experimental, up to 0.5: draw slightly ahead for less visual delay (small overshoot when stopping)
gc_after_tick	1	Free memory after each game tick has been sent to the screen, so it doesn't delay the image (0 = inside the tick, as in 1.3)
max_margin	0 (Switch: 6)	Cap, in ms, of how far behind the game the image is shown (0 = automatic, 4-30 ms). A cap keeps input delay constant in busy rooms
performance	0	1 = performance overlay always on, written to performance.txt
joystick_acceleration	1	Analog movement: speed follows how far the stick is pushed (full speed at ¾ of its travel). 0 = always full speed
touch_controls	1	Android / Switch: 0 turns off the touch joysticks, HUD taps and pause button
render_height	0	Android: rendering resolution (0 = automatic; 1080, 720, 600, 540, 480)
mods	(empty)	The mods that are on, by id, separated by commas: only until mods/mods.txt exists
bind_keyboard, bind_controller, controller_move_stick, touch_move_side, touch_joystick_size, touch_joystick_type	(empty)	Written by the rebinding screen; empty = original controls

Save files:

Platform	Folder
Windows	The saves folder next to IsaacRecomp.exe
Linux	The saves folder next to isaac
Android	IsaacRecomp/saves/ in the internal storage (up to 1.3.1: IsaacRecomp/, copied over by 1.4.0)
Switch	sd:/switch/isaac/saves/ (up to 1.3.1: sd:/switch/isaac/, copied over by 1.4.0)

There is one save file per version (isaac_og_save.sol, isaac_save.sol, isaac_eternal_save.sol), and the format is the same everywhere, so you can copy saves between PC, Android and Switch. Up to 1.3.0 the PC versions saved in %APPDATA%\IsaacRecomp\WrathOfTheLamb\ (Windows) or ~/.local/share/IsaacRecomp/WrathOfTheLamb/ (Linux); 1.3.1 copies those saves to saves the first time it starts.

❓ FAQ

<details> <summary><b>Do I need Flash installed?</b></summary>

No. The recomp doesn't use Flash Player, Ruffle or any emulator. The game's code has been translated to C++ and runs as a native program.

</details> <details> <summary><b>Can I use my saves from the Steam version?</b></summary>

No. Flash stores unlocks in a different format and location, so the recomp starts from scratch. Saves made by the recomp, however, can be copied freely between PC, Android and Switch.

</details> <details> <summary><b>Does it change the difficulty or game speed?</b></summary>

No. The logic runs at exactly 30 ticks per second, just like the original: same physics, same fire rates and same enemy patterns. The 60 fps are purely visual. Analog movement only lets you move slower than the original when the stick is barely pushed; top speed is unchanged, and joystick_acceleration = 0 turns it off.

</details> <details> <summary><b>Android: where do I put the game? It says it can't find it.</b></summary>

In a folder called IsaacRecomp at the root of the internal storage (the same level as Download or DCIM), not inside Download. Also check that the app has All files access in Android's settings. log.txt in that folder says where it looked.

</details> <details> <summary><b>Android: the update won't install over the previous version.</b></summary>

If the new APK was signed with a different key, Android refuses to update it. Uninstall the old app first: saves and settings are kept, because they live in the IsaacRecomp folder, not inside the app.

</details> <details> <summary><b>It stutters or runs slowly on my device. What can I do?</b></summary>

Press F3 in game (L3 + R3 on Switch; on Android, set performance = 1 in isaac.ini). The performance overlay appears and a performance.txt file is written to the save folder. Open an issue and attach that file: it shows exactly which part is taking the time.

</details> <details> <summary><b>What about Rebirth / Afterbirth / Repentance?</b></summary>

No. Those are a different game, rewritten from scratch in C++. This project is only for the original Flash game.

</details>

📬 Contact

Suggestions, bug reports or rights-holder requests (including DMCA notices): whylurking@gmail.com

You can also open an issue for bugs and suggestions.

⚖️ Legal notice

• This repository contains no game assets: no graphics, sounds, music or game files. The program reads them at startup from the Isaac.exe of your own Steam copy.
• You must buy the original game to play, on Steam. If you enjoy this project, please support the original creators.
• The Switch version is homebrew software. It contains no code or assets belonging to Nintendo, and it does not include or facilitate anything to bypass the console's protections.
• The Android version is distributed only as an APK on this page; it is not published on any app store.
• This is a free, non-profit fan project with no affiliation to Edmund McMillen, Florian Himsl, Nicalis, Valve or Nintendo. The Binding of Isaac is © Edmund McMillen and Florian Himsl. All trademarks belong to their respective owners.
• If you are a rights holder and have any concern, please write to the contact address above and it will be addressed promptly.

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🔬 For the curious: how it's made

This part is technical. If you just want to play, everything you need is above.

Static recompilation from AVM1 to C++

The game is written in ActionScript 2, which runs on Flash 8's AVM1 virtual machine. Instead of interpreting it, this project translates every block of bytecode into a C++ function that is compiled together with the engine:

flowchart LR
    A["Isaac.exe<br/>(Zinc projector)"] -->|decrypt + zlib| B["Original SWF"]
    B -->|FFDec: root script| C["HD patch<br/>(patchhd.py)"]
    C --> D["Patched SWF"]
    D -->|tools/gen.py| E["Generated C++<br/>(one namespace per game)"]
    E --> F["isaac / IsaacRecomp.exe / isaac.nro / IsaacRecomp.apk"]
    G["Native engine<br/>(rt/: SDL2 + OpenGL 3.3 / ES 3.0)"] --> F
    A -.->|at runtime:<br/>graphics, sounds, music| F

	Original	Wrath of the Lamb	Eternal Edition
Frame scripts	495	679	712
Functions	180	225	245
Button actions	54	62	62
Generated C++ lines	87,255	116,915	146,142
Preloaded graphics	1,883	2,336	2,357

• No interpreter. All 47 AVM1 opcodes the game uses are translated to C++. The VM stack is resolved at compile time whenever possible, so operands live in local variables instead of a dynamic stack.
• Faithful Flash Player 8 semantics, ported from Ruffle. This covers scopes, for-in order, the per-frame action queue, gotoAndStop rewinding with PlaceObject merging, and script depths.
• Three games, one executable. Each version is generated into its own namespace (fl::og, fl::wotl, fl::eternal) with its own script tables. At load time, the version is identified by the SWF's CRC-32 and its code is bound in.
• Two stage sizes. PC uses a 1440×1080 stage; Switch and Android use a 960×720 one (gen720/) to save memory on the game's bitmaps.

Reading the game from Isaac.exe

The Steam version is an MDM Zinc projector packed with UPX. After the PE image there is a container holding the SWF: CWS, zlib level 6, prefixed by its length and encrypted with an XOR key k(i) = a + 0x21·i. The value of a changes with each executable (0xd5 original, 0x83 WotL, 0xa1 Eternal) and is derived from the CWS header. The decrypted SWF is byte-for-byte identical to the original.

Engine

• Two main threads. The logic runs at a fixed 30 Hz and publishes an immutable snapshot of the display list every tick. The renderer draws at the display's refresh rate, interpolating between the last two snapshots. Scale and rotation are interpolated separately, and jumps are detected so they aren't smeared.
• Vector rasterization with PlutoVG, spread across CPU cores and packed into a 2048² texture atlas.
• No antialiasing seams. Flash rasterizes each shape in one pass, but painting each fill separately lets the background bleed through shared edges. A saturate operator was added to PlutoVG that paints front to back with dst += src · min(coverage, 1 − dst.a). <p align="center"><img src="docs/media/seams.png" alt="Antialiasing seams before and after" width="80%"></p>
• GPU BitmapData. Layers the game only displays are drawn as GPU operations into an FBO. Their pixels only come back to the CPU if the code reads them.
• Asynchronous preloading during the opening cutscene, from a per-game built-in list that grows with what each player sees.
• Mark-and-sweep garbage collector with a time-budgeted, incremental sweep, so it never makes a tick miss its deadline.
• Analog movement. The game only knows eight digital directions. A native wrapper around its movement routine scales the player's acceleration by how far the stick is pushed (dead zone, then 40 % → 100 % at ¾ of the stick's travel), so top speed and the game's physics stay untouched.
• Audio is mixed at the output device's own sample rate with cubic interpolation, so there is a single resampling step.
• Nintendo Switch. Logic, rendering and rasterization are pinned to separate cores with tuned thread priorities, because Horizon doesn't time-slice threads of equal priority. Frame pacing uses GPU fences so that no more than one frame is queued.
• Android. Built as a native library started by SDL's Java activity. The OpenGL ES 3.0 renderer produces the same image as the desktop one. Resolution adjusts automatically to keep 60 fps; logic and rendering run on the big CPU cores; blur effects have a per-frame budget for mobile GPUs; and audio uses AAudio in low-latency mode.

Testing

• Deterministic headless mode (--headless): input scripts, screenshots at given ticks, and a random-input seed (--monkey).
• Regression: the same seed produces the same logic sequence across engine versions, and screenshots are compared pixel by pixel.
• Differential tests against the original (tools/difftest/): a patched Ruffle runs the same game file with the same input, random numbers and clock, and the game state (every enemy, tear and pickup, the player, the room) is compared frame by frame. Every enemy, boss, item, pill, card and trinket of the three versions plays identically, and the original 800x600 game file plays identically to the high-resolution one.
• Stability: tens of thousands of ticks of random input on all three versions, with no failures.
• Mobile code on PC: the OpenGL ES renderer (ISAAC_GLES=1) and the touch controls (ISAAC_TOUCH=1, plus scripted synthetic touches) can be exercised on a desktop build.

Building from source

# Linux
cmake -G Ninja -B build && ninja -C build

# Windows (MinGW-w64 cross-compile)
cmake -G Ninja -B build-win -DCMAKE_TOOLCHAIN_FILE=cmake/mingw64.cmake \
      -DCMAKE_PREFIX_PATH=<SDL2-mingw>/x86_64-w64-mingw32 && ninja -C build-win

# Nintendo Switch (devkitPro: devkitA64 + libnx + switch-sdl2 + switch-mesa)
export DEVKITPRO=/opt/devkitpro
cmake -G Ninja -B build-switch -DCMAKE_TOOLCHAIN_FILE=cmake/switch.cmake && ninja -C build-switch

# Android (Android SDK + NDK r27c, or open android/ in Android Studio)
cd android && ./gradlew assembleRelease

The Android APK is also built automatically by GitHub Actions (Actions → Android APK); see android/README.md. Design notes, and how to regenerate the code from the SWFs, are in NOTES.md.

Credits

• whylurking: creator of the recomp.
• Artificial Intelligence (Claude, by Anthropic): development assistance.
• Edmund McMillen and Florian Himsl: creators of The Binding of Isaac.
• g333nza: the original save converter.
• Ruffle: reference for AVM1 semantics.
• JPEXS FFDec: decompiler used by the tooling.
• SDL2, PlutoVG, stb_image, dr_mp3, libnx and Mesa: their licenses ship with each package.

<p align="center">_{Made with 😢 and plenty of tears by whylurking.}</p>