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skills/creative/touchdesigner-mcp/references/particles.md

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+# Particles Reference
+
+Particle systems in TouchDesigner — modern POPs (Particle Operators) and the legacy particleSOP path.
+
+For instancing static geometry (without per-instance lifetime/velocity), see `geometry-comp.md`. For GLSL-driven feedback simulations (no particle abstraction), see `operator-tips.md` (Feedback TOP section).
+
+Always call `td_get_par_info` for the op type before setting params. Param names below reflect TD 2025.32 — verify before relying on them.
+
+---
+
+## Two Paths: POPs vs. SOPs
+
+| | **POP family** (modern) | **particleSOP** (legacy) |
+|---|---|---|
+| GPU? | Yes (compute) | No (CPU) |
+| Particle count | 100k+ comfortably | ~5k before slowdown |
+| API style | Source / Force / Solver / Render chain | Single op with many params |
+| Use for | New projects, anything intensive | Quick demos, low counts, TD < 2023 |
+
+**Default to POPs.** Only fall back to particleSOP if a POP variant of an op you need doesn't exist.
+
+---
+
+## POP Pipeline Overview
+
+A POP system is a chain of operators inside a `geometryCOMP`:
+
+```
+popSourceTOP / popSourceSOP   ← spawn new particles
+        ↓
+popForceTOP (gravity, wind, etc.)
+        ↓
+popForceTOP (attractor, vortex, ...)
+        ↓
+popDeleteTOP (lifetime, bounds)
+        ↓
+popSolverTOP                  ← integrates velocity, updates positions
+        ↓
+[render via geometryCOMP / glslMAT instancing]
+```
+
+POP buffers carry standard channels: `P` (position), `v` (velocity), `life`, `id`, `Cd` (color), plus any custom channels you add.
+
+---
+
+## Minimal POP Setup
+
+```python
+# Create a geometry COMP to hold the POP network
+geo = root.create(geometryCOMP, 'particles_geo')
+
+# 1. Source — emit particles from a point
+src = geo.create(popSourceTOP, 'src')
+src.par.birthrate = 500          # per second
+src.par.life = 4.0                # seconds
+
+# 2. Gravity force
+grav = geo.create(popForceTOP, 'gravity')
+grav.par.forcetype = 'gravity'
+grav.par.fy = -9.8
+
+# 3. Lifetime cleanup
+delp = geo.create(popDeleteTOP, 'cull')
+delp.par.condition = 'lifeleq'    # delete when life <= 0
+delp.par.value = 0
+
+# 4. Solver
+solv = geo.create(popSolverTOP, 'solver')
+solv.par.timestep = 'frame'
+
+# Wire: source → force → delete → solver
+src.outputConnectors[0].connect(grav.inputConnectors[0])
+grav.outputConnectors[0].connect(delp.inputConnectors[0])
+delp.outputConnectors[0].connect(solv.inputConnectors[0])
+```
+
+The `popSolverTOP` output IS the live particle buffer. Render it via `glslMAT` instancing on a small SOP (sphere, point) as the "shape" of each particle.
+
+---
+
+## Common Forces
+
+| Force type | Effect | Common params |
+|---|---|---|
+| `gravity` | Constant directional pull | `fx`, `fy`, `fz` |
+| `wind` | Constant velocity addition | `wx`, `wy`, `wz` |
+| `drag` | Velocity damping over time | `dragstrength` |
+| `noise` | Curl-noise turbulence | `noiseamp`, `noisefreq`, `noiseseed` |
+| `attractor` | Pull toward a point | `position`, `strength`, `falloff` |
+| `vortex` | Swirl around an axis | `axis`, `strength` |
+| `point` (custom) | GLSL-evaluated arbitrary force | via `popforceadvancedTOP` |
+
+Stack multiple `popForceTOP`s in series — each modifies velocity additively.
+
+---
+
+## Lifecycle Patterns
+
+### Continuous emission (e.g. smoke plume)
+
+```python
+src.par.birthrate = 800
+src.par.life = 6.0       # variance via 'lifevariance'
+src.par.lifevariance = 1.5
+```
+
+### Burst emission (e.g. explosion)
+
+```python
+src.par.birthrate = 0    # no continuous emission
+src.par.burst.pulse()    # one burst on demand (verify param name)
+src.par.burstcount = 5000
+src.par.life = 1.5
+```
+
+### Beat-triggered burst
+
+Wire a `triggerCHOP` (from audio or MIDI) to pulse the burst:
+
+```python
+op('/project1/audio_kick_trigger').outputConnectors[0].connect(...)
+# Then via a chopExecuteDAT, on each kick:
+def offToOn(channel, sampleIndex, val, prev):
+    op('/project1/particles_geo/src').par.burst.pulse()
+    return
+```
+
+---
+
+## Rendering Particles
+
+### Point Sprites (simplest)
+
+```python
+# Inside the geometryCOMP, render the solver output directly
+# The geo's first SOP child becomes the geometry
+# But for POPs, we typically render via glslMAT on a small "shape"
+
+# Simple billboard sphere per particle:
+shape = geo.create(sphereSOP, 'shape')
+shape.par.rad = 0.05
+shape.par.rows = 6; shape.par.cols = 6   # low-poly to keep it fast
+
+# Material that uses POP buffer for instancing
+mat = root.create(glslMAT, 'particle_mat')
+# Configure mat.par.instancingTOP = solver output (verify param name)
+```
+
+The exact instancing setup varies by TD version — call `td_get_hints(topic='popInstancing')` (or `popRender` / `instancing` — try a few).
+
+### GPU Sprites via glslcopyPOP
+
+For dense smoke/fire-like effects, use a `glslcopyPOP` that writes per-particle color/size from a compute shader, then render as point sprites with additive blending in a `renderTOP`.
+
+---
+
+## Collisions
+
+```python
+# Collision detection against an SOP
+coll = geo.create(popCollideTOP, 'ground_coll')
+coll.par.collidewithsop = '/project1/ground_geo'  # path to colliding SOP
+coll.par.bounce = 0.3
+coll.par.friction = 0.1
+# Insert between force and solver
+```
+
+For plane/box collisions only, use `popPlaneCollideTOP` (cheaper).
+
+---
+
+## Custom Per-Particle Data
+
+Add a custom channel via `popAttribCreateTOP` (or by writing through `glslcopyPOP`):
+
+```python
+# Add a "phase" attribute initialized random per-particle, used in render shader
+attr = geo.create(popAttribCreateTOP, 'add_phase')
+attr.par.attribname = 'phase'
+attr.par.value0 = 'rand(@id)'   # expression in TD's POP attribute language
+```
+
+Then in the render shader, `texture(sTDPOPInputs[0].phase, ...)` (or whichever sampler convention your TD version uses — verify with `td_get_docs(topic='pops')`).
+
+---
+
+## Legacy particleSOP (Use Sparingly)
+
+For quick demos or low-count systems:
+
+```python
+# Inside a geo
+psrc = geo.create(addSOP, 'point_src')      # source: a single point
+psrc.par.points = '0 0 0'
+
+part = geo.create(particleSOP, 'particles')
+part.par.life = 3.0
+part.par.birthrate = 100
+part.par.gravityy = -9.8
+part.par.windx = 0.5
+part.inputConnectors[0].connect(psrc)
+```
+
+CPU-bound. Beyond ~5,000 active particles you'll see frame drops.
+
+---
+
+## Pitfalls
+
+1. **Particles don't appear** — usually a render-side issue. Check via `td_get_screenshot` on the solver output (renders the buffer as a TOP-like view in newer TD). Then check the `geometryCOMP`'s render path.
+2. **Burst won't fire** — verify the `burst` param is a pulse, not a toggle. Pulses must use `.pulse()`, not `= True`.
+3. **Particles teleport on first frame** — uninitialized velocity. Set `popSourceTOP.par.initialvelocityX/Y/Z` or zero them explicitly.
+4. **Gravity feels wrong** — TD's "1 unit" depends on your scene scale. Start with `fy = -1.0` and scale up rather than using real-world 9.8.
+5. **High birthrate = stuttering** — birthrate is per-second, not per-frame. At 60fps, `birthrate = 6000` is 100/frame which is fine; `birthrate = 600000` will tank.
+6. **POP solver order matters** — forces apply in the order they appear in the chain. Putting gravity AFTER drag dampens gravity itself; usually not what you want.
+7. **Instancing param name varies** — `mat.par.instancingTOP` vs. `mat.par.instanceop` vs. `mat.par.instances` differs across TD versions. Always check `td_get_par_info(op_type='glslMAT')`.
+8. **Cooking dependency loops** — POP solvers create implicit time-loops. The "cook dependency loop" warning is expected and harmless for POPs.
+9. **CHOP-driven force values** — when a force param is expression-bound to a CHOP (e.g., audio-reactive gravity), make sure the CHOP cooks before the solver. If not, force lags by one frame.
+
+---
+
+## Performance Targets
+
+| Particle count | Setup | Frame budget @ 60fps |
+|---|---|---|
+| < 1k | particleSOP fine | trivial |
+| 1k - 10k | POPs, simple forces | ~2-5ms |
+| 10k - 100k | POPs, GPU-only forces | ~5-15ms |
+| 100k+ | `glslcopyPOP`, custom compute | ~10-25ms |
+| 1M+ | Custom GPU buffer, no POP framework | depends on shader |
+
+Use `td_get_perf` to find which op in the POP chain is the bottleneck.
+
+---
+
+## Quick Recipes
+
+| Goal | Pipeline |
+|---|---|
+| Smoke plume | `popSourceTOP` (point) → gravity + wind + noise → `popDeleteTOP` (life) → solver → glslMAT instancing |
+| Beat-triggered burst | `triggerCHOP` (audio) → chopExecuteDAT pulses `popSourceTOP.par.burst` |
+| Fireworks shell | Burst at point → drag + gravity → secondary burst on lifetime threshold |
+| Snow/rain | Continuous emission across XZ plane (high y), gravity + small wind, infinite life box-deleted |
+| Sparks | Burst, very short life (0.3s), bright additive render, motion blur via feedback |
+| Audio particles | Birthrate driven by audio envelope, color driven by frequency band |