Bullet Physics

What it is

Bullet Physics is a professional-grade physics simulation library for real-time collision detection, rigid body dynamics, soft body dynamics, and vehicle simulation. It is widely used in games, VR, robotics, and film production.

Bullet provides APIs to simulate physics worlds with rigid bodies, soft bodies, vehicles, and constraints. It includes a broad-phase collision detection system, efficient solvers, and supports GPU acceleration via OpenCL.

Installation

sudo apt install libbullet-dev

Getting started

The smallest useful thing you can do with it, and what each part means.

Hello World physics simulation

cpp

12345678910111213141516171819202122232425262728293031323334353637383940414243444546474849505152#include <btBulletDynamicsCommon.h>
#include <iostream>

int main() {
    // Broadphase
    btBroadphaseInterface* broadphase = new btDbvtBroadphase();

    // Collision configuration and dispatcher
    btDefaultCollisionConfiguration* collisionConfig = new btDefaultCollisionConfiguration();
    btCollisionDispatcher* dispatcher = new btCollisionDispatcher(collisionConfig);

    // Solver
    btSequentialImpulseConstraintSolver* solver = new btSequentialImpulseConstraintSolver;

    // Dynamics world
    btDiscreteDynamicsWorld* dynamicsWorld = new btDiscreteDynamicsWorld(dispatcher, broadphase, solver, collisionConfig);
    dynamicsWorld->setGravity(btVector3(0, -9.81f, 0));

    // Ground plane
    btCollisionShape* groundShape = new btStaticPlaneShape(btVector3(0, 1, 0), 1);
    btDefaultMotionState* groundMotionState = new btDefaultMotionState(btTransform(btQuaternion(0,0,0,1), btVector3(0,-1,0)));
    btRigidBody::btRigidBodyConstructionInfo groundRigidBodyCI(0, groundMotionState, groundShape, btVector3(0,0,0));
    btRigidBody* groundRigidBody = new btRigidBody(groundRigidBodyCI);
    dynamicsWorld->addRigidBody(groundRigidBody);

    // Falling sphere
    btCollisionShape* sphereShape = new btSphereShape(1);
    btDefaultMotionState* sphereMotionState = new btDefaultMotionState(btTransform(btQuaternion(0,0,0,1), btVector3(0,50,0)));
    btScalar mass = 1;
    btVector3 inertia(0,0,0);
    sphereShape->calculateLocalInertia(mass, inertia);
    btRigidBody::btRigidBodyConstructionInfo sphereRigidBodyCI(mass, sphereMotionState, sphereShape, inertia);
    btRigidBody* sphereRigidBody = new btRigidBody(sphereRigidBodyCI);
    dynamicsWorld->addRigidBody(sphereRigidBody);

    // Simulation loop
    for (int i=0; i<300; i++) {
        dynamicsWorld->stepSimulation(1/60.f, 10);
        btTransform trans;
        sphereRigidBody->getMotionState()->getWorldTransform(trans);
        std::cout << "Sphere height: " << trans.getOrigin().getY() << std::endl;
    }

    // Cleanup
    delete dynamicsWorld;
    delete solver;
    delete dispatcher;
    delete collisionConfig;
    delete broadphase;

    return 0;
}

Creates a simple world with gravity, a ground plane, and a falling sphere simulated in Bullet Physics.

Advanced usage

Where the library earns its place over a simpler alternative.

Adding constraints

cpp

btHingeConstraint* hinge = new btHingeConstraint(*bodyA, *bodyB, pivotInA, pivotInB, axisInA, axisInB);
dynamicsWorld->addConstraint(hinge);

Adds a hinge constraint between two rigid bodies, simulating joints like doors or elbows.

Soft body simulation

cpp

btSoftBody* softBody = btSoftBodyHelpers::CreateRope(worldInfo, btVector3(0,10,0), btVector3(10,10,0), 8, 1);
dynamicsWorld->addSoftBody(softBody);

Creates a rope-like soft body simulation using Bullet’s soft body module.

Vehicle simulation

cpp

btRaycastVehicle::btVehicleTuning tuning;
btRaycastVehicle* vehicle = new btRaycastVehicle(tuning, chassis, raycaster);
dynamicsWorld->addVehicle(vehicle);

Demonstrates how to simulate a vehicle with wheels and suspension in Bullet.

Using GPU acceleration

cpp

// Requires Bullet built with OpenCL support
// Use btOpenCLSoftBodySolver for GPU-accelerated soft body simulation

Enables GPU acceleration for large-scale simulations when Bullet is compiled with OpenCL.

Errors and fixes

The failures you are most likely to hit, and what actually resolves them.

Objects fall through the floor: Ensure collision shapes are set correctly and the ground plane is added as a static rigid body.
Simulation instability: Use smaller timesteps, increase solver iterations, or simplify collision geometry.
High memory usage: Remove unused rigid bodies and shapes, and reuse collision shapes across multiple objects.

Best practices

Use appropriate collision shapes (box, sphere, convex hull) for performance over exact meshes.
Always call `stepSimulation` with a fixed timestep for stable results.
Release memory carefully to avoid leaks in complex scenes.
Use constraints sparingly; too many can hurt performance.
Profile and optimize for broadphase vs narrowphase collision detection.

Background

Why it exists, and what it was reacting to.

Bullet Physics was created by Erwin Coumans to provide an open-source, high-performance physics engine for real-time applications. It became popular in the gaming industry and is also used in robotics simulations, VR environments, and even Hollywood films. The library is modular, efficient, and integrates well with OpenGL, Vulkan, and game engines.

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