Raycasting to the Entire Surface of an Object in Three.js

To efficiently raycast from the camera and cover the entire surface of an object in Three.js, the best approach depends on your specific use case. Here are the three most efficient methods:

✅ Best Approaches for Full Object Surface Raycasting

Approach	Best For	Performance
1️⃣ Randomized Ray Sampling	Full object coverage with speed	⚡ Fast
2️⃣ Screen-Space Grid Sampling	Balanced coverage & precision	⚡⚡ Moderate
3️⃣ Vertex-Based Raycasting	Guaranteed full surface hit	⚡⚡⚡ Slower

1️⃣ Efficient Randomized Ray Sampling (Fast & Covers Most Areas)

A fast method to approximate full coverage by randomly casting rays from the camera toward the object's bounding box.

const raycaster = new THREE.Raycaster();
const camera = yourCamera;
const object = yourObject;
 
const numRays = 1000; // Adjust for more coverage
const boundingBox = new THREE.Box3().setFromObject(object); // Get object bounds
 
for (let i = 0; i < numRays; i++) {
  // Generate a random point in front of the object
  const randomPoint = new THREE.Vector3(
    THREE.MathUtils.lerp(boundingBox.min.x, boundingBox.max.x, Math.random()),
    THREE.MathUtils.lerp(boundingBox.min.y, boundingBox.max.y, Math.random()),
    boundingBox.max.z + 5 // Slightly in front of the object
  );
 
  // Compute ray direction from the camera
  const direction = new THREE.Vector3()
    .subVectors(randomPoint, camera.position)
    .normalize();
  raycaster.set(camera.position, direction);
 
  // Check for intersections
  const intersects = raycaster.intersectObject(object, true);
  if (intersects.length > 0) {
    console.log("Hit at:", intersects[0].point);
  }
}

✅ Pros:

Covers most of the object surface quickly.
Works well for irregular shapes.

❌ Cons:

Not guaranteed to hit every part of the object.
Randomness means some areas may get missed.

2️⃣ Grid-Based Screen Sampling (More Precise & Balanced)

Casts rays from a grid of pixels on the screen, ensuring even coverage across the object.

const raycaster = new THREE.Raycaster();
const camera = yourCamera;
const object = yourObject;
const width = window.innerWidth;
const height = window.innerHeight;
const step = 10; // Adjust step size for more/less coverage
 
for (let x = 0; x < width; x += step) {
  for (let y = 0; y < height; y += step) {
    // Convert screen coordinates to normalized device coordinates (-1 to 1)
    const ndcX = (x / width) * 2 - 1;
    const ndcY = -(y / height) * 2 + 1;
 
    // Set the ray from the camera
    raycaster.setFromCamera({ x: ndcX, y: ndcY }, camera);
 
    // Check for intersections
    const intersects = raycaster.intersectObject(object, true);
    if (intersects.length > 0) {
      console.log("Hit at:", intersects[0].point);
    }
  }
}

✅ Pros:

Covers the whole visible surface of the object.
More even coverage than random sampling.

❌ Cons:

More expensive than random sampling.
Depends on screen resolution (higher res = more rays).

3️⃣ Raycasting From Every Vertex (Best Coverage, but Slowest)

For guaranteed full surface coverage, cast rays from every vertex of the object back toward the camera.

const raycaster = new THREE.Raycaster();
const camera = yourCamera;
const object = yourObject;
const vertices = object.geometry.attributes.position;
 
for (let i = 0; i < vertices.count; i++) {
  // Get vertex position in world space
  const vertex = new THREE.Vector3();
  vertex.fromBufferAttribute(vertices, i);
  object.localToWorld(vertex);
 
  // Compute direction from vertex to camera
  const direction = new THREE.Vector3()
    .subVectors(camera.position, vertex)
    .normalize();
  raycaster.set(vertex, direction);
 
  // Check if ray hits the object
  const intersects = raycaster.intersectObject(object, true);
  if (intersects.length > 0) {
    console.log("Hit at:", intersects[0].point);
  }
}

✅ Pros:

Guaranteed to cover the entire object.
Works independently of the camera view.

❌ Cons:

Slowest method (lots of rays).
Dense meshes will have too many vertices → needs downsampling.

🚀 Best Approach Based on Use Case

Use Case	Best Method
Fastest way to cover most areas	🔹 Randomized Ray Sampling
Balanced speed & accuracy	🔹 Grid-Based Screen Sampling
Guaranteed full surface coverage	🔹 Raycasting from Every Vertex

🔹 For most cases, use Randomized Ray Sampling.
🔹 If you need precise coverage, go for Screen-Space Grid Sampling.
🔹 If you must hit every vertex, use Vertex Raycasting (but optimize for performance).

Note: This is article is generated by ChatGPT. I found it meaningful, so I've clipped it here. Fair warning, trust but verify. 🚀