unity-physicscore2d-stacking - SKILL.md Agent Skill

name: unity-physicscore2d-stacking description: Stacking stability, structural physics (arches, card houses), and domino chain reactions

Unity PhysicsCore2D Stacking Expert

You are now acting as a Unity PhysicsCore2D stacking expert, specialized in stable stacking, structures, and cascading physics.

Overview

Stacking physics requires careful tuning for stability:

Stack stability - Preventing collapse
Structural formations - Arches, towers, bridges
Domino effects - Chain reactions
Confined stacking - Containers and piles
Balanced structures - Precarious arrangements

Repository Examples

Reference examples from the PhysicsExamples2D repository:

Stacking Stability

Factors Affecting Stability

Contact area - Larger contact = more stable
Center of mass - Lower is more stable
Mass distribution - Bottom-heavy preferred
Friction - Higher friction increases stability
Shape geometry - Flat surfaces stack better
Solver iterations - More iterations = more stable

Stability Techniques

Shape Optimization

Use flat contact surfaces
Avoid sharp corners
Round edges slightly (beveled polygons)
Larger contact patches
Appropriate shape complexity

Material Properties

Higher friction on stacking surfaces
Low restitution (no bouncing)
Appropriate mass distribution
Consistent material between layers

Physics Settings

Increase solver iterations (8-16)
Lower timestep if needed
Enable position correction
Tune contact tolerance
Adjust sleep thresholds

Structural Formations

Arches

Self-supporting curved structures:

Keystone at top provides stability
Lateral forces balance
Requires precise placement
Foundation must be stable
Use appropriate mass ratios

Creation tips:

Start with foundation blocks
Place side supports
Add curved arch segments
Place keystone last
Allow settling time

Towers

Vertical stacks:

Align centers of mass vertically
Use uniform or tapering masses
Larger base for stability
Minimize wobble during creation
Consider crossbracing

Bridges

Span structures:

Support at both ends
Distribute load evenly
Use arch or beam design
Tension and compression balance
Test with dynamic loads

Card Houses

Precarious lean-to structures:

Friction critical for stability
Angle of lean important
Balance forces carefully
Very sensitive to disturbance
Requires high solver quality

Domino Chains

Domino Setup

Chain reaction sequences:

Proper spacing (typically domino height / 2)
Consistent domino properties
Stable placement on surface
Trigger mechanism
Environmental considerations

Chain Reaction Design

Test reliability of toppling
Account for bounce and scatter
Use guides or channels
Multiple parallel chains
Branching and merging paths

Advanced Domino Effects

Spirals - Curved domino paths
Stairs - Multi-level chains
Splits - One domino triggers multiple
Delays - Slow sections
Jumps - Gaps in chain

Confined Stacking

Container Physics

Stacking within boundaries:

Wall friction affects settling
Pressure distribution
Particle packing behavior
Flow and avalanche effects
Stable vs. chaotic packing

Piling Dynamics

Objects falling into container:

Drop height affects energy
Order affects final configuration
Shape affects packing density
Mass affects pressure
Friction affects angle of repose

Best Practices

Creation Process

Bottom-up - Always build from base
Settling time - Let physics stabilize
Incremental - Add pieces gradually
Positioning - Precise initial placement
Testing - Verify stability before continuing

Stability Tuning

Start with high friction (0.5-0.8)
Use low restitution (0.0-0.1)
Increase solver iterations for tall stacks
Enable sleeping for stable stacks
Test with disturbances

Common Pitfalls

Creating entire stack at once
Misaligned centers of mass
Insufficient friction
Too few solver iterations
Sharp corners causing instability
Excessive initial velocities

Performance Considerations

Large stacks are expensive
Use sleeping for stable regions
Consider static bodies for permanent structures
Limit active collision pairs
Profile with target stack size

Gameplay Applications

Building Games

Player-constructed structures:

Provide stability feedback
Show center of mass
Highlight unstable connections
Undo/redo for iteration
Save/load structures

Destruction Games

Knocking down structures:

Score based on destruction
Chain reactions increase score
Precision vs. power tradeoffs
Multiple attempt strategies
Progressive difficulty

Puzzle Games

Stability as puzzle mechanic:

Required formations
Limited pieces
Precise placement challenges
Physics-based solutions
Time or move limits

Advanced Techniques

Active Stabilization

Helping unstable stacks:

Temporary constraints
Gradual activation
Damping during creation
Collision filtering during build
Motor drives for correction

Procedural Stacking

Automated structure creation:

Algorithmic placement
Stability testing
Iterative adjustment
Random variations
Validated generation

Destruction Analysis

Studying collapse:

Identify failure points
Force distribution
Progressive collapse
Energy dissipation
Debris patterns

Testing and Validation

Stability Tests

Apply small impulses
Test with time acceleration
Environmental disturbances
Collision from projectiles
Sustained loads

Stress Testing

Maximum stack height
Overloading structures
Worst-case configurations
Edge case geometries
Performance under load

Related Skills

When users need information about:

Material properties - Use unity-physicscore2d-materials
Collision behavior - Use unity-physicscore2d-collision
Shape optimization - Use unity-physicscore2d-shapes-advanced
Physics settings - Use unity-physicscore2d-settings (if available)
Performance - Use unity-physicscore2d-performance

Worked Examples

All examples below assume the standard PhysicsCore2D OnEnable/OnDisable lifecycle. See the umbrella skill unity-physicscore2d, section "Creating and Destroy Physics Objects", for the canonical lifecycle pattern.

examples/Arch.cs — stone arch built from trapezoidal voussoirs + keystone + 4 box weights; friction-sensitive structural stack.
examples/CardHouse.cs — 5-story card house using extremely thin rectangles balanced at angles; stress test for stable thin-shape stacking.
examples/DominoChain.cs — five domino shelves (alternating tip directions) triggered by a small ApplyLinearImpulse to the first/last domino of each row.
examples/Pyramid.cs — large pyramid stack of dynamic boxes (default 1830 bodies); slight corner radius for stacking stability + chamber walls catch any collapse.