Jérôme Bousquié
437 lines
14 KiB
Markdown
437 lines
14 KiB
Markdown
# Étape 28 — Système de Particules : Étape A (Pool)
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> **Objectif** : Créer l'infrastructure GPU du pool de particules (buffer + pipeline render + draw).
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> C'est la brique de base sur laquelle les drivers (GPU/CPU/Manual) seront construits.
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> **Référence** : `docs/tech/ARCHI_PARTICULES.md` (§2, §3, §6, §7, §8.2, §8.3, §12, §13)
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---
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## Contexte
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La phase 6 "Post-MVP" a couvert les effets post-process (bloom, DoF, fog, MSAA) et le PBR.
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On passe maintenant au **système de particules** — une nouvelle catégorie de fonctionnalité
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(simulation + rendu) qui suit l'architecture Pool ≠ Driver décrite dans `ARCHI_PARTICULES.md`.
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Les effets restants de la phase 6 (6.6 CSM, 6.7 SSAO, 6.14-6.16 Area lights / Volumetric)
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seront repris **après** le système de particules (phase 7).
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---
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## Scope de cette étape (A)
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| Fait | Non fait (étapes suivantes) |
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|------|---------------------------|
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| Struct `Particle` (64 bytes, Pod) | Driver GPU (compute + spawn) — Étape B |
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| `ParticlePoolConfig` + `BlendingMode` | Driver CPU (simulation Rust) — Étape C |
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| `ParticlePool` (buffer + pipeline + bind group) | Driver Manual + handle — Étape D |
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| Pipeline render (billboard instancé) | Intégration Renderer (frame loop) — Étape E |
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| Vertex shader (quad via vertex_index + billboard) | Presets + Example — Étape F |
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| Fragment shader (texture × color) | Tests WGSL + layout — Étape G |
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| Texture par défaut (disque 16×16) | |
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| Méthodes `Scene::create_particle_pool` | |
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| Pool inactif par défaut (zéro draw sans driver) | |
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> **Cette étape produit un pool qui EXISTE mais ne draw rien** (pas de driver = pas de count > 0).
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> Le draw sera activé à l'étape E (intégration Renderer). On peut néanmoins tester le pipeline
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> en forçant un count artificiel dans un test.
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---
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## Décisions (rappel de ARCHI_PARTICULES.md)
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| # | Décision | Détail |
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|---|----------|--------|
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| D1 | Pool ≠ Driver | Le pool est la ressource GPU. Le driver est swappable. |
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| D2 | 64 bytes/particule | pos(12)+pad+vel(12)+pad+life+max_life+size+size_growth+angle+angular_vel+color(16) |
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| D3 | Billboard camera-facing | Quad orienté vers la caméra (axes right/up de la view matrix) |
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| D4 | Quad via `@builtin(vertex_index)` | Pas de vertex buffer. 4 sommets générés en shader. |
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| D5 | `draw(4, max_count)` + early-out | Le vertex shader skip les instances au-delà de `count_buffer` |
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| D6 | Blend figé au pipeline | 1 mode par pool (Additive ou Alpha) |
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| D7 | Depth test oui, depth write non | Transparence correcte |
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| D8 | Texture par défaut : disque 16×16 | Si `texture: None` |
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| D9 | Pool inactif si pas de driver | Zéro compute, zéro draw |
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---
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## Fichiers à créer / modifier
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```
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lib/src/
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├── core/
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│ ├── mod.rs # + pub mod particles
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│ └── particles.rs # NOUVEAU : ParticlePool + ParticlePoolConfig + BlendingMode
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├── resources/
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│ ├── mod.rs # + re-export Particle
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│ └── particle.rs # NOUVEAU : struct Particle (64 bytes, Pod)
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├── shaders/
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│ ├── mod.rs # + PARTICLE_BILLBOARD_SHADER
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│ └── particle_billboard.wgsl # NOUVEAU : vs_main + fs_main
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├── scene/
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│ └── scene.rs # + particle_pools: HashMap<String, Arc<ParticlePool>>
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│ # + create_particle_pool()
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└── prelude.rs # + re-exports
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lib/tests/
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└── wgsl_validate.rs # + test particle_billboard
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lib/examples/
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└── particles.rs # (Étape F, pas cette étape)
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```
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---
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## Détail des implémentations
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### 1. `resources/particle.rs`
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```rust
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use bytemuck::{Pod, Zeroable};
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/// 64 bytes per particle. Mirror of the WGSL `Particle` struct.
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#[repr(C)]
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#[derive(Copy, Clone, Pod, Zeroable, Default)]
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pub struct Particle {
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pub pos: [f32; 3], // offset 0
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pub _pad0: f32, // offset 12
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pub vel: [f32; 3], // offset 16
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pub _pad1: f32, // offset 28
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pub life: f32, // offset 32
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pub max_life: f32, // offset 36
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pub size: f32, // offset 40
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pub size_growth: f32, // offset 44
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pub angle: f32, // offset 48
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pub angular_vel: f32, // offset 52
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pub color: [f32; 4], // offset 56
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}
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impl Particle {
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pub const SIZE: u64 = std::mem::size_of::<Self>() as u64; // must be 64
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}
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```
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**Test** : `assert_eq!(size_of::<Particle>(), 64)`, `assert_eq!(align_of::<Particle>(), 16)`.
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### 2. `core/particles.rs`
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```rust
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pub enum BlendingMode {
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Additive,
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Alpha,
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}
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pub struct ParticlePoolConfig {
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pub max_count: u32,
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pub texture: Option<String>, // ID dans scene.textures
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pub blending: BlendingMode,
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}
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pub struct ParticlePool {
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pub(crate) buffer: wgpu::Buffer,
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pub(crate) pipeline: wgpu::RenderPipeline,
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pub(crate) bind_group: wgpu::BindGroup,
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pub(crate) sampler: wgpu::Sampler,
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pub(crate) count_buffer: wgpu::Buffer,
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pub max_count: u32,
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pub blending: BlendingMode,
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// Driver (Étape B/C/D) :
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pub(crate) driver: Option<Box<dyn ParticleDriver>>,
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pub(crate) active: bool,
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}
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```
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**Construit** par `Scene::create_particle_pool` qui a accès au `device`, `queue`,
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`format`, et aux textures. Le pipeline est compilé immédiatement.
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### 3. `shaders/particle_billboard.wgsl`
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```wgsl
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// Particle billboard shader (vertex + fragment).
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// Quad generated via @builtin(vertex_index) — no vertex buffer.
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// Instance data read from storage buffer.
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struct Particle {
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pos: vec3<f32>, pad0: f32,
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vel: vec3<f32>, pad1: f32,
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life: f32, max_life: f32,
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size: f32, size_growth: f32,
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angle: f32, angular_vel: f32,
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color: vec4<f32>,
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}
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struct CameraParams {
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view: mat4x4<f32>,
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proj: mat4x4<f32>,
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}
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struct VsOut {
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@builtin(position) clip: vec4<f32>,
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@location(0) frag_color: vec4<f32>,
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@location(1) uv: vec2<f32>,
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}
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@group(0) @binding(0) var<uniform> camera: CameraParams;
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@group(0) @binding(1) var<storage, read> particles: array<Particle>;
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@group(0) @binding(2) var<uniform> count_buf: f32;
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const QUAD: array<vec2<f32>, 4> = array<vec2<f32>, 4>(
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vec2(-0.5, -0.5),
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vec2( 0.5, -0.5),
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vec2( 0.5, 0.5),
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vec2(-0.5, 0.5),
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);
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@vertex
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fn vs_main(
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@builtin(vertex_index) vi: u32,
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@builtin(instance_index) ii: u32,
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) -> VsOut {
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var out: VsOut;
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if f32(ii) >= count_buf {
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out.clip = vec4(0.0, 0.0, -2.0, 1.0);
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out.frag_color = vec4(0.0);
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out.uv = vec2(0.0);
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return out;
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}
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let p = particles[ii];
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let q = QUAD[vi];
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let c = cos(p.angle);
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let s = sin(p.angle);
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let rot = vec2(q.x * c - q.y * s, q.x * s + q.y * c) * p.size;
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let right = vec3(camera.view[0][0], camera.view[1][0], camera.view[2][0]);
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let up = vec3(camera.view[0][1], camera.view[1][1], camera.view[2][1]);
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let world = p.pos + right * rot.x + up * rot.y;
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out.clip = camera.proj * camera.view * vec4(world, 1.0);
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out.frag_color = p.color;
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out.uv = q + vec2(0.5);
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return out;
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}
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@group(0) @binding(3) var samp: sampler;
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@group(0) @binding(4) var tex: texture_2d<f32>;
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@fragment
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fn fs_main(in: VsOut) -> @location(0) vec4<f32> {
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let t = textureSample(tex, samp, in.uv);
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return in.frag_color * t;
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}
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```
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### 4. Bind group layout (render)
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| Binding | Type | Contenu | Visibility |
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|---------|------|---------|------------|
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| 0 | Uniform (min 112 B) | CameraParams (view + proj) | VERTEX |
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| 1 | Storage (RO) | particle_data | VERTEX |
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| 2 | Uniform (min 4 B) | count_buffer | VERTEX |
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| 3 | Sampler | Sampler | FRAGMENT |
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| 4 | Texture (2D) | Texture particule | FRAGMENT |
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### 5. Pipeline descriptor
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```rust
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wgpu::RenderPipelineDescriptor {
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vertex: wgpu::VertexStage {
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module: shader,
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entry_point: "vs_main",
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buffers: &[], // PAS de vertex buffer
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},
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fragment: Some(wgpu::FragmentStage {
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module: shader,
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entry_point: "fs_main",
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}),
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primitive: wgpu::PrimitiveState {
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topology: wgpu::PrimitiveTopology::TriangleList,
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// Indices : pas de index buffer → on utilise draw(4, N)
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// MAIS : 4 sommets sans indices = 2 triangles ? NON.
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// draw(4, N) drawe 4 triangles (4 indices implicites 0,1,2,3) = 1 triangle + 1 degénéré.
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// IL FAUT un index buffer ! Ou utiliser draw_indexed.
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// → Voir GOTCHA ci-dessous.
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..Default::default()
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},
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color_states: [wgpu::ColorState {
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format,
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alpha_blend: blend_alpha,
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color_blend: blend_color,
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write_mask: wgpu::ColorWrites::ALL,
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}],
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depth_stencil: Some(wgpu::DepthStencilState {
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format: depth_format,
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depth_write_enabled: false,
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depth_compare: wgpu::CompareFunction::LessEqual,
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..Default::default()
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}),
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multisample,
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..
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}
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```
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### ⚠️ GOTCHA : Topologie du quad billboard
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**Problème** : `draw(4, N)` sans index buffer drawe 4 **vertices** en `TriangleList`,
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ce qui fait 4/3 = 1 triangle + 1 vertex orphelin. Ce n'est PAS un quad.
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**Solutions** :
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| Option | Pro | Contre |
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|--------|-----|--------|
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| A : `draw(6, N)` + 6 sommets (quad = 2 tris, 6 verts) | Pas d'index buffer | 6 vertices au lieu de 4 (2 dupliqués) |
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| B : Index buffer (6 indices) + `draw_indexed(6, N, 0, 0)` | 4 vertices seulement | 1 petit buffer index (24 bytes) partagé |
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| C : `@builtin(vertex_index)` avec 6 values dans le const | Pas d'index buffer, pas de vertex buffer | Le const a 6 entries au lieu de 4 |
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**Décision : Option C** — 6 entries dans le const QUAD, `draw(6, max_count)`.
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```wgsl
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// 6 entries = 2 triangles (0-1-2, 3-4-5) formant un quad
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const QUAD: array<vec2<f32>, 6> = array<vec2<f32>, 6>(
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vec2(-0.5, -0.5), // 0
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vec2( 0.5, -0.5), // 1
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vec2( 0.5, 0.5), // 2
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vec2(-0.5, -0.5), // 3
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vec2( 0.5, 0.5), // 4
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vec2(-0.5, 0.5), // 5
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);
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```
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→ `draw(6, max_count)`. Pas de vertex buffer, pas d'index buffer. Cohérent avec
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le pattern fullscreen triangle du TM/bloom (qui utilise `draw(3, 1)`).
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### 6. Texture par défaut (disque 16×16)
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Générée en Rust au build du pool (si `config.texture == None`) :
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```rust
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fn default_disc_texture() -> Vec<u8> {
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let size = 16;
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let mut data = vec![0u8; size * size * 4];
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let center = (size as f32 - 1.0) / 2.0;
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for y in 0..size {
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for x in 0..size {
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let dx = (x as f32 - center) / center;
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let dy = (y as f32 - center) / center;
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let dist = (dx * dx + dy * dy).sqrt();
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let alpha = (1.0 - dist).clamp(0.0, 1.0) as u8 * 255;
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let i = (y * size + x) * 4;
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data[i] = 255; // R
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data[i+1] = 255; // G
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data[i+2] = 255; // B
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data[i+3] = alpha; // A
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}
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}
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data
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}
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```
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### 7. `Scene::create_particle_pool`
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```rust
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impl Scene {
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pub fn create_particle_pool(&mut self, id: &str, config: ParticlePoolConfig) -> Result<(), String> {
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if self.particle_pools.contains_key(id) {
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return Err(format!("particle pool '{}' already exists", id));
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}
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// Résoudre la texture
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let (texture_view, sampler, is_owned) = match &config.texture {
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Some(tex_id) => {
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let tex = self.textures.get(tex_id)
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.ok_or_else(|| format!("texture '{}' not found", tex_id))?;
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(tex.view.clone(), tex.sampler.clone(), false)
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}
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None => {
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// Créer la texture disque 16×16
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let (view, sampler) = self.gpu.create_default_disc_texture();
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(view, sampler, true)
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}
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};
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// Construire le pool (buffer + pipeline + bind group)
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let pool = ParticlePool::new(
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&self.gpu.device,
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&self.gpu.queue,
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self.gpu.format,
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self.gpu.depth_format,
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self.gpu.msaa,
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&config,
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texture_view,
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sampler,
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);
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self.particle_pools.insert(id.to_string(), Arc::new(pool));
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Ok(())
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}
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}
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```
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### 8. Prelude
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```rust
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// Dans prelude.rs :
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pub use crate::core::particles::{ParticlePoolConfig, BlendingMode};
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pub use crate::resources::particle::Particle;
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```
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---
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## Blend states
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| Mode | color_ops.src | color_ops.dst | alpha_ops.src | alpha_ops.dst |
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|------|--------------|--------------|---------------|---------------|
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| **Additive** | One | One | One | One |
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| **Alpha** | SrcAlpha | OneMinusSrcAlpha | One | OneMinusSrcAlpha |
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---
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## Tests
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### Unit tests (`particles.rs`)
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| Test | Vérifie |
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|------|---------|
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| `particle_size_is_64` | `size_of::<Particle>() == 64` |
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| `particle_align_is_16` | `align_of::<Particle>() == 16` |
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| `particle_offsets` | Offsets de chaque champ |
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| `pool_config_default_max_count` | Valeur raisonnable |
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| `default_disc_texture_size` | 16×16×4 bytes |
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| `default_disc_center_is_opaque` | Center pixel alpha = 255 |
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| `default_disc_corner_is_transparent` | Corner pixel alpha = 0 |
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### WGSL validation (`wgsl_validate.rs`)
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| Test | Vérifie |
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|------|---------|
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| `particle_billboard_compiles` | Naga compile le shader |
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| `particle_billboard_entry_points` | Contient `vs_main` + `fs_main` |
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| `particle_billboard_no_compute` | Pas d'entry point compute (cette étape) |
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---
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## Vérification de non-régression
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- [ ] `cargo check -p wsg-lib --all-targets` → 0 errors, 0 warnings
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- [ ] `cargo test -p wsg-lib` → tous les tests existants passent (127+)
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- [ ] Les examples existants (demo, pbr, bloom, etc.) compilent et fonctionnent
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- [ ] Aucun changement dans `renderer.rs` (le pool n'est pas encore intégré au frame loop)
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- [ ] `Scene` a un nouveau champ `particle_pools` mais il est vide par défaut → zéro coût
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---
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## Critères d'acceptation
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1. ✅ `Particle` compile, 64 bytes, Pod, offsets corrects
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2. ✅ `particle_billboard.wgsl` compile par Naga (test WGSL)
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3. ✅ `ParticlePool::new` crée buffer + pipeline + bind group sans erreur
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4. ✅ La texture disque 16×16 est générée correctement
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5. ✅ `Scene::create_particle_pool` fonctionne (test unitaire avec mock device)
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6. ✅ Le pool est inactif (pas de draw) tant qu'aucun driver n'est attaché
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7. ✅ Zéro warning, tous les tests verts
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8. ✅ Prelude expose les types
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---
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## Étape suivante (B)
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Driver GPU : compute shader `particle_update.wgsl` + `GpuEmitterConfig` +
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spawn CPU + dispatch + `Scene::attach_gpu_emitter`.
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