PhysECS

Physecs is a real-time 3D rigid body physics engine using a sequential impulses TGS solver and built on top of EnTT ECS.

Usage

Components

Physecs is intended to share the same transform component as your application, defined in Transform.h. It must be the same as the transform component in the main application, and include the fields glm::vec3 position and glm::quat orientation.

struct TransformComponent {
    glm::vec3 position;
    glm::quat orientation;
    glm::vec3 scale;
};

Additionally Physecs defines two components RigidBodyCollisionComponent and RigidBodyDynamicComponent.

struct RigidBodyCollisionComponent {
    std::vector<Collider> colliders;
};

struct RigidBodyDynamicComponent {
    float invMass;
    glm::vec3 com;
    glm::vec3 velocity;
    glm::mat3 invInertiaTensor;
    glm::vec3 angularVelocity;
    bool isKinematic;
    glm::mat3 invInertiaTensorWorld;
};

RigidBodyCollisionComponent defines the shapes the entity uses for collision, and RigidBodyDynamicComponent defines the properties of the entity used for simulating motion. If an entity has a RigidBodyCollisionComponent but not a RigidBodyDynamicComponent it is treated as a static body.

Currently the supported shapes are:

• Sphere
• Capsule
• Box
• Convex mesh
• Triangle mesh (static only)

Joints

Physecs also supports joints. The following joints are included:

• Fixed
• Revolute
• Spherical
• Gear
• Prismatic
• Servo
• Universal (U-joint)

But new joints can be added by subclassing the Joint class.

Simulation

A Physecs scene takes an entt::registry in it's constructor.

physecs::Scene physicsScene = physecs::Scene(registry);

In order to advance the simulation by an amount of time, call the void simulate(float timeStep) function on it. You can change the number of substeps and iterations on the physics scene to create more or less fidelity.

Check out the demo for an example of usage.

Demo

To build the demo, enable the BUILD_DEMO option in CMake.

Dependencies

• EnTT 3.12.2
• GLM 0.9.9.8
• OpenGL 3.3 (Demo)
• SDL2 (Demo)
• TinyGltf (Demo)
• STB image (Demo)
• C++ standard library

Requirements

C++ 17

Why build physics on top of EnTT and GLM?

I am developing this physics library for a specific game I am making and my engine uses EnTT and GLM.

Future plans

• Fix bodies tilting when bouncing off a flat surface when there is multiple contact points
• Add multithreading
• Continuous collision detection

References

These are the some of the materials I used to learn about physics simulation and collision detection and other helpful resources:

• https://box2d.org/publications/
• https://box2d.org/posts/2024/02/solver2d/
• Christer Ericson: "Real-Time Collision Detection"
• Philip J. Schneider, David H. Eberly: "Geometric Tools for Computer Graphics"
• David H. Eberly: "3D Game Engine Design: A Practical Approach to Real-Time Computer Graphics"
• https://media.steampowered.com/apps/valve/2015/DirkGregorius_Contacts.pdf
• https://allenchou.net/game-physics-series/
• https://github.com/kevinmoran/GJK
• https://www.codercorner.com/MeshContacts.pdf