Build a Complete Physics-Based Game System

# CHAPTER 20

Build a Complete Physics-Based Game System #

1. Introduction #

You have spent 19 chapters exploring the deep, mathematical underpinnings of game physics. You have mastered Vectors, Newton's laws of motion, Collision Matrices, and multiplayer synchronization. However, theory is only the beginning. True mastery comes from integration—forcing distinct physics systems to interact without breaking the engine. In this final chapter, we will outline the architecture for the Capstone Project: "Physics Sandbox." This will be a complete vertical slice of a game featuring a responsive character, a drivable vehicle, hitscan and projectile weaponry, and interactive environmental puzzles. It is time to build a world.

2. Project Requirements #

The sandbox will feature:

• The Player: A Kinematic Character Controller with perfect ground detection, sloped movement, and Coyote Time.

• The Vehicle: A Rigidbody car utilizing WheelColliders and an artificially lowered Center of Mass.

• Combat: A rifle that utilizes instant Raycasts (Hitscan) for bullets, and physical Rigidbodies for a grenade launcher.

• Environment: A trigger-based water volume that applies Drag and Buoyancy to falling objects.

• Optimization: A strict Collision Matrix and Non-Allocating memory functions to maintain 60 FPS.

3. Step 1: The Kinematic Player Controller #

We begin with the player. We will NOT use a standard Rigidbody.

• PlayerMotor.cs:

• Utilize a CharacterController component.

• Read input (x, 0, z) and Normalize the vector to prevent diagonal exploits.

• Shoot a SphereCast downward from the feet to determine isGrounded.

• If grounded, reset velocity.y to -2f to stick to slopes.

• If jump is pressed, instantly set velocity.y = jumpForce.

• Multiply gravity by DeltaTime and add to velocity.y every frame.

• Call controller.Move(velocity * Time.deltaTime).

4. Step 2: The Physics-Driven Vehicle #

Next, we build a heavy, interactive car.

• The Setup: A Rigidbody box (Mass: 1500kg). Four empty GameObjects with WheelCollider components.

• CarController.cs:

• In Awake(), execute rb.centerOfMass = new Vector3(0, -0.5f, 0) to prevent the car from rolling over on tight turns.

• In FixedUpdate(), apply motorTorque to the rear wheels based on Vertical Input.

• Apply steerAngle to the front wheels based on Horizontal Input.

• Write a loop that syncs the visual tire meshes to the invisible WheelColliders.

5. Step 3: The Interaction (Entering the Car) #

The Player must be able to drive the Car.

• Put a BoxCollider (Is Trigger) near the car door.

• When the Player presses 'E' inside the trigger:

1. 1. Disable the Player's Mesh and CharacterController.

1. 2. Parent the Camera to the Car.

1. 3. Enable the CarController.cs script.

6. Step 4: Hybrid Weapon Systems #

The player needs tools to interact with the sandbox.

• The Laser Rifle (Hitscan):

• Use Physics.Raycast(camera.position, camera.forward, out hit, 100f).

• Check if the hit object has a Rigidbody.

• If yes, use AddForceAtPosition to realistically knock the object backward based on the exact point the laser hit!

• The Grenade Launcher (Projectile):

• Instantiate a grenade Prefab.

• Grab its Rigidbody and set velocity = barrel.forward * 20f.

• Use an IEnumerator Coroutine to wait 3 seconds.

• Use Physics.OverlapSphereNonAlloc to find nearby rigidbodies.

• Apply AddExplosionForce to scatter crates across the room.

7. Step 5: The Environmental Water Volume #

Create a lake to test our buoyancy math.

• Create a large BoxCollider set to Is Trigger.

• WaterVolume.cs:

• On OnTriggerEnter, set the entering Rigidbody's drag to 3.0f to simulate water resistance.

• In OnTriggerStay, calculate Depth = WaterSurfaceY - object.position.y.

• Apply rb.AddForce(Vector3.up * Depth * buoyancyMultiplier) to make objects bob naturally to the surface.

• On OnTriggerExit, return the drag to 0.1f.

8. Step 6: Global Physics Optimization #

Before we declare the game finished, we must protect the CPU.

1. 1. Open the Engine's Collision Matrix.

1. 2. Create layers: Player, Vehicle, Debris, Water.

1. 3. Uncheck interactions between Debris and Debris (falling boxes shouldn't calculate collisions against other falling boxes, saving thousands of calculations).

1. 4. Verify the Fixed Timestep is set to 0.02 (50 times a second).

1. 5. Ensure all fast-moving projectiles (grenades) are set to Continuous Dynamic collision to prevent tunneling through thin walls.

9. The Final Playtest #

1. 1. Hit Play.

1. 2. The Player moves flawlessly up ramps without bouncing, thanks to the Kinematic controller.

1. 3. You shoot a laser at a stack of crates. The Raycast perfectly calculates the surface normal and knocks the top crate off.

1. 4. You press 'E' to enter the vehicle. The low Center of Mass allows you to drift around corners at high speeds.

1. 5. You drive the car into the lake. The Trigger Volume detects the car, cranks up the drag, and applies depth-based buoyancy, causing the 1500kg car to slowly float to the surface.

10. Your Journey Continues #

Congratulations! You have successfully architected a highly complex, interconnected physics sandbox. You have proven that you understand not just how to push a box, but how to control the very fabric of virtual reality.

Game Physics is an endless, fascinating rabbit hole. From here, your path diverges based on your passion. You can dive into C++ and study the raw Navier-Stokes equations for fluid dynamics. You can explore compute shaders to calculate million-particle simulations on the GPU. Or, you can become a Game Designer, utilizing "Game Feel" and Arcade Physics to craft the next iconic platformer.

You have mastered the rules of the virtual world. Now go break them.

Course Complete.