---
title: "Box2D Tutorials"
subtitle: "Ray Casting"
date: 2024-12-16T22:00:00.000Z
url: https://phaser.io/tutorials/box2d-tutorials/ray-casting
site: Phaser - https://phaser.io
copyright: © 2026 Phaser Studio Inc. All rights reserved.
---

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# Box2D Tutorials

*Ray Casting*

Ray casting is often used to find out what objects are in a certain part of the world. A ray is just a straight line, and we can use a function provided by Box2D to check if the line crosses a shape. We can also find out what the normal is at the point the line hits the shape.

In Box2D v3.0, ray casting is done through the world object using these functions:

```javascript
// Cast a ray and get all intersections
b2World_CastRay(worldId, origin, translation, filter, resultCallback, context);

// Cast a ray and get only the closest intersection
b2World_CastRayClosest(worldId, origin, translation, filter);
```

The ray is defined by an origin point and a translation vector. The filter parameter lets you control which shapes are tested against.

![two points connected by a ray that extends further to intersect a polygon before it ends](raycasting-input.png)

If there is an intersection, the result contains the intersection point and normal.

![two points connected by a ray with the intersection point and normal indicated](raycasting-output.png)

### Example

To try out ray casting, let's set up a scene with a fenced area and some shapes floating inside in a zero-gravity environment:

```javascript
// Create world
const gravity = new b2Vec2(0, 0);
const worldDef = b2DefaultWorldDef();
worldDef.gravity = gravity;
const worldId = b2CreateWorld(worldDef);

// Create static body for walls
const bodyDef = b2DefaultBodyDef();
bodyDef.type = STATIC;
bodyDef.position = new b2Vec2(0, 0);
const staticBodyId = b2CreateBody(worldId, bodyDef);

// Create wall shapes
const shapeDef = b2DefaultShapeDef();
shapeDef.density = 0;

// Ground
const groundSegment = new b2Segment(new b2Vec2(-20, 0), new b2Vec2(20, 0));
b2CreateSegmentShape(staticBodyId, shapeDef, groundSegment);

// Ceiling
const ceilingSegment = new b2Segment(new b2Vec2(-20, 40), new b2Vec2(20, 40));
b2CreateSegmentShape(staticBodyId, shapeDef, ceilingSegment);

// Left wall
const leftWallSegment = new b2Segment(new b2Vec2(-20, 0), new b2Vec2(-20, 40));
b2CreateSegmentShape(staticBodyId, shapeDef, leftWallSegment);

// Right wall
const rightWallSegment = new b2Segment(new b2Vec2(20, 0), new b2Vec2(20, 40));
b2CreateSegmentShape(staticBodyId, shapeDef, rightWallSegment);

// Create dynamic bodies
bodyDef.type = DYNAMIC;
bodyDef.position = new b2Vec2(0, 20);
shapeDef.density = 1;

// Create boxes
const boxShape = b2MakeBox(2, 2);
for (let i = 0; i < 5; i++) {
    const bodyId = b2CreateBody(worldId, bodyDef);
    b2CreatePolygonShape(bodyId, shapeDef, boxShape);
}

// Create circles
const circleShape = { radius: 2 };
for (let i = 0; i < 5; i++) {
    const bodyId = b2CreateBody(worldId, bodyDef);
    b2CreateCircleShape(bodyId, shapeDef, circleShape);
}
```

![a bounded region with a number of shapes inside](raycasting-scene-shapes.png)

Now let's create a rotating ray to cast through the scene:

```javascript
let currentRayAngle = 0;

function update() {
    // Update ray angle (one revolution every 20 seconds)
    currentRayAngle += (Math.PI * 2) / (20 * 60);
    
    // Calculate ray points
    const rayLength = 25;
    const origin = new b2Vec2(0, 20);
    const translation = new b2Vec2(
        rayLength * Math.sin(currentRayAngle),
        rayLength * Math.cos(currentRayAngle)
    );

    // Set up query filter
    const filter = b2DefaultQueryFilter();

    // Cast ray and get closest hit
    const result = b2World_CastRayClosest(worldId, origin, translation, filter);

    if (result.hit) {
        // Calculate intersection point
        const hitFraction = result.fraction;
        const intersectionPoint = new b2Vec2(
            origin.x + hitFraction * translation.x,
            origin.y + hitFraction * translation.y
        );

        // Draw ray up to intersection
        drawLine(origin, intersectionPoint);
        drawPoint(intersectionPoint);

        // Draw normal at intersection point
        const normalEnd = new b2Vec2(
            intersectionPoint.x + result.normal.x,
            intersectionPoint.y + result.normal.y
        );
        drawLine(intersectionPoint, normalEnd);
    } else {
        // No hit - draw full ray
        const endpoint = new b2Vec2(
            origin.x + translation.x,
            origin.y + translation.y
        );
        drawLine(origin, endpoint);
    }
}
```

![a white line enters the scene from outside the walls and intersects several shapes](raycasting-scene-ray.png)

To create reflected rays, we can use the normals at each intersection to bounce the ray repeatedly in this recursive function:

```javascript
function drawReflectedRay(origin, translation, depth = 0) {
    if (depth > 8) return; // Limit reflection depth
    
    const filter = b2DefaultQueryFilter();
    const result = b2World_CastRayClosest(worldId, origin, translation, filter);
    
    if (result.hit) {
        // Calculate intersection point
        const hitFraction = result.fraction;
        const intersectionPoint = new b2Vec2(
            origin.x + hitFraction * translation.x,
            origin.y + hitFraction * translation.y
        );

        // Draw ray to intersection
        drawLine(origin, intersectionPoint);

        if (hitFraction < 1.0) {
            // Calculate reflection vector
            const dot = 2 * (
                translation.x * result.normal.x + 
                translation.y * result.normal.y
            );
            
            const remainingTranslation = new b2Vec2(
                translation.x - dot * result.normal.x,
                translation.y - dot * result.normal.y
            );

            // Recurse with reflected ray
            drawReflectedRay(
                intersectionPoint,
                remainingTranslation,
                depth + 1
            );
        }
    } else {
        // No hit - draw full ray
        const endpoint = new b2Vec2(
            origin.x + translation.x,
            origin.y + translation.y
        );
        drawLine(origin, endpoint);
    }
}
```

And use it in the update function:

```javascript
function update() {
    currentRayAngle += (Math.PI * 2) / (20 * 60);
    
    const rayLength = 25;
    const origin = new b2Vec2(0, 20);
    const translation = new b2Vec2(
        rayLength * Math.sin(currentRayAngle),
        rayLength * Math.cos(currentRayAngle)
    );

    drawReflectedRay(origin, translation);
}
```

![a white line bounces off a number of shapes before coming to an end](raycasting-recurse.png)

#### Credits

This tutorial is adapted from an original piece of work created by Chris Campbell and is used under license.

---

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