Getting Started with ReactPhysics3D: A Beginner’s GuideReactPhysics3D is a lightweight, open-source physics engine written in C++ designed for real-time simulations in games, interactive applications, and simulations. It focuses on ease of use, stability, and performance while providing a clean API for rigid-body dynamics, collision detection, and joint constraints. This guide walks you through what ReactPhysics3D offers, how to set it up, core concepts, step-by-step examples, common pitfalls, and tips for expanding your simulations.
Why choose ReactPhysics3D?
- Lightweight and easy to integrate — ReactPhysics3D is designed to be embedded into existing projects without heavy dependencies.
- Real-time performance — Optimized for simulations at game update rates.
- Simple API — Well-structured C++ interfaces make it approachable for beginners.
- Cross-platform — Works on Windows, macOS, Linux, and can be integrated with engines or graphics libraries such as OpenGL, SDL, or custom renderers.
- Open-source — You can inspect, modify, and contribute to the codebase.
Installing and building ReactPhysics3D
ReactPhysics3D is distributed with CMake build scripts, making integration straightforward. The typical steps are:
- Get the source:
- Clone the repository from GitHub: git clone https://github.com/DanielChappuis/reactphysics3d.git
- Build with CMake:
- Create a build directory: mkdir build && cd build
- Configure: cmake ..
- Build: cmake –build . –config Release
- Link into your project:
- Use the generated static or shared library (.a/.lib or .so/.dll) and include the headers from the src/include directory.
- Alternatively, add the ReactPhysics3D project as a subdirectory in your CMakeLists to build it as part of your project.
Example CMake snippet to include ReactPhysics3D as a subproject:
add_subdirectory(path/to/reactphysics3d) target_link_libraries(your_game PRIVATE reactphysics3d) target_include_directories(your_game PRIVATE ${PROJECT_SOURCE_DIR}/path/to/reactphysics3d/src) Core concepts
Understanding a few core concepts will help you effectively use ReactPhysics3D.
- Rigid bodies — Objects that do not deform; they have mass, inertia, and transform.
- Collision shapes — Primitive shapes used for collision detection (box, sphere, capsule, convex mesh).
- World (PhysicsWorld) — The container for bodies, constraints, and collision detection; it steps the simulation forward.
- Colliders — Attach collision shapes to rigid bodies with local transforms.
- Joints — Constraints connecting two bodies (hinge, fixed, slider, etc.).
- Collision callbacks — Events for collision start, persist, and end; useful for gameplay logic.
A minimal example: Creating a world and a falling box
Below is a simple C++ example that demonstrates creating a physics world, adding a static ground and a dynamic falling box, stepping the simulation, and printing the box’s position.
#include <reactphysics3d/reactphysics3d.h> #include <iostream> int main() { // Create physics common object (memory allocator, etc.) rp3d::PhysicsCommon physicsCommon; // Create the physics world with default settings rp3d::PhysicsWorld::WorldSettings settings; settings.gravity = rp3d::Vector3(0, -9.81, 0); rp3d::PhysicsWorld* world = physicsCommon.createPhysicsWorld(settings); // Create ground (static body) rp3d::Transform groundTransform(rp3d::Vector3(0, -1, 0), rp3d::Quaternion::identity()); rp3d::RigidBody* ground = world->createRigidBody(groundTransform); rp3d::BoxShape* groundShape = physicsCommon.createBoxShape(rp3d::Vector3(50, 1, 50)); ground->addCollider(groundShape, rp3d::Transform::identity()); // Create dynamic box rp3d::Transform boxTransform(rp3d::Vector3(0, 5, 0), rp3d::Quaternion::identity()); rp3d::RigidBody* box = world->createRigidBody(boxTransform); box->setType(rp3d::BodyType::DYNAMIC); rp3d::BoxShape* boxShape = physicsCommon.createBoxShape(rp3d::Vector3(1, 1, 1)); rp3d::Collider* boxCollider = box->addCollider(boxShape, rp3d::Transform::identity()); // Set mass (inertia calculated automatically) rp3d::decimal mass = 1.0; rp3d::Vector3 localInertia; boxShape->computeLocalInertia(mass, localInertia); box->setMass(mass); // Simulate for a few steps const float timeStep = 1.0f / 60.0f; for (int i = 0; i < 180; ++i) { world->update(timeStep); rp3d::Transform t = box->getTransform(); rp3d::Vector3 pos = t.getPosition(); std::cout << "Step " << i << " Box position: " << pos.x << ", " << pos.y << ", " << pos.z << " "; } // Cleanup physicsCommon.destroyPhysicsWorld(world); return 0; } Notes:
- Use PhysicsCommon to create/destroy objects and shapes.
- Body mass and inertia are used to compute dynamics; set the mass on dynamic bodies.
- Always call world->update(dt) each frame.
Collision detection and contact handling
ReactPhysics3D provides callbacks to notify you of collisions. Implement rp3d::ContactListener and register it with the world to receive onContact(), onContactAdded(), and onContactRemoved() events. Typical uses include playing sound effects, spawning particles, or handling game logic when objects collide.
Example skeleton:
class MyContactListener : public rp3d::ContactListener { public: void onContact(const rp3d::CollisionCallbackData& callbackData) override { // called each simulation step for existing contacts } void onContactAdded(const rp3d::CollisionCallbackData& callbackData) override { // new contact } void onContactRemoved(const rp3d::CollisionCallbackData& callbackData) override { // contact removed } }; Register:
MyContactListener* listener = new MyContactListener(); world->setEventListener(listener); Joints and constraints
Common joints in ReactPhysics3D:
- Fixed joint — keeps two bodies at a fixed relative transform.
- Hinge joint — rotation around a single axis (like a door).
- Slider joint — allows translation along one axis.
- Ball-and-socket joint — spherical joint with three rotational degrees of freedom.
Example: creating a hinge joint between two bodies requires joint info specifying local anchor points and axes.
Integrating with rendering
ReactPhysics3D is responsible for physics only. For visuals:
- Maintain parallel transform data for your renderable objects.
- After world->update(dt), copy the rigid body transforms to your renderer.
- Convert rp3d::Transform to your engine’s matrix or position/rotation format.
Example conversion (pseudo):
- Position: rp3d::Vector3 -> vec3
- Rotation: rp3d::Quaternion -> mat4 (use quaternion-to-matrix function)
Performance tips
- Use simple collision shapes (boxes, spheres, capsules) when possible; convex meshes are more expensive.
- Reduce number of active colliders by deactivating sleeping bodies.
- Use appropriate broad-phase settings for large worlds.
- Keep fixed time steps for deterministic behavior; accumulate frame time and call world->update(fixedDelta) in a loop.
Common pitfalls
- Forgetting to set body type to DYNAMIC for movable objects.
- Not setting mass on dynamic bodies (default mass may be zero).
- Mixing units — pick meters/seconds consistently.
- Using very small or very large mass/inertia values that cause instability.
- Updating physics with variable large dt values — use fixed steps.
Useful extensions and integrations
- Integrate with input systems and game logic by using collision callbacks.
- Use convex decomposition tools to simplify complex meshes.
- Combine with animation systems for ragdolls — use joints and constraints to simulate limbs.
Further resources
- Official ReactPhysics3D GitHub repository for source, examples, and documentation.
- Example projects demonstrating integration with OpenGL and game engines.
- Community forums and issue tracker for troubleshooting.
This guide provides a practical starting point for using ReactPhysics3D. Once comfortable with the basics (world, rigid bodies, shapes, and joints), experiment with collision callbacks, compound shapes, and joint configurations to build more realistic and interactive simulations.
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