Box2D-MT is an experimental library that adds multi-threading to Box2D.

It has no external dependencies and aims to be no less deterministic than Box2D.

Before using Box2D-MT you should be familiar with Box2D and multi-threading.

To enable multi-threading, create a thread-pool and pass it to your world's constructor.

b2ThreadPool tp;
b2World world(gravity, &tp);

Now when you step the world, the work will be split among all of your cpu's cores. The thread pool must remain in scope until the world is destroyed.

If you were doing anything interesting in your callbacks, things probably got a little crashy. This is discussed below.

Box2D uses various callback functions to let the user respond to events. In Box2D-MT, some of these functions are called from multiple threads at the same time, so your callback implementations must practice thread-safety.

Here are some guidelines for relevant callbacks (these are a work in progress and probably not 100% accurate):

b2ContactFilter::ShouldCollide: Treat all bodies and joints as read-only. Don't access any contacts.

b2ContactListener::BeginContact, b2ContactListener::EndContact, b2ContactListener::Presolve: Treat all bodies and joints as read-only. The provided contact is safe for reads and writes. Don't access any other contacts.

b2ContactListener::PostSolve: The provided contact is safe for reads and writes. A dynamic body that is part of the provided contact is safe for reads and writes, except for the body's flags, which are read-only. Joints attached to the dynamic body are safe for reads and writes. A static body that is part of the provided contact is read-only, except for it's flags, which must not be accessed at all. Don't access any other bodies, joints, or contacts.

A simple way of avoiding these dangers is to queue up events and process them on a single thread when the world is done stepping. There's no need for thread-safe data-structures or mutexes (aqcuiring a mutex in a callback can kill performance ^{[citation needed]}). You can use an array of per thread data of size b2_maxThreads, and use b2GetThreadId() to index the array.

A later version of Box2D-MT should provide an option to automatically defer callbacks.

Box2D is a 2D physics engine for games.

For help with Box2D, please visit http://www.box2d.org. There is a forum there where you may post your questions.

Please see Building.txt to learn how to build Box2D and run the testbed.

To run the demos, set Testbed as your startup project and press F5. Some test bed commands are:

• r to reset the current test
• SPACE to launch a bomb
• ← → keys to pan
• x and z to zoom in/out
• use the mouse to click and drag objects

Please do not submit pull requests with new features. Instead, please file an issue first for discussion. For bugs, I prefer detailed bug reports over pull requests.

• Continuous collision detection
• Contact callbacks: begin, end, pre-solve, post-solve
• Convex polygons and circles
• Multiple shapes per body
• One-shot contact manifolds
• Dynamic tree broadphase
• Efficient pair management
• Fast broadphase AABB queries
• Collision groups and categories

• Continuous physics with time of impact solver
• Persistent body-joint-contact graph
• Island solution and sleep management
• Contact, friction, and restitution
• Stable stacking with a linear-time solver
• Revolute, prismatic, distance, pulley, gear, mouse joint, and other joint types
• Joint limits, motors, and friction
• Momentum decoupled position correction
• Fairly accurate reaction forces/impulses

• Small block and stack allocators
• Centralized tuning parameters
• Highly portable C++ with no use of STL containers

• OpenGL with GLFW
• Graphical user interface with imgui
• Easily switch between tests using GUI
• Test framework for easily adding new tests
• Mouse picking and the bomb!
• CMake build system files

• User manual
• Doxygen document with code comments
• Active user forum

Box2D is developed by Erin Catto, and has the zlib license. While the zlib license does not require acknowledgement, we encourage you to give credit to Box2D in your product.