Project 4: 2D Collision and Physics Slice

A collision sandbox with AABB broadphase, narrowphase resolution, and debug vector overlays.

Quick Reference

Attribute Value
Difficulty Level 3
Time Estimate 2 weeks
Main Programming Language C# (.NET 8) + MonoGame
Alternative Programming Languages F#, C++ (raylib), Godot C#
Coolness Level Level 4
Business Potential Level 1
Prerequisites Deterministic loop basics, debugging discipline, content pipeline fundamentals
Key Topics Broadphase culling, Contact resolution, Fixed timestep physics

1. Learning Objectives

  1. Translate one concrete production question into a testable implementation plan.
  2. Implement and validate the feature in a MonoGame runtime context.
  3. Instrument success and failure paths with actionable diagnostics.
  4. Produce a repeatable demo artifact for portfolio or interview use.

2. All Theory Needed (Per-Concept Breakdown)

Broadphase culling

Fundamentals Broadphase culling is central to this project because it defines the non-negotiable behavioral contract for the feature. You should be able to describe valid inputs, legal state transitions, and expected outputs under normal and failure conditions.

Deep Dive into the concept Treat Broadphase culling as a boundary-setting mechanism. Start by defining the smallest deterministic scenario that proves the feature works. Stress that scenario under altered timing, altered content inputs, and altered user actions. If behavior changes unexpectedly, document hidden coupling and sequence assumptions. Keep transitions explicit and observable via logs or debug panels. Connect each transition to an event record so regression analysis is possible after refactors.

Contact resolution

Fundamentals Contact resolution ensures the project scales from local prototype behavior to repeatable system behavior.

Deep Dive into the concept Use Contact resolution to reason about data flow ownership and mutation timing. Document where writes occur, when validation runs, and how rollback behaves if a write fails.

Fixed timestep physics

Fundamentals Fixed timestep physics connects this project to shipping reality by forcing you to think about operational constraints early.

Deep Dive into the concept Define one production-like failure mode related to Fixed timestep physics and build a mitigation checklist. The solution is complete when you can demonstrate both a golden path and a controlled failure path.

3. Project Specification

3.1 What You Will Build

A physics validation scene for 2D AABB collision detection and resolution with deterministic debug visualizations and repeatable scenarios.

Visible game deliverable:

  • Debug view shows AABB bounds, normals, and penetration vectors
  • Statistics panel displays broadphase pairs and narrowphase contacts
  • Scenario selector runs stack stability and projectile tunnel tests

3.2 Functional Requirements

  1. Implement broadphase pair pruning for active bodies.
  2. Resolve narrowphase contacts with stable depenetration.
  3. Render collision diagnostics (bounds, normals, penetration depth).
  4. Provide deterministic scenario scripts for regression validation.

3.3 Non-Functional Requirements

  • Performance: Must remain inside project-appropriate frame budget.
  • Reliability: Must recover from at least one injected failure mode.
  • Usability: Outcome must be observable by a reviewer in under two minutes.

3.4 Example Usage / Output

[PHYS] broadphase_pairs=142 contacts=37
[PHYS] stack_jitter_metric=0.003 PASS
[PHYS] projectile_tunnel_events=0 PASS

3.5 Data Formats / Schemas / Protocols

  • Event record: {timestamp, module, action, result}
  • Feature state snapshot: {version, state, counters, flags}

3.6 Edge Cases

  • Thin collider at high velocity.
  • Multiple simultaneous corner contacts.
  • Resting bodies with small floating-point drift.

3.7 Real World Outcome

This is a game-facing outcome you can see and play immediately.

What you will see in the game window:

  • Debug view shows AABB bounds, normals, and penetration vectors
  • Statistics panel displays broadphase pairs and narrowphase contacts
  • Scenario selector runs stack stability and projectile tunnel tests

Project 4 2D Collision and Physics Slice Window Mockup

How you interact:

  • 1 runs stack stability test
  • 2 runs high-speed projectile test
  • N toggles contact normal overlays

3.7.1 How to Run (Copy/Paste)

$ dotnet restore
$ dotnet build
$ dotnet run --project src/Game -- --scene physics-lab

3.7.2 Golden Path Demo (Deterministic)

  1. Start the scene and confirm all HUD panels load.
  2. Perform the three core interactions listed above.
  3. Verify the success signal appears without warnings.

3.7.3 If CLI: exact transcript

$ dotnet run --project src/Game -- --scene physics-lab
[PHYS] broadphase_pairs=142 contacts=37
[PHYS] stack_jitter_metric=0.003 PASS
[PHYS] projectile_tunnel_events=0 PASS

3.7.7 If GUI / Desktop

+------------------------------------------------------+
| physics-lab                                   [F1 HUD] |
|------------------------------------------------------|
| PLAYFIELD: gameplay objects and interactions         |
| HUD: key metrics + status badges                    |
| STATUS: success/failure cues and prompts            |
+------------------------------------------------------+

4. Solution Architecture

4.1 High-Level Design

Scene Script -> Broadphase -> Narrowphase -> Resolver -> Debug Draw

Scene Script -> Broadphase -> Narrowphase -> Resolver -> Debug Draw

4.2 Key Components

Component Responsibility Key Decisions
CollisionBroadphase Filters potential overlap pairs Spatial partitioning to avoid O(n^2) scans
ContactResolver Computes stable separation and velocity response Order contacts deterministically
PhysicsDebugView Visualizes collision internals Show contacts and normals per frame

4.4 Algorithm Overview

  1. Validate preconditions.
  2. Apply deterministic transition.
  3. Emit feedback and telemetry.
  4. Persist if required.

5. Implementation Guide

5.3 The Core Question You’re Answering

“How do you prevent tunneling and jitter while keeping the simulation explainable?”

5.4 Concepts You Must Understand First

  1. Broadphase culling
  2. Contact resolution
  3. Fixed timestep physics

5.5 Questions to Guide Your Design

  1. Which contact ordering rule keeps resolution stable?
  2. How will you test tunneling deterministically?
  3. What metrics best describe collision solver quality?

5.6 Thinking Exercise

Trace one full success path and one failure path on paper before implementation.

5.7 The Interview Questions They’ll Ask

  1. Why did you pick this architecture boundary?
  2. Which failure mode did you prioritize first and why?
  3. How does your instrumentation accelerate debugging?
  4. How would you scale this feature to a larger game?

5.8 Hints in Layers

  • Hint 1: Stabilize one invariant before feature expansion.
  • Hint 2: Add diagnostics before optimization.
  • Hint 3: Keep platform calls at system boundaries.
  • Hint 4: Re-run deterministic scenario after each refactor.

5.9 Books That Will Help

Topic Book Chapter
Core concept “Game Physics Engine Development by Ian Millington” Relevant concept chapter
Reliability “Release It!” Failure handling chapters
Architecture “Clean Architecture” Boundary and dependency chapters

6. Testing Strategy

  1. Golden path completes and emits success signal.
  2. Injected failure path recovers without crash.
  3. Re-run scenario after restart and confirm consistency.

7. Common Pitfalls & Debugging

  • Hidden initialization order coupling
  • Time-coupled behavior tied to render rate
  • Missing fallback behavior on platform call failure

8. Extensions & Challenges

  • Beginner: add one extra diagnostics panel metric.
  • Intermediate: add replay capture for event flow.
  • Advanced: add automated stress test harness.

9. Real-World Connections

This project mirrors shipping feature-module work in real indie and mid-size game teams.

10. Resources

  • Steamworks official docs
  • MonoGame docs
  • Gemini image generation docs (for asset-related projects)

11. Self-Assessment Checklist

  • I can explain the feature invariant and prove it in a demo.
  • I can trigger and handle one deterministic failure scenario.
  • I can describe tradeoffs and future scaling choices.

12. Submission / Completion Criteria

Minimum Viable Completion:

  • Feature works in deterministic golden path.
  • One controlled failure path is handled gracefully.
  • Core diagnostics are visible and documented.

Full Completion:

  • All minimum criteria plus edge-case coverage and regression checks.

Excellence:

  • Includes polished instrumentation and clear productionization notes.