2nd/10_Wiki/Topics/Frontend/Indirect Draw.md

---
id: wiki-2026-0508-indirect-draw
title: Indirect Draw
category: 10_Wiki/Topics
status: verified
canonical_id: self
aliases: [Indirect Drawing, GPU-Driven Rendering, drawIndirect]
duplicate_of: none
source_trust_level: A
confidence_score: 0.9
verification_status: applied
tags: [graphics, gpu, webgpu, vulkan, rendering, performance]
raw_sources: []
last_reinforced: 2026-05-10
github_commit: pending
tech_stack:
  language: wgsl
  framework: webgpu
---

# Indirect Draw

## 매 한 줄
> **"매 indirect draw 는 draw call args 의 GPU buffer 의 read — CPU roundtrip 없이 GPU 의 self-dispatch"**. 2026 의 GPU-driven rendering pipeline 의 foundation: Vulkan/D3D12/Metal/WebGPU 의 support. 매 culling, LOD, instancing 의 GPU 에서 결정 → CPU draw-call overhead 의 elimination.

## 매 핵심

### 매 vs Direct Draw
- **Direct**: `draw(vertexCount, instanceCount, firstVertex, firstInstance)` — args from CPU.
- **Indirect**: `drawIndirect(buffer, offset)` — args read from GPU buffer.
- **Multi-draw indirect (MDI)**: thousands of draws from one CPU command.

### 매 Args Layout (WebGPU)
```
struct DrawIndirectArgs {
  vertexCount: u32,
  instanceCount: u32,
  firstVertex: u32,
  firstInstance: u32,
}
struct DrawIndexedIndirectArgs {
  indexCount: u32,
  instanceCount: u32,
  firstIndex: u32,
  baseVertex: i32,
  firstInstance: u32,
}
```

### 매 Pipeline (GPU-driven)
1. Compute shader: per-object frustum/occlusion cull → write visible list.
2. Compute shader: write indirect args buffer (instanceCount=0 for culled).
3. `drawIndexedIndirect` (or MDI) reads buffer → renders only visible.

### 매 응용
1. Massive instanced scenes (foliage, crowds, particles).
2. GPU-driven culling (frustum, occlusion via Hi-Z).
3. LOD selection on GPU.
4. Variable-rate / batched rendering (cluster culling, Nanite-style).

## 💻 패턴

### WebGPU Indirect Draw Setup
```ts
// Args buffer (visible after compute)
const indirectBuffer = device.createBuffer({
  size: 16,  // 4 u32
  usage: GPUBufferUsage.INDIRECT | GPUBufferUsage.STORAGE | GPUBufferUsage.COPY_DST,
});

// Initialize: 36 verts, 1000 instances, offset 0/0
device.queue.writeBuffer(indirectBuffer, 0,
  new Uint32Array([36, 1000, 0, 0]));

// In render pass
pass.setPipeline(pipeline);
pass.setVertexBuffer(0, vertices);
pass.drawIndirect(indirectBuffer, 0);
```

### Culling Compute Shader (WGSL)
```wgsl
struct DrawArgs { vertexCount: u32, instanceCount: u32,
                  firstVertex: u32, firstInstance: u32 }

@group(0) @binding(0) var<storage, read>       objects: array<Object>;
@group(0) @binding(1) var<storage, read_write> drawArgs: DrawArgs;
@group(0) @binding(2) var<storage, read_write> visibleInstances: array<u32>;
@group(0) @binding(3) var<uniform>              camera: Camera;

@compute @workgroup_size(64)
fn cullCS(@builtin(global_invocation_id) gid: vec3<u32>) {
  let i = gid.x;
  if (i >= arrayLength(&objects)) { return; }
  let obj = objects[i];
  if (frustumTest(obj.bounds, camera.frustum)) {
    let slot = atomicAdd(&drawArgs.instanceCount, 1u);
    visibleInstances[slot] = i;
  }
}
```

### Reset Pass (clear instanceCount)
```ts
// Each frame, before culling, zero out instanceCount
device.queue.writeBuffer(indirectBuffer, 4, new Uint32Array([0]));
```

### Multi-Draw Indirect (Vulkan)
```cpp
// Draw N different meshes from one buffer
vkCmdDrawIndexedIndirect(cmd, indirectBuf, 0,
                         /*drawCount*/ N,
                         /*stride*/ sizeof(VkDrawIndexedIndirectCommand));

// Or with count buffer (drawCount is itself on GPU)
vkCmdDrawIndexedIndirectCount(cmd, indirectBuf, 0,
                              countBuf, 0, /*maxDraws*/ N,
                              sizeof(VkDrawIndexedIndirectCommand));
```

### Three.js (R175+ has WebGPU)
```js
import { WebGPURenderer, BatchedMesh } from 'three';
const renderer = new WebGPURenderer();
// BatchedMesh internally uses indirect draw + instancing
const batched = new BatchedMesh(maxInstances, maxVerts, maxIndices);
batched.addGeometry(geom1);
batched.addGeometry(geom2);
// One draw call, GPU handles per-instance state
```

### Hi-Z Occlusion Culling (sketch)
```wgsl
// Sample Hi-Z mip — fastest mip where bounding sphere covers >1 texel
fn occluded(bsphere: vec4<f32>) -> bool {
  let screenRect = projectToScreen(bsphere);
  let mip = computeMip(screenRect);
  let depth = textureSampleLevel(hiZ, samp, screenRect.center, mip).r;
  return bsphereMinDepth(bsphere) > depth;
}
```

## 매 결정 기준
| 상황 | Approach |
|---|---|
| <100 unique objects | Direct draw / instancing — overhead 의 not worth |
| 1k-1M instances | Indirect draw + GPU cull |
| Many distinct meshes | Multi-draw indirect (Vulkan/D3D12); WebGPU 의 batched |
| Foliage/crowd | Indirect + GPU LOD selection |
| Mobile / low-end | Direct draw (compute overhead 의 watch) |

**기본값**: large dynamic scene 의 GPU-driven indirect pipeline. Small scene 의 direct draw.

## 🔗 Graph
- 부모: [[Graphics Pipeline]]
- 변형: [[GPU-Driven Rendering]]
- 응용: [[Frustum Culling]] · [[Nanite]]
- Adjacent: [[WebGPU]] · [[Vulkan]] · [[Compute Shader]]

## 🤖 LLM 활용
**언제**: GPU-driven pipeline 의 design, culling 의 implement, draw-call overhead 의 reduce.
**언제 X**: simple scene 의 indirect draw 의 over-engineering — direct 의 fine.

## ❌ 안티패턴
- **CPU readback of indirect buffer**: 매 stall. GPU 의 self-contained 의 keep.
- **Per-frame full buffer rewrite**: defeats purpose. 매 GPU compute 의 update.
- **No Hi-Z for occlusion**: false positives — Hi-Z 또는 conservative AABB 의 사용.
- **Indirect for tiny scenes**: compute dispatch overhead > savings.
- **WebGL fallback assumed**: WebGL 의 no indirect draw — WebGPU required.

## 🧪 검증 / 중복
- Verified (WebGPU spec, Vulkan spec, GPU Gems / Activision Nanite paper).
- 신뢰도 A.

## 🕓 Changelog
| 날짜 | 변경 |
|---|---|
| 2026-05-08 | Phase 1 |
| 2026-05-10 | Manual cleanup — indirect draw / GPU-driven rendering full content |