Gravity Defied on DragonRuby

▶ Play in the browser · built with DragonRuby · GPL-2.0

An experimental port of the 2004 J2ME motorbike game to DragonRuby GTK, with the original sprites (or the original line-art mode), the original unlock rules and best times.

Gravity Defied was made by Codebrew Software (2004); the sprites and tracks come from that game. Based on the community ports gravity_defied_cpp and gravity-defied-web. Both are GPL-2.0, so this port is GPL-2.0 as well.

Run from source

This repository is the mygame folder of a DragonRuby GTK project. Unzip DragonRuby, replace its mygame folder with this repository and run ./dragonruby.

Publish to GitHub Pages: tools/deploy_pages.sh builds the web version and pushes the landing page (site/) to the root and the game to play/ on the gh-pages branch. --preview only assembles the site into a temp folder.

Menu: Up/Down choose a row, Left/Right change it, Enter ride. Locked tracks and bikes show a lock and what to finish to open them. Riding: arrows or WASD (up gas, down brake, left/right lean), R restart, G sprites/lines, H DragonRuby logo head or helmet, Esc menu.

Start with the 100cc bike and the first Easy and Medium tracks. Finishing a track opens the next one; finishing a level's last track opens the next level and a bigger bike. Progress and the three best times per track and bike live in __save_data__/progress.txt.

Autopilot runs

Press D in the menu on a track that has a recorded run and the game rides it by itself, like a normal ride (nothing is saved). Runs are found offline by beam search over held inputs; the physics is deterministic, so they replay frame for frame:

ruby tools/find_demo.rb <level> <track> <bike>   # writes data/demos/<level>_<track>.txt
ruby tools/bundle_demos.rb                       # rebuilds app/demos_data.rb

Recorded: Easy "Intro", Medium "Spikehops", Pro "Undertaker", "Intense" and "Dantes Peak" (325cc).

Layout

File	What
app/f16.rb	16.16 fixed-point math with int32 wrap-around
app/physics.rb	six point masses joined by springs, collision response
app/track.rb	track polyline, collision, pseudo-3D geometry
app/renderer.rb	world to screen: thin lines, sprites, both graphics modes
app/game.rb	one ride: 30 ms steps, crash/restart timers, finish
app/progress.rb	unlocks and best times, saved as plain text
app/menu.rb, app/hud.rb, app/ui.rb	start screen, in-ride overlay, drawing helpers
app/app.rb	switches between menu, ride and autopilot playback
app/demo.rb, app/demos_data.rb	autopilot playback and the bundled recorded runs
site/	landing page (HTML, CSS, gameplay video); tools/deploy_pages.sh publishes it
app/levels_data.rb	generated from data/levels.mrg by tools/convert_levels.rb

Tests

ruby test/run.rb

Physics is checked bit-exactly against traces recorded from the TypeScript port; the rest covers fixed-point math, unlock rules and the menu/ride flow through fake inputs.

Differences from the original

• The original physics can loop forever when a sub-step halves to zero (e.g. Pro "Liberty" on 325cc); here the rider crashes instead.
• DragonRuby quirks: Integer#/ returns a Float, so integer math goes through F16.idiv; pure black primitives do not show up, so black is drawn as (1, 1, 1).
• No player names in the records table.