epic: GPU Particle System

[Hekbas]

Goal

Fully GPU-driven particle system — emit, simulate, and render particles entirely on the GPU via compute shaders and indirect draw. No CPU per-particle work.

Checklist

Compute Passes

• Emit pass (compute) — spawn particles from emitter configs, initialize position/velocity/lifetime in particle SSBO
• Simulate pass (compute) — integrate positions, apply forces/gravity, decay lifetime, kill dead particles
• Compact pass (compute) — stream compaction to remove dead particles (atomic append or parallel prefix sum)
• Dead particle recycling via free-list in SSBO

Rendering

• Render as point sprites or camera-facing billboards
• vkCmdDrawIndirect with GPU-computed draw count (from compact pass)
• Alpha blending + soft particle depth fade
• Optional: lit particles via Forward+ light clusters (Phase 18)

Emitter System

• Component::ParticleEmitter — shape (point/sphere/cone/box), rate, lifetime, speed range, color gradient, size over life
• Burst mode + continuous emission
• World-space vs local-space simulation toggle

Editor

• Inspector for emitter properties with live preview
• Particle count stats in profiler
• Serialization in scene format

Verification

• 100K+ particles at stable 60fps
• Particles correctly die and respawn
• No GPU memory growth over time (free-list recycles correctly)
• Correct draw count from GPU (no CPU readback)

Dependencies

• Phase 12 (compute + indirect draw infrastructure)
• Phase 18 (Forward+ for lit particles, optional)