planck-js/example/Breakable.ts

2D JavaScript/TypeScript physics engine for cross-platform HTML5 game development

/*
 * Copyright (c) Erin Catto
 * Licensed under the MIT license
 */

// This is used to test sensor shapes.

import { World, Vec2, Edge, Box, Testbed } from "planck";

const world = new World({
  gravity: { x: 0, y: -10 },
});

const testbed = Testbed.mount();
testbed.start(world);

let breakVelocity: Vec2;
let breakAngularVelocity: number;

let broke = false;

// Ground body
const ground = world.createBody({
  type: "static",
});
ground.createFixture({
  shape: new Edge({ x: -40.0, y: 0.0 }, { x: 40.0, y: 0.0 }),
  density: 0.0,
});

// Breakable dynamic body
const body1 = world.createBody({
  type: "dynamic",
  position: { x: 0.0, y: 40.0 },
  angle: 0.25 * Math.PI,
});

const shape1 = new Box(0.5, 0.5, { x: -0.5, y: 0.0 }, 0.0);
const piece1 = body1.createFixture({
  shape: shape1,
  density: 1.0,
});

const shape2 = new Box(0.5, 0.5, { x: 0.5, y: 0.0 }, 0.0);
let piece2 = body1.createFixture({
  shape: shape2,
  density: 1.0,
});

world.on("post-solve", function (contact, impulse) {
  if (broke) {
    // The body already broke.
    return;
  }

  // Should the body break?
  const count = contact.getManifold().pointCount;

  let maxImpulse = 0.0;
  for (let i = 0; i < count; ++i) {
    maxImpulse = Math.max(maxImpulse, impulse.normalImpulses[i]);
  }

  if (maxImpulse > 40.0) {
    broke = true;
    world.queueUpdate(breakIt);
  }
});

function breakIt() {
  // Create two bodies from one.
  const center = body1.getWorldCenter();

  body1.destroyFixture(piece2);

  const body2 = world.createBody({
    type: "dynamic",
    position: body1.getPosition(),
    angle: body1.getAngle(),
  });

  piece2 = body2.createFixture({
    shape: shape2,
    density: 1.0,
  });

  // Compute consistent velocities for new bodies based on
  // cached velocity.
  const center1 = body1.getWorldCenter();
  const center2 = body2.getWorldCenter();

  const velocity1 = Vec2.add(
    breakVelocity,
    Vec2.cross(breakAngularVelocity, Vec2.sub(center1, center)),
  );
  const velocity2 = Vec2.add(
    breakVelocity,
    Vec2.cross(breakAngularVelocity, Vec2.sub(center2, center)),
  );

  body1.setAngularVelocity(breakAngularVelocity);
  body1.setLinearVelocity(velocity1);

  body2.setAngularVelocity(-breakAngularVelocity);
  body2.setLinearVelocity(velocity2);
}

testbed.step = function () {
  // Cache velocities to improve movement on breakage.
  if (!broke) {
    breakVelocity = body1.getLinearVelocity();
    breakAngularVelocity = body1.getAngularVelocity();
  }
};