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unity-physicscore2d-filtering

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Collision filtering, layer-based interactions, custom collision filters, and trigger areas

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Unity PhysicsCore2D Filtering Expert

You are now acting as a Unity PhysicsCore2D filtering expert, specialized in collision filtering, layers, and trigger areas.

Overview

PhysicsCore2D provides flexible collision filtering to control which objects interact:

  • Layer-based filtering - Using collision layers and masks
  • Custom filters - Programmatic collision filtering
  • Trigger areas - Sensors that detect overlaps without collision
  • Collision groups - Group-based filtering
  • Callback filtering - Runtime collision decisions

Repository Examples

Reference examples from the PhysicsExamples2D repository:

Collision Filtering Concepts

Layer-Based Filtering

Use PhysicsMask to control layer interactions:

  • Assign shapes to specific layers
  • Define which layers can collide
  • Efficient and performant
  • Configured at shape creation

Example layers:

  • Player (layer 0)
  • Enemy (layer 1)
  • Environment (layer 2)
  • Projectile (layer 3)

Custom Collision Filters

Implement custom filtering logic:

  • Filter based on object properties
  • Dynamic filtering conditions
  • Tag-based filtering
  • Distance-based filtering

Trigger Areas

Shapes that detect overlaps without physical collision:

  • Mark shapes as sensors/triggers
  • Receive overlap events
  • No collision response
  • Useful for detection zones

Common uses:

  • Pickup detection
  • Area triggers
  • Goal zones
  • Damage areas
  • Sensor zones

Collision Groups

Group objects for collective filtering:

  • Positive groups collide normally
  • Negative groups never collide (same group)
  • Zero group uses layer filtering
  • Useful for complex filtering rules

Filtering Strategies

Player-Enemy Filtering

  • Players collide with environment
  • Players collide with enemies
  • Enemies collide with environment
  • Enemies don't collide with each other

Projectile Filtering

  • Projectiles ignore shooter
  • Projectiles collide with targets
  • Projectiles ignore each other
  • Time-based filter enabling

Ragdoll Filtering

  • Ragdoll pieces don't self-collide
  • Ragdolls collide with environment
  • Connected pieces may overlap
  • Filter using negative groups

One-Way Platforms

  • Collide when approaching from above
  • No collision from below
  • Custom filter callback
  • Check relative velocity

Implementation Best Practices

Layer Design

  • Plan layer hierarchy early
  • Use meaningful layer names
  • Document layer interactions
  • Keep layer count reasonable (16-32 layers typical)

Performance

  • Prefer layer filtering over custom filters
  • Cache filter masks when possible
  • Minimize filter complexity
  • Use triggers sparingly

Trigger Areas

  • Keep trigger geometry simple
  • Process trigger events efficiently
  • Unregister callbacks when disabled
  • Consider using larger triggers vs. many small ones

Filter Callbacks

  • Keep callbacks fast
  • Avoid allocations in callbacks
  • Cache frequently used data
  • Return early when possible

Common Filtering Patterns

Ignore Self

Prevent object's shapes from colliding with each other:

  • Use negative collision group
  • All shapes share same negative group ID
  • Common for ragdolls and vehicles

Temporary Invincibility

Make object temporarily non-collidable:

  • Change to non-colliding layer
  • Use custom filter with timer
  • Re-enable after duration

Faction-Based

Filter based on team or faction:

  • Store faction ID in user data
  • Custom filter checks faction
  • Same faction may or may not collide

Progressive Collision

Enable collision after spawning:

  • Start with non-colliding layer
  • Switch to normal layer after delay
  • Prevents stuck spawns

Related Skills

When users need information about:

  • Layer helpers - Use unity-physicscore2d-layers (if available)
  • Trigger queries - Use unity-physicscore2d-queries
  • User data for filtering - Use unity-physicscore2d-helpers
  • Collision callbacks - Use unity-physicscore2d-collision or unity-physicscore2d-events

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/BasicContactFiltering.cs — minimal IContactFilterCallback that suppresses circle-vs-circle contacts while letting circles still collide with the ground chain.
  • examples/CustomContactFilter.cs — color-grouped boxes filtered by PhysicsUserData so only same-group bodies collide; demonstrates thread-safe identity passing.
  • examples/PostSimulateFiltering.cs — PhysicsEvents.PostSimulate filtering via PhysicsShape.GetContacts() with predicate filters on contact normal angle and impulse magnitude; includes a reusable ContactExtensions helper.