How to Perform Ray Cast in Planck.js?

Ray casting is a powerful feature in planck.js (a 2D physics engine for JavaScript) that allows you to project a line segment through a physics world to detect any intersecting fixtures. This article provides a comprehensive guide on how to implement ray casting, including a quick overview of the world.rayCast() method, how to utilize the callback function to handle hit results, and a complete code example to get you started.

Understanding Ray Casting in Planck.js

Ray casting works by drawing an imaginary straight line between a starting point (\(p_1\)) and an ending point (\(p_2\)). The physics engine calculates which fixtures intersect this line and provides detailed information about the intersection point, the surface normal vector, and the fraction along the ray where the hit occurred.

In planck.js, this is achieved using the world.rayCast method, which requires three arguments: the starting point vector, the ending point vector, and a callback function.

world.rayCast(point1, point2, callback);

Implementing the Ray Cast Callback

The core of ray casting lies in the callback function. The physics engine triggers this callback whenever the ray intersects a fixture. The callback function accepts four arguments:

• fixture: The planck.Fixture that the ray intersected.
• point: The planck.Vec2 coordinates where the ray intersected the fixture.
• normal: The planck.Vec2 surface normal vector at the intersection point.
• fraction: A number between 0 and 1 indicating how far along the ray the intersection occurred.

Controlling Ray Behavior with Return Values

Your callback function must return a number to tell the engine how to proceed with the remaining fixtures along the ray:

• **Return -1**: Filter out this fixture and continue casting the ray as if no hit occurred.
• **Return 0**: Terminate the ray cast immediately.
• **Return fraction**: Clip the ray to the current intersection point. The engine will only look for closer intersections.
• **Return 1**: Do not clip the ray; continue finding all intersections along the original path.

Full Code Example

Here is a practical example demonstrating how to cast a ray and capture the closest intersecting fixture in a planck.js world.

const planck = require('planck-js');

// 1. Create a world and a vector helper
const world = planck.World();
const Vec2 = planck.Vec2;

// (Assume bodies and fixtures have been added to the world here)

// 2. Define the ray start and end points
const p1 = Vec2(0.0, 0.0);
const p2 = Vec2(10.0, 0.0);

// 3. Variables to store the closest hit data
let closestFixture = null;
let closestPoint = null;
let closestNormal = null;
let closestFraction = 1.0; // Start at max length

// 4. Perform the ray cast
world.rayCast(p1, p2, function(fixture, point, normal, fraction) {
  // Update closest hit data
  closestFixture = fixture;
  closestPoint = point;
  closestNormal = normal;
  closestFraction = fraction;

  // Clip the ray to this point to only find closer hits
  return fraction;
});

// 5. Use the results
if (closestFixture) {
  console.log(`Hit detected at: X: ${closestPoint.x}, Y: ${closestPoint.y}`);
} else {
  console.log('No intersection detected.');
}