Incorrect collision detection in physics-based interactions

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What is Collision Detection?

Collision detection is a core part of a physics engine. It determines whether two (or more) objects are intersecting (overlapping) or are about to collide, so that the engine can respond — by stopping movement, triggering physics responses, or firing events.

It typically involves two stages:

1. Broad phase – quickly checks potential collisions using bounding volumes (e.g. AABB).
2. Narrow phase – does precise checks with shapes (like spheres, boxes, polygons, meshes).

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What Is “Incorrect” Collision Detection?

“Incorrect” collision detection can mean:

• False positives: Detecting a collision that didn’t actually happen
• False negatives: Missing a collision that did happen
• Tunneling: Fast-moving objects pass through others without detection
• Jittering: Objects constantly flicker or bounce due to rapid contact/resolve cycles
• Sticky surfaces: Objects get stuck or don’t separate correctly after contact
• Overlap without resolution: Colliders pass through each other and stay overlapped

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Common Causes of Incorrect Collision Detection

1. High Object Speed (Tunneling)

• If an object moves too far between frames, it may skip past other colliders.
• Fixes:
  • Use continuous collision detection (CCD) instead of discrete
  • Increase physics update rate (smaller fixedDeltaTime)
  • Use raycasting or sweeps for fast projectiles

2. Low Physics Update Rate

• If your physics timestep is too long (e.g. low FPS), collisions can be missed or jittery.
• Fix:
  • Set a smaller fixed time step (FixedUpdate in Unity or substeps in Unreal)

3. Improper Collider Configuration

• Incorrect shapes, scales, or hierarchy (e.g., colliders not matching visuals)
• Fix:
  • Double-check all collider types (box, sphere, mesh, etc.)
  • Make sure colliders are properly scaled and aligned with objects

4. Improper Layer or Mask Settings

• In some engines, collision layers or masks control which objects interact.
• Fix:
  • Ensure correct collision layers/masks are set to allow proper detection

5. Rigidbodies Missing or Incorrect

• If colliders don’t have appropriate rigidbody components, they may not behave as expected.
• Fix:
  • Static objects: collider only
  • Dynamic objects: collider + dynamic rigidbody
  • Kinematic objects: collider + kinematic rigidbody (for controlled motion)

6. Precision Issues or Floating-Point Errors

• In large worlds or at tiny scales, floating-point precision can cause detection problems.
• Fix:
  • Keep simulations near world origin
  • Use reasonable object scales (not too small or too big)

7. Mesh Colliders vs. Primitive Colliders

• Mesh colliders can be less accurate and more expensive.
• Fix:
  • Use primitive colliders (box, capsule, sphere) where possible
  • Use convex meshes for dynamic objects

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Debugging & Fixing Tips

• Visualize Colliders: Most engines have a debug view to show colliders in the editor or at runtime.
• Log Collisions: Log collision callbacks to see if/when they’re triggered.
• Raycast or Sweep Test: Manually check expected collisions.
• Sub-Stepping: Some engines support sub-stepping for more accurate results.

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Engine-Specific Notes

Unity

• Collision Detection Mode: Set to Continuous or Continuous Dynamic for fast-moving objects.
• Use FixedUpdate for physics interactions.
• Use Physics.autoSimulation = false for custom stepping if needed.

Unreal Engine

• Check collision presets and response channels.
• Use CCD (bUseCCD = true) for fast objects.
• Consider using Sweep with movement functions.

Box2D / 2D Engines

• Fast-moving bullets should use raycasts or bullet mode.
• Ensure proper collision filters (category/mask bits).

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✅ Best Practices Summary

Problem	Fix
Tunneling	Use continuous collision detection or raycasts
Missed collisions	Increase physics update rate
Jittering	Stabilize contact resolution, avoid stacking
Overlapping	Ensure correct collider types and rigidbody use
Performance issues	Use simple colliders and layers

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