xanxys / ammo.js

Direct port of the Bullet physics engine to JavaScript using Emscripten

ammo.js

Demos

• Cubes
• SoftBody-Rope
• SoftBody-Cloth
• SoftBody-Volume
• Heightmap
• Vehicle new!

Overview

Example code to give you an idea of the API: https://github.com/kripken/ammo.js/blob/master/examples/webgl_demo/ammo.html#L14

ammo.js is a direct port of the Bullet physics engine to JavaScript, using Emscripten. The source code is translated directly to JavaScript, without human rewriting, so functionality should be identical to the original Bullet.

Note: ammo.js has just been updated to a new porting approach. If you find some part of the Bullet API that is not supported that you need, please see kripken#60

'ammo' stands for "Avoided Making My Own js physics engine by compiling bullet from C++" ;)

ammo.js is zlib licensed, just like Bullet.

Discussion takes place on IRC at #emscripten on Mozilla's server (irc.mozilla.org)

Instructions

builds/ammo.js contains a prebuilt version of ammo.js. This is probably what you want.

You can also build ammo.js yourself, as follows:

• Get Emscripten

  http://emscripten.org

  and set it up. See

  http://kripken.github.io/emscripten-site/docs/getting_started/

• Run the build script,

  python make.py

  which should generate builds/ammo.js.

• Optionally, run the automatic tests,

  python test.py

Usage

The most straightforward thing is if you want to write your code in C++, and run that on the web. If so, then compile your code into LLVM, link it with bullet, and compile that to JavaScript using emscripten. (The easiest way to link it is to add your .bc file to the llvm-link command in make.py.)

If, on the other hand, you want to write code in JavaScript, you can use the autogenerated binding code. A complete example appears in

examples/hello_world.js

That is HelloWorld.cpp from Bullet, translated to JavaScript. Other examples in that directory might be useful as well. In particular see the WebGL demo code in

examples/webgl_demo/ammo.html

Bindings API

ammo.js autogenerates its API from the Bullet source code, so it should be basically identical. There are however some differences and things to be aware of:

• See https://github.com/kripken/emscripten/wiki/WebIDL-Binder for a description of the bindings tool we use here, which includes instructions for how to use the wrapped objects.

• All ammo.js elements should be accessed through Ammo.*. For example, Ammo.btVector3, etc., as you can see in the example code.

• Member variables of structs and classes can be accessed through setter and getter functions, that are prefixed with |get_| or |set_|. For example,

  rayCallback.get_m_rayToWorld()

  will get m_rayToWorld from say a ClosestRayResultCallback. Native JavaScript getters and setters could give a slightly nicer API here, however their performance is potentially problematic.

• Functions returning or getting float& or btScalar& are converted to float. The reason is that float& is basically float* with nicer syntax in C++, but from JavaScript you would need to write to the heap every time you call such a function, making usage very ugly. With this change, you can do |new btVector3(5, 6, 7)| and it will work as expected. If you find a case where you need the float& method, please file an issue.

• Not all classes are exposed, as only what is described in ammo.idl is wrapped. Please submit pull requests with extra stuff that you need and add.

• There is experimental support for binding operator functions. The following might work:

  Operator	Name in JS
  =	op_set
  +	op_add
  -	op_sub
  *	op_mul
  /	op_div
  []	op_get
  ==	op_eq

Troubleshooting

• It's easy to forget to write |new| when creating an object, for example

  var vec = Ammo.btVector3(1,2,3); // This is wrong! Need 'new'!

  This can lead to error messages like the following:

  Cannot read property 'a' of undefined Cannot read property 'ptr' of undefined

  This is an annoying aspect of JavaScript, sadly.

Reporting Issues

If you find a bug in ammo.js and file an issue, please include a script that reproduces the problem. That way it is easier to debug, and we can then include that script in our automatic tests.

Release Process

Pushing a new build in builds/ammo.js should be done only after the following steps:

• Build using python make.py which generates builds/temp.js

• Make sure it passes all automatic tests using python test.py (That uses builds/temp.js by default, you can also pass a flag saying which build to use.) Note that it uses SpiderMonkey by default, and SPIDERMONKEY_ENGINE is defined in ~/.emscripten, see the script contents for details.

• Make sure that the stress test benchmark did not regress compared to the old build. That number is printed out at the end of running the tests.

• Run the WebGL demo in examples/webgl_demo and make sure it looks ok, using something like firefox examples/webgl_demo/ammo.html (chrome will need a webserver as it doesn't like file:// urls)

Upstream Version

Bullet 2.82 patched with raycast fix from 2.83