Tutorial 4: Collision Detection

Use engine collision helpers: Body2D + Collision for platformers, World solids / Trigger for top-down.

What you'll learn

• AABB via Collision / Body2D.step
• Platforms as Body2D.Solid
• Top-down blocking tiles and trigger volumes

Do not invent a second physics/entity system. Prefer examples/java/platformer and examples/java/rpg.

Platformer body

Body2D body = new Body2D(x, y, 16, 16);
body.setVelocityX(3f);
if (input.isKeyPressed(KeyEvent.VK_SPACE)) {
    body.jump();
}
body.step(platforms); // List<? extends Body2D.Solid>

Platforms implement Body2D.Solid (getX/Y/Width/Height). See examples/java/platformer Platform + Player.

One-shot playtest

# playtests/jump.script
0 press SPACE
1 release SPACE
4 idle

PlaytestScript.loadClasspath("playtests/jump.script")
    .play(harness, (SimulatedInput) game.getManager().getInputHandler());
assertTrue(game.getPlayer().getY() < groundedY);

Top-down

• Solids — world.setTile(tx, ty, World.SOLID) or GameSpec world.solid=5,7;6,7
• Pickups / doors — world.addTrigger(new Trigger(x, y, w, h, actor -> { ... }))

World world = GameSpec.loadClasspath("gamespec.properties").createWorld();
world.fillBorder(World.SOLID);
world.addTrigger(new Trigger(coinX, coinY, 16, 16, a -> score++));

Low-level AABB

boolean hit = Collision.aabb(ax, ay, aw, ah, bx, by, bw, bh);
Collision.resolveAabb(...); // see COOKBOOK / Body2D internals

Legacy

TransmuteCore.ecs.Object / TiledLevel still exist; prefer World / Body2D for new work (AUTHORING.md).

Next

Tutorial 5: State Management