physics-rendering-expert

name	physics-rendering-expert
description	Real-time rope/cable physics using Position-Based Dynamics (PBD), Verlet integration, and constraint solvers. Expert in quaternion math, Gauss-Seidel/Jacobi solvers, and tangling detection. Activate on 'rope simulation', 'PBD', 'Position-Based Dynamics', 'Verlet', 'constraint solver', 'quaternion', 'cable dynamics', 'cloth simulation', 'leash physics'. NOT for fluid dynamics (SPH/MPM), fracture simulation (FEM), offline cinematic physics, molecular dynamics, or general game physics engines (use Unity/Unreal built-ins).
allowed-tools	Read,Write,Edit,Bash,mcp__firecrawl__firecrawl_search,WebFetch
category	AI & Machine Learning
tags	["physics","pbd","verlet","simulation","constraints"]
pairs-with	[{"skill":"metal-shader-expert","reason":"GPU-accelerated physics rendering"},{"skill":"native-app-designer","reason":"Physics in app animations"}]

Physics & Rendering Expert: Rope Dynamics & Constraint Solving

Expert in computational physics for real-time rope/cable dynamics, constraint solving, and physically-based simulations.

When to Use This Skill

Use for:

• Real-time rope/cable/chain simulation (leashes, climbing ropes)
• Position-Based Dynamics (PBD) implementation
• Constraint solvers (Gauss-Seidel, Jacobi)
• Quaternion/dual-quaternion rotation math
• Verlet integration for particle systems
• Tangle detection (multi-rope collisions)

Do NOT use for:

• Fluid dynamics → specialized SPH/MPM solvers
• Fracture simulation → requires FEM or MPM
• Offline cinematic physics → different constraints
• Unity/Unreal physics → use built-in systems

Expert vs Novice Shibboleths

Topic	Novice	Expert
Constraint approach	Uses spring forces (F=ma)	Uses PBD (directly manipulates positions)
Why PBD	"Springs work fine"	Springs require tiny timesteps; PBD is unconditionally stable
Solver choice	"Just iterate until done"	Gauss-Seidel for chains, Jacobi for GPU
Iterations	20+ iterations	5-10 is optimal; diminishing returns after
Rotation	Uses Euler angles	Uses quaternions (no gimbal lock)
Integration	Forward Euler	Verlet (symplectic, energy-conserving)

Common Anti-Patterns

Force-Based Springs for Stiff Constraints

What it looks like	Why it's wrong
force = k * (distance - rest_length) with high k	High k requires tiny dt for stability; low k gives squishy ropes
Instead: Use PBD - directly move particles to satisfy constraints

Euler Angles for Rotation

What it looks like	Why it's wrong
rotation = vec3(pitch, yaw, roll)	Gimbal lock at 90° pitch; unstable composition
Instead: Use quaternions - 4 numbers, no gimbal lock, stable SLERP

Over-Iteration

What it looks like	Why it's wrong
solver_iterations = 50	Diminishing returns after 5-10; wastes cycles
Instead: Use 5-10 iterations; if more needed, use XPBD compliance

Single-Threaded Gauss-Seidel for Large Systems

What it looks like	Why it's wrong
Gauss-Seidel on 1000+ constraints	Gauss-Seidel is inherently sequential
Instead: Use Jacobi solver for GPU parallelization

Quick Reference

Why PBD Beats Force-Based Physics

• Unconditionally stable (large timesteps OK)
• Direct control over constraint satisfaction
• No spring constants to tune
• Predictable behavior

Solver Choice

Solver	Parallelizable	Convergence	Use Case
Gauss-Seidel	No	Fast	Chains, ropes
Jacobi	Yes (GPU)	Slower	Large meshes, cloth

Rotation Representation

• 3D rotation → Quaternion (never Euler)
• Rotation + translation → Dual quaternion
• Skinning/blending → Dual quaternion (no candy-wrapper artifact)

Performance Targets

System	Budget	Notes
Single rope (100 particles)	<0.5ms	5 iterations sufficient
Three-dog leash (60 particles)	<0.7ms	Include tangle detection
Cloth (1000 particles)	<2ms	Use Jacobi on GPU

Evolution Timeline

Era	Key Development
Pre-2006	Mass-spring systems, stability issues
2006-2015	PBD introduced (Müller et al.), unconditional stability
2016-2020	XPBD adds compliance for soft constraints
2021-2024	ALEM (2024 SIGGRAPH), BDEM, neural physics
2025+	XPBD standard, hybrid CPU/GPU, learned corrections

Decision Trees

Choosing constraint solver:

• Sequential structure (rope/chain)? → Gauss-Seidel
• Large parallel system (cloth/hair)? → Jacobi (GPU)
• Need soft constraints? → XPBD with compliance

Choosing integration:

• Position-only needed? → Basic Verlet
• Need velocity for forces? → Velocity Verlet
• High accuracy required? → RK4 (but PBD usually sufficient)

Integrates With

• metal-shader-expert - GPU compute shaders for Jacobi solver
• native-app-designer - Visualization and debugging UI

Reference Files

File	Contents
references/core-algorithms.md	PBD loop, Verlet, quaternions, solver implementations
references/tangle-physics.md	Multi-rope collision, Capstan friction, TangleConstraint

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Remember: Real-time physics is about stability and visual plausibility, not physical accuracy. PBD with 5-10 iterations at 60fps looks great and runs fast.