🏍️ ReGravity Defied

An open-source port of the classic J2ME motorcycle-trials game Gravity Defied

Written in C — bare-metal arm-none-eabi on GBA (no SDK, just the hardware), plus native Nintendo DS / DSi (devkitARM + libnds), Nintendo 3DS / 2DS (devkitARM + libctru, with stereoscopic 3D), and Sony PSP (pspdev / pspsdk) builds, all driven from one shared game core.

🔗 github.com/antoxa2584x/regravity_defied

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✨ Features

🎯 Four targets, one core	A portable game core behind a small platform layer builds a bare-metal GBA .gba, a native DS/DSi .nds, a 3DS/2DS .3dsx, and a Sony PSP EBOOT.PBP — identical physics, menus, and progression on all four.
🏁 Authentic physics	Verlet-integrated bike dynamics ported from the original — 3 engine leagues (100cc · 175cc · 220cc).
🧍 Articulated rider	A jointed sprite (legs, torso, arm, helmet) that leans with the bike.
🎞️ Native-res rendering	Software rasteriser into a per-target framebuffer, fixed 60 Hz simulation fully decoupled from the frame rate, filling each screen 1:1 — 240×160 (GBA), 256×192 (DS), 400×240 (3DS), 480×272 (PSP).
🖥️ Dual-screen	On DS and 3DS the bottom screen carries the track-detail card and a live in-game progress minimap instead of crowding the playfield.
🥽 Stereoscopic 3D	The 3DS top screen renders once per eye with per-layer parallax (track, bike, flags pop at different depths), dialed in live by the 3D slider.
🎨 Customization	Recolor the helmet, rider suit, and bike from a shared palette, with a live preview; choices persist with your save.
🔓 Progression	The first track is open by default; finishing a track unlocks the next, and clearing a league unlocks the next league.
⏱️ Best times & cursor	Per-track scoreboard plus the last track opened per league, persisted per platform — GBA battery SRAM, or an SD-card / Memory Stick save file on DS/3DS/PSP.
⚡ Fast navigation	Hold Up/Down (or the analog stick) to auto-scroll long league/level lists.
🔊 Sound	Crash SFX on every target — GBA DirectSound, DS ARM7 soundPlaySample, 3DS ndsp, PSP sceAudio.
⚙️ Settings	Pick tilt buttons (D-pad or L/R shoulders), toggle sound, reset progress (with confirmation), and an About screen.
🎛️ Platform-aware UI	Menus center and scale to each screen — the PSP draws the interface at 2× and shows its ✗ / ◯ buttons in prompts instead of A/B.
🧩 Mod packs	Drop a levels/*.mrg file in and each build emits a separate ROM per mod with its name baked onto the menu.

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🚀 Quick Start

1 · Install the toolchain (WSL / Linux)

sudo apt update
sudo apt install gcc-arm-none-eabi binutils-arm-none-eabi make

🐍 Regenerating the asset/sound headers is optional — they're committed. If you want to, install Python with pillow and miniaudio:

pip install pillow miniaudio

2 · Build (GBA)

make            # one ROM per levels/*.mrg  →  release/gba/ReGravity_Defied_<mod>_v<ver>.gba
make assets     # regenerate src/generated/gd_assets.h + gd_sound.h from assets/
make debug      # build with mGBA debug logging (-DDEBUG)
make clean      # remove build/gba and release/gba

Every target writes its ROMs to release/<platform>/ (objects go under build/<platform>/).

3 · Play (GBA)

Run a release/gba/ReGravity_Defied_*.gba in any GBA emulator (mGBA, VisualBoyAdvance, no$gba) or on real hardware via a flashcart. SRAM saves require an emulator/flashcart with save support.

💡 WSL tip: if a Windows emulator has the ROM open, the .gba file is locked and make can't overwrite it — close the emulator before rebuilding.

4 · Build & play (Nintendo DS / DSi)

The DS build needs the devkitPro DS toolchain (devkitARM + libnds + calico + libfat):

# install devkitPro pacman per https://devkitpro.org/wiki/Getting_Started, then:
sudo dkp-pacman -S nds-dev
export DEVKITPRO=/opt/devkitpro

make -f Makefile.nds          # one .nds per levels/*.mrg  →  release/nds/ReGravity_Defied_<mod>_v<ver>.nds
make -f Makefile.nds clean

Run a release/nds/ReGravity_Defied_*.nds in a DS emulator (melonDS, DeSmuME) or on real hardware — flashcart on DS/DS Lite, or directly on a homebrew-enabled DSi/3DS (the .nds is flagged DSi-compatible). Saves are written to the SD card as regravity.sav.

🆕 The DS/DSi target is new: it builds, links, and packages a valid .nds, but hasn't yet been verified on hardware/emulator. The GBA build remains the primary, battle-tested target.

5 · Build & play (Nintendo 3DS / 2DS — with stereoscopic 3D)

The 3DS build needs the devkitPro 3DS toolchain (devkitARM + libctru):

# install devkitPro pacman per https://devkitpro.org/wiki/Getting_Started, then:
sudo dkp-pacman -S 3ds-dev
export DEVKITPRO=/opt/devkitpro

make -f Makefile.3ds          # one .3dsx per levels/*.mrg  →  release/3ds/ReGravity_Defied_<mod>_v<ver>.3dsx
make -f Makefile.3ds icon     # (optional) generate the HOME-menu app icon (needs Pillow)
make -f Makefile.3ds clean

Run a release/3ds/ReGravity_Defied_*.3dsx in Citra or on a homebrew-enabled 3DS/2DS (copy to the SD card and launch from the Homebrew Launcher). Saves are written to the SD card root as regravity_defied.sav.

The 3DS is the second dual-screen target, so it reuses the DS layout — the track-detail card and the in-game progress minimap live on the bottom screen. On top of that it drives the autostereoscopic top screen: the flat 2D scene is rendered once per eye with per-layer horizontal parallax (the track sits at the screen plane, the bike floats toward you, and the flags pop out the most — a pop-up-book depth). Slide the 3D slider up to dial in the effect; at zero the image is a comfortable flat 2D. The Circle Pad mirrors the D-pad for steering.

🆕 The 3DS target is new: it builds against libctru and packages a valid .3dsx, but hasn't yet been verified on hardware/emulator. The GBA build remains the primary, battle-tested target.

6 · Build & play (Sony PSP)

The PSP build needs the pspdev toolchain (psp-gcc + pspsdk + mksfoex + pack-pbp):

# install pspdev per https://github.com/pspdev/pspdev (or `brew install pspdev`),
# then make sure psp-config is on PATH:
make -f Makefile.psp          # per levels/*.mrg: a game folder + a .zip of it
make -f Makefile.psp icon     # (optional) regenerate the 144×80 Game-menu icon (needs Pillow)
make -f Makefile.psp clean

Each mod is emitted as a ready-to-run game folder, release/psp/ReGravity_Defied_<mod>_v<ver>/EBOOT.PBP, with a 144×80 ICON0.PNG (generated from assets/icon.jpg) so the entry shows artwork in the Game menu — plus a same-named .zip of that folder (ReGravity_Defied_<mod>_v<ver>.zip, containing <game>/EBOOT.PBP) for distribution. Run an EBOOT.PBP in the PPSSPP emulator, or install it on a homebrew-enabled PSP (see Install). The PSP has a single screen, so it uses the GBA single-screen layout (with the interface drawn at 2× and reflowed for the larger panel), rendered at the LCD's native 480×272. Cross accelerates, Circle/Square brake, the D-pad or analog stick lean/steer, and the shoulder triggers are L/R — and the on-screen prompts show the PSP's ✗ / ◯ glyphs instead of A/B. Saves are written to the Memory Stick at ms0:/PSP/SAVEDATA/regravity_defied.sav.

🆕 The PSP target is new: it builds against pspsdk and packages a valid EBOOT.PBP, but hasn't yet been verified on hardware/emulator. The GBA build remains the primary, battle-tested target.

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📥 Install on hardware

Each build lands in release/<platform>/. Pick your mod's ROM and follow the steps for your device. All targets run in an emulator too (see each build section above).

Game Boy Advance — release/gba/*.gba

1. Copy the .gba onto a GBA flashcart's SD/CF card (EZ-Flash, EverDrive-GBA, …).
2. Launch it from the cart's menu.
3. Saves need a flashcart with SRAM save support; progress is stored in the ROM's battery SRAM.

Nintendo DS / DSi — release/nds/*.nds

• DS / DS Lite: copy the .nds to a DS flashcart (R4, etc.) and launch from its menu.
• DSi / 3DS (homebrew): the .nds is DSi-flagged — run it from the SD card via TWiLight Menu++ or nds-bootstrap.
• Saves are written to the SD card as regravity.sav.

Nintendo 3DS / 2DS — release/3ds/*.3dsx

1. On a homebrew-enabled console, copy the .3dsx to the SD card's /3ds/ folder.
2. Launch it from the Homebrew Launcher.
3. Saves are written to the SD card root as regravity_defied.sav.

Sony PSP — release/psp/*.zip

1. On a CFW / homebrew-enabled PSP, extract the .zip into ms0:/PSP/GAME/ on the Memory Stick (it already contains a ReGravity_Defied_<mod>_v<ver>/EBOOT.PBP folder, so it lands at ms0:/PSP/GAME/ReGravity_Defied_<mod>_v<ver>/EBOOT.PBP).
2. The entry appears in the PSP's Game menu with its icon; launch it there.
3. Saves are written to ms0:/PSP/SAVEDATA/regravity_defied.sav.

Equivalently, copy the unzipped release/psp/ReGravity_Defied_<mod>_v<ver>/ folder directly into ms0:/PSP/GAME/ — the .zip is just that folder, packaged.

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🎮 Controls

Button	Action
A	Accelerate · confirm
B	Brake · back
← / →	Lean (D-pad tilt mode)
L / R	Lean (shoulder tilt mode)
↑ / ↓	Navigate menus · hold to scroll faster
START	Select · start
SELECT	In-game: track menu · League menu: settings

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🗂️ Project Structure

The source tree is split by responsibility: a portable core, the generated asset headers, and one platform backend folder per target. The core talks to each backend only through the small interface in core/platform.h, so adding a target is a new platform/<name>/ folder plus a Makefile.

src/
├── core/                    portable game code + the platform interface (built for every target)
│   ├── main.c               game loop, state machine, menus, progression
│   ├── physics.c/.h         bike physics + rider rendering
│   ├── graphics.c/.h        drawing primitives + back buffer
│   ├── level.c/.h           track decoding and rendering
│   ├── save.c/.h            save logic (progress, best times, cursor)
│   ├── platform.h           color/keys/timing + platform_init/keys/timer/vsync interface
│   └── sound.h              SFX interface
│
├── generated/               committed, tool-generated (do not hand-edit)
│   ├── gd_assets.h           sprites/images  (tools/convert_assets.py)
│   ├── gd_sound.h            crash SFX PCM   (tools/convert_sound.py)
│   └── gd_sprites.h          flag sprites
│
└── platform/                one hardware backend per target (platform_/graphics_/sound_/save_)
    ├── gba/   …_gba.c + gba.h (register map) + crt0.s (startup) + gba.ld (linker script)
    ├── nds/   …_nds.c         LCDC framebuffer · libnds sound · libfat SD save
    ├── 3ds/   …_3ds.c         libctru gfx + stereo 3D · ndsp sound · stdio SD save
    └── psp/   …_psp.c         GU 480×272 present · sceAudio SFX · Memory Stick save

Makefile  GBA  ·  Makefile.nds  DS/DSi  ·  Makefile.3ds  3DS/2DS  ·  Makefile.psp  PSP
build/<platform>/      intermediate objects        release/<platform>/   final ROMs
assets/                source PNGs + sound/ + icon.jpg; tools/*.py bake them into src/generated/ + the DS/3DS icons
levels/*.mrg           track-data mod packs — one ROM is built per file

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🛠️ How the Mod Build Works

Each levels/<name>.mrg is compiled into its own build/gba/<name>/ directory with MOD_NAME baked in, embedded via objcopy, and linked into a standalone release/gba/ReGravity_Defied_<name>_v<ver>.gba. The mod name shows up on the league screen, so you can ship a classic ROM, a 1000-track ROM, or your own pack side by side.

Makefile.nds, Makefile.3ds and Makefile.psp mirror this exactly for the DS, 3DS and PSP, producing one .nds / .3dsx / game-folder EBOOT.PBP per mod under release/<platform>/.

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🎯 GBA vs. DS vs. 3DS vs. PSP — what differs

The game logic, rendering primitives, menus and physics are identical on all four — only the thin hardware backends change:

	Game Boy Advance	Nintendo DS / DSi	Nintendo 3DS / 2DS	Sony PSP
Toolchain	bare-metal arm-none-eabi, custom crt0.s + gba.ld	devkitARM + libnds + calico	devkitARM + libctru	pspdev + pspsdk
CPU	ARM7TDMI @ 16.78 MHz	ARM9 @ 67 MHz (+ ARM7 for sound)	ARM11 @ 268 MHz	MIPS Allegrex @ 222–333 MHz
Display	Mode 3 bitmap, DMA blit	LCDC framebuffer, native 256×192 full-screen	320×240 centered on the 400-wide top screen; BGR555→RGB565 rotated present	VRAM framebuffer, native 480×272 (512 stride), 5551 present
Second screen	—	detail card + in-game minimap	detail card + in-game minimap	—
Stereoscopic 3D	—	—	per-layer parallax, rendered once per eye, scaled by the 3D slider	—
Frame timing	fixed 60 Hz sim; ~30 fps render	fixed 60 Hz sim; 60 fps render	fixed 60 Hz sim; 60 fps render	fixed 60 Hz sim; 60 fps render
Sound	DirectSound FIFO (hand-fed)	soundPlaySample (ARM7)	libctru ndsp	sceAudio SRC (worker thread)
Save	battery SRAM	regravity.sav on SD via libfat	regravity_defied.sav on SD via stdio	regravity_defied.sav on Memory Stick

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📜 License

Open source, released for educational purposes. This is a fan reimplementation of Gravity Defied — the original game and its assets belong to their respective owners.

_{Built with C, fixed-point math, and a lot of respect for the 16.78 MHz ARM7TDMI.}