PHYSICS ENGINE INTEGRATION

What is a physics engine?

A physics engine is a computer software that provides an approximate simulation of certain physical systems.
It is used in the domains of computer graphics, video games and CGI film.
Phsics engine provides real-time simulations.
A physics engine is essentially a big calculator that does mathematics needed to simulate physics.
A physics engine is a software component that simulates the laws of physics in a virtual environment, such as a video game world.

* There are two general classes of physics engines:

- Real-time: These engines are typically used in games and other interactive applications and use simpler calculations with less accuracy to compute in time for the game to respond.

- High-precision: These engines require more processing power to calculate very precise physics for movies and scientific simulations. → What does a physics engine do?

Physics engine performs calculations to simulate anything that happens in a game.
Examples: A character jumps, the ball hits the corner, the car moves faster...

A physics engine is capable of:
- Motion simulation: Calculating the velocity, acceleration, force...
- Collision detection: Observing whether objects collide or not.
- Collision response: Determining the next step after a collusion.
- Application of laws of physics: Simpified versions of concepts such as gravity, friction...

How does a physics engine work?

Even though physics engine workflow may differ in different games, there is a common pattern that we can point out easily.

1. Apply forces
2. Integrate motion
3. Collision detection
4. Collision resolution
5. Update state
6. Repeat

First, gravity is applied with other physical effects. Then the velocity is updated along with the position. The objects are checked to see if they are touching each other. If there is a collision, the objects are adjusted so that they don't collide with each other. State is updated by sending the new positions to the engine. These steps are repeated, usually for a fixed timestep or every frame.

Why is integration needed?

The integration of a physics engine into a game engine is a complex process. It requires optimizing the engine's computational resources and ensuring that it can handle the various interactions and movements within the game world.
The physics engine works mutually with other components of the game engine to create a smooth and immersive gameplay experience.Together, these components create a cohesive world that responds to the player's actions and inputs.
Overall, the integration of a physics engine into a game engine is a crucial part of the game development process. It ensures that the game environment is interactive and realistic, contributing to the overall success of the game.

How to integrate physics engine in games?

Integration is performed via an API (application programming interface). This API allows the game to communicate with the physics system by creating physics objects, running simulations, and retrieving results.
Through this API, the integration process generally involves several steps:

1) Creating a physics world: Basically, form an area which will contain physics objects of the game.

2) Mapping physics objects: Physics engine does not have all of the game's objects. With mapping, all the objects in the game get corresponding counterparts in the physics engine.

3) Simulation step: Apply the physics forces to each fixed timestep/frame with this step. Compute necessary calculations for the simulation.

4) Synchronizing results: Once the simulation is over, write the new positions. This step ensures the calculated physics makes a difference in the game.

5) Handling events and interactions: Deal with the events and interactions created by physics.

Create physics world → Map objects → Run simulation → Sync results → Handle events

There are two commonly encountered situations about the integration of physics engine:

* Using an external physics engine and following the 5 steps mentioned above. This approach is rather considered low-level.
* Using high-level built-in physics systems, where the game engine has the physics engine already integrated in its structure.

Future trends in physics engine integration:

With advancing technology and the contributions of AI, physics engines create more realistic simulations. Furthermore, in today's modeled systems, physics calculations are performed on the GPU instead of the CPU, and the parallelism provided by the GPU allows the engine to run faster.

Conclusion:

Physics engine integration is a critical part of game engine architecture. It connects the physics simulation with the rest of the engine, enabling realistic movement and interaction. Proper integration ensures consistency, performance, and synchronization between different systems in the game.

References:

https://winter.dev/articles/physics-engine/
https://www.cs.cornell.edu/courses/cs3152/2022sp/labs/program4/
https://www.reddit.com/r/gamedev/comments/4gu0ke/integrators_in_physics_engines_rk4_improved_euler/
https://bluebirdinternational.com/physics-engine/