How to Step the Physics World in planck.js?

Advancing the physics simulation time step in planck.js requires calling the world.step() method within a continuous game or animation loop. This article explains how to configure the time step, manage velocity and position iterations for precision, and implement a robust game loop using requestAnimationFrame to ensure smooth rigid-body physics simulations in JavaScript.

Understanding the World Step Method

In planck.js (a 2D physics engine rewritten from Box2D for JavaScript), the simulation moves forward in time by updating the positions and velocities of all active bodies. This is handled by the World.step() function, which takes three primary arguments:

world.step(timeStep, velocityIterations, positionIterations);

1. Time Step (timeStep)

The amount of time the simulation should advance, measured in seconds. For a smooth simulation, this value should remain constant rather than variable. A typical value is 1/60 (representing 60 frames per second).

2. Velocity Iterations (velocityIterations)

The number of constraint solver calculations dedicated to calculating forces and velocities. Higher values prevent bodies from clipping through each other but require more CPU power. A standard recommendation is 8.

3. Position Iterations (positionIterations)

The number of calculations dedicated to resolving overlapping bodies and positioning them correctly. A standard recommendation is 3.

Implementing the Animation Loop

To make your physics world dynamic, you must call the step function repeatedly. Using a fixed time step inside a requestAnimationFrame loop is the standard approach for web development.

Here is a basic implementation of a physics loop:

import { World } from 'planck';

// Initialize the physics world
const world = new World();

const timeStep = 1 / 60; // 60 FPS
const velocityIterations = 8;
const positionIterations = 3;

function update() {
  // Advance the physics world
  world.step(timeStep, velocityIterations, positionIterations);

  // Clear your canvas and render your bodies here
  // renderMyGraphics();

  // Request the next frame
  requestAnimationFrame(update);
}

// Start the loop
requestAnimationFrame(update);

Best Practices for Stable Simulations