2nd/10_Wiki/Topics/DevOps_and_Security/Raycasting.md

---
id: wiki-2026-0508-raycasting
title: Raycasting
category: 10_Wiki/Topics
status: verified
canonical_id: self
aliases: [ray casting, ray-object intersection, picking, ray-sphere, ray-triangle]
duplicate_of: none
source_trust_level: A
confidence_score: 0.95
verification_status: applied
tags: [graphics, geometry, ray-tracing, picking, collision]
raw_sources: []
last_reinforced: 2026-05-10
github_commit: pending
tech_stack:
  language: TypeScript/GLSL
  framework: Three.js, WebGPU, three-mesh-bvh
---

# Raycasting

## 매 한 줄
> **"매 ray 와 매 geometry 의 intersection test"**. 매 1968 Arthur Appel 의 매 first hidden surface paper 매 origin, 매 1992 Wolfenstein 3D 의 매 game engine signature. 매 2026 의 매 universal primitive: mouse picking, hit-test, AR placement, lighting, AI vision-cone, BIM section. 매 Raycasting ≠ Ray Tracing — 매 single ray (no recursion) vs 매 recursive light path.

## 매 핵심

### 매 raycasting vs raytracing
| | Raycasting | Ray Tracing |
|---|---|---|
| Recursion | 매 single hit | 매 reflection / refraction recursive |
| Cost | 매 O(log N) per ray (BVH) | 매 50-1000x heavier |
| Use | Picking, collision | Photorealistic render |

### 매 Ray = origin + t·direction
- t > 0: 매 forward.
- nearest hit: 매 minimum t > 0.
- ray vs primitive: 매 sphere / plane / triangle / AABB / OBB.

### 매 acceleration structure
- BVH (Bounding Volume Hierarchy): 매 dominant 매 2026.
- KD-tree: 매 static scene 매 slightly faster build.
- Octree: 매 voxel world.
- Spatial hash: 매 dynamic scene.

### 매 응용
1. Mouse picking (Three.js Raycaster).
2. AR object placement (hit-test with depth).
3. AI line-of-sight / vision cone.
4. Bullet physics (sweep test).
5. Audio occlusion (raycast for muffle).
6. BIM 의 section plane / clipper.

## 💻 패턴

### 1. Three.js mouse picking
```ts
import * as THREE from 'three';

const raycaster = new THREE.Raycaster();
const mouse = new THREE.Vector2();

window.addEventListener('pointerdown', (e) => {
  mouse.x = (e.clientX / innerWidth) * 2 - 1;
  mouse.y = -(e.clientY / innerHeight) * 2 + 1;
  raycaster.setFromCamera(mouse, camera);
  const hits = raycaster.intersectObjects(scene.children, true);
  if (hits.length) console.log('Hit:', hits[0].object.name, hits[0].point);
});
```

### 2. Ray-sphere intersection (analytic)
```ts
function raySphere(ro: V3, rd: V3, center: V3, r: number): number {
  const oc = sub(ro, center);
  const b = dot(oc, rd);
  const c = dot(oc, oc) - r * r;
  const h = b * b - c;
  if (h < 0) return -1;
  const t = -b - Math.sqrt(h);
  return t >= 0 ? t : -1;
}
```

### 3. Ray-triangle (Möller-Trumbore)
```ts
function rayTriangle(ro: V3, rd: V3, a: V3, b: V3, c: V3): number {
  const e1 = sub(b, a), e2 = sub(c, a);
  const p = cross(rd, e2);
  const det = dot(e1, p);
  if (Math.abs(det) < 1e-8) return -1;
  const inv = 1 / det;
  const tv = sub(ro, a);
  const u = dot(tv, p) * inv;
  if (u < 0 || u > 1) return -1;
  const q = cross(tv, e1);
  const v = dot(rd, q) * inv;
  if (v < 0 || u + v > 1) return -1;
  return dot(e2, q) * inv;
}
```

### 4. BVH-accelerated picking (three-mesh-bvh)
```ts
import { computeBoundsTree, acceleratedRaycast } from 'three-mesh-bvh';

THREE.BufferGeometry.prototype.computeBoundsTree = computeBoundsTree;
THREE.Mesh.prototype.raycast = acceleratedRaycast;

mesh.geometry.computeBoundsTree();   // 매 once
// 매 매 raycast 100x+ faster
```

### 5. AR hit-test (WebXR)
```ts
const session = await navigator.xr.requestSession('immersive-ar', {
  requiredFeatures: ['hit-test']
});
const refSpace = await session.requestReferenceSpace('viewer');
const hitSource = await session.requestHitTestSource({ space: refSpace });

session.requestAnimationFrame(function frame(t, frame) {
  const results = frame.getHitTestResults(hitSource);
  if (results.length) {
    const pose = results[0].getPose(refSpace);
    placeReticleAt(pose.transform.matrix);
  }
  session.requestAnimationFrame(frame);
});
```

### 6. Vision cone (AI agent)
```ts
function canSee(agent: Agent, target: V3, world: BVH): boolean {
  const dir = normalize(sub(target, agent.pos));
  const angle = Math.acos(dot(dir, agent.forward));
  if (angle > agent.fov / 2) return false;
  const dist = length(sub(target, agent.pos));
  if (dist > agent.sightRange) return false;
  const hit = world.raycastFirst(agent.pos, dir);
  return !hit || hit.t >= dist - 0.01;
}
```

### 7. WebGPU compute-shader raycast
```wgsl
@compute @workgroup_size(64)
fn cs_raycast(@builtin(global_invocation_id) id: vec3u) {
  let ray = rays[id.x];
  var t_min = 1e30;
  var hit_idx = -1;
  for (var i = 0u; i < tri_count; i++) {
    let t = ray_triangle(ray, tris[i]);
    if (t > 0.0 && t < t_min) { t_min = t; hit_idx = i32(i); }
  }
  results[id.x] = Hit(t_min, hit_idx);
}
```

## 매 결정 기준
| 상황 | Approach |
|---|---|
| < 1k triangles | Naive Three.js raycaster 충분 |
| 1k-100k triangles | three-mesh-bvh (BVH on CPU) |
| 100k-10M, dynamic | refit BVH per frame + worker |
| 10M+ static | WebGPU compute + GPU BVH |
| Voxel world (Minecraft-ish) | DDA / Amanatides-Woo (매 grid traversal) |
| AR placement | WebXR hit-test API (매 system-provided) |

**기본값**: Three.js + three-mesh-bvh 매 web 의 standard. 매 dynamic 매 BVH refit. 매 GPU compute 매 last resort.

## 🔗 Graph
- 부모: [[Computational Geometry]] · [[Computer Graphics]]
- 응용: [[Collision Detection]]
- Adjacent: [[KD-Tree]]

## 🤖 LLM 활용
**언제**: 매 3D scene 매 user input mapping (click/touch/AR). 매 line-of-sight / occlusion query. 매 sweep collision 1-shot.
**언제 X**: 매 2D UI hit-test (DOM event 매 충분). 매 dense per-pixel intersection — 매 GPU rasterization 매 더 fast.

## ❌ 안티패턴
- **매 frame 의 brute-force intersect 모든 triangle**: 매 100k tri scene 매 60fps 의 X — 매 BVH 필수.
- **BVH refit 의 X 매 dynamic mesh**: 매 stale tree → 매 missed hits.
- **Far plane 무시**: 매 무한 ray 매 매 distant unimportant geom hit.
- **Ray direction 매 unnormalized**: 매 t value 매 distance 의 X — 매 모든 distance compare 매 broken.
- **Single-precision float 의 self-intersection**: 매 origin offset (`+ 0.001 * normal`) 매 epsilon 처리.

## 🧪 검증 / 중복
- Verified (Möller-Trumbore 1997 paper, Three.js 2026 source, three-mesh-bvh 0.7+).
- 신뢰도 A.

## 🕓 Changelog
| 날짜 | 변경 |
|---|---|
| 2026-05-08 | Phase 1 |
| 2026-05-10 | Manual cleanup — Möller-Trumbore + BVH + WebXR hit-test + WebGPU compute |